View Full Version : 350 Mag MPi overheating

Last Tango
03-25-2011, 10:53 PM
2005 series 350 Mag MPi with Bravo One drive, 191 hours, factory stock.

No prior overheats that could not be traced to either a plastic bag floating in the water, or a temporary and minor influx of mud in shallow water. All cleared long ago. No overtemps in the past two years.

A couple outings ago the boat showed a slight tendency to run up a few degrees when coming off of plane. However, it would come down to normal within a few minutes of idling. Then the last time out, it ran hotter and hotter right from the beginning. The digital temp gauge showed 184 while at 3300 rpm's. Normally it stays right at 159 degrees once it warms up. When I came off plane and slowed down, the alarm went off and the digital temp briefly read 224. The analog gauge quickly pegged. Fortunately, I was back at my dock when that happened.

So, a very competent mechanic looked it over and we agreed that it could be either the thermostat, the impeller, or the curvy hose that chokes up inside Bravo Ones. So he replaced all three. He ran it on the hose and water came out of all the right places while idling, and he didn't think it was running too hot. He did put in a 170 deg thermostat and he stated that it stayed within that range. The old impeller was worn only slightly from normal use. No missing blades or chunks. Thermostat looked normal for 5 years in brakish water. The hose in the drive was extremely constricted from the outside from the expected 5 years of corrosion build-up on the outside of the hose that eventually crushes them down. I was told by several sources that this is a common problem.

No oil in the water. No water in the oil. Oil did not seem too hot, even with the overtemp readings. Full synthetic Mercury spec oil. Only 12 hours since that was last changed with the filter.
Brand new drive belt at that same time as oil change. Belt turning, all aux items rotating, no funny noises, engine does not smell hot or even feel particularly hot. No hot engine clicking sounds or boiling water sounds. no smoke or odors of any kind. Engine compartment temp normal.

So, I put it back in the water and off I went for a test ride. It began to overheat immediately. The digital temp gauge went up at a pretty consistent rate, and I turned back to the dock after only 5 minutes running. Digital temp got to 184 as I was coming off plane. Alarm went off, and the analog temp gauge pegged. At the the dock at idle, no water coming out of the through hull tips.

Back on the hose and up on the stand, the water came through all the right places, although the tips seemed a bit weak to me. But that could have been the angle of the boat on the stand. I have the permanently open Y-style Quick 'n' Quiet exhaust. No Captains Choice switch. Water always comes through both the tips and the exhaust port on the drive. Water was running freely from the outdrive port. Engine temp began to rise well through the 170 thermostat to 175 and I cut it off. Water coming out of the tips was only luke warm to the touch.

So water can be pushed through the system under pressure, but it won't do it on its own.

Water pump in the drive?
Water pump housing in the engine?
Thermostat housing blocked?
Exhaust Manifolds?

I did lots of searches here before posting. Looks like it could be a number of (expensive) things at this point.

What say you?

(P.S. I have to be very careful what I wish for because it often comes true. In this case I have been waiting impatiently for 5 years for the risers and exhaust manifolds to clog up so I could order a new stainless steel EMI Thunder exhaust manifold and riser package. Did my Genie come through for me right on schedule?)

03-26-2011, 12:45 AM
Probably not the case based on your thorough description, but I thought I would chime in.

On my 97 MAG 350 Bravo I, the water pump with the impeller is plastic and has never given me an ounce of trouble. I change the impeller from time to time and this is a trailer boat that only gets launched at ramps. It always pumps without issue. I do always check the pipes at startup to make sure water is pumping before I leave the dock.

My Formula, with two 2004 350 MAG Bravo III's have brass water pumps and each pump has two blue "drain" plugs. This boat is launched each year with slings. Several times now, I've had it running on land via the hose and muffs for spring recommissioning and when it gets droped in at launch an air pocket prevents the pumps from getting primed with water and I get a pretty quick overheat condition. I've since learned to pull the lower blue plugs to purge the air and get water up to the impeller. Upon firing off the engines I pull the top plugs to make sure they're pumping pressure.

The first time before I knew what to look for / do (purge the air), I very quickly lost two impellers and had to learn to change them out while the boat was in the water since it's $100 to have it pulled and relaunched

Anyway, I hust thought I would throw this out there..

03-26-2011, 07:18 AM
My vote check the risers they could be getting plugged which would not flow enough water for higher speed, worth checking, it is normal for raw cooled engine to show a temperature rise when coming of plane only to cool down as you describe. I have had to deal with that issue many times with new customers, even having to go for a ride or two to demonstrate it.


fogducker III
03-26-2011, 09:24 AM
Just a thought, how do you flush the motor when on land? Is there a through hull fitting to attach the garden hose or "muffs"?

If it is a through hull, check the cap and washer that block it off at the hull as well as the line that feeds the water pump. On one occasion I left the cap off and the pump pulled air rather then water through, easier to pull air then water, motor heated up REAL quick.....yours seems to take a little time to over heat so perhaps there is a "small" air leak.......:confused:

Failing that it might be old impeller pieces in the lines?

Morgan's Cloud
03-26-2011, 04:56 PM
, , . The hose in the drive was extremely constricted from the outside from the expected 5 years of corrosion build-up on the outside of the hose that eventually crushes them down. I was told by several sources that this is a common problem.

Are you confirming that the grommet in the trans plate that transfers water from the outside hose to the inside hose is OK ?

In your case this sounds like a classic example of it.

Last Tango
03-27-2011, 07:16 AM
No through-hull fitting for the land flush. The earmuffs (and a special nose plug) fit over the bottom of the drive above the skeg and torpedo. There are a vertical series of intake holes on each side of the lower drive unit sort of like gills on a fish. The muff covers those holes and the hose connects to the muff. The nose plug blocks water from just pouring out a hole on the leading edge of the torpedo. All of the holes feed the same "plenum" area in the lower drive unit for the water pick-up. The nose cone hole provides a "ram air" effect for water to enter the drive when under way.

Since I was not present when the hose in the outdrive was replaced, I cannot confirm if the grommet was plugged, replaced, or even inspected. However, since the drive was removed to replace the internal hose, I would hope the mechanic inspected that since the whole reason for going in in the first place was to search for blockages.

I usually experience a few degress of temp increase when first coming off of plane. But the recent activity far exceeded any normal observations. I do know it takes the cooling system a few minutes to cool the block down after running on plane and then reducing the amount of water flow when coming down to idle. But that increse was usually only a few degrees for a few minutes (amazing how persnicketty I can be about those details since I installed the SmartCraft System Monitor a few years ago. Analog gauges just don't provide that level of detail).

The mechanic has not had an opportunity to get back to my boat, yet. (it has only been a one actual work day since the test drive) Probably go in again this coming week. And I will be available to be there to ask questions (LOL! JUST what he wants! LOL!)

03-27-2011, 09:41 AM
I am assuming he replaced the "s" hose so that means he cleared both ends...they do the same thing. Problem is the motor end is a PITA to get to on some boats.

If he just shortened the hose to repair the bell housing side then your problem is probably the other end at the motor.

Any air leak on the intake hose/fittings/transom nipple will cause an overheat.

Surge temp when pulled back is normal as previously said.

Last Tango
06-17-2011, 09:35 AM
So here is what we know so far:
RANMAN gets an A+ for his suggestion about checking the blue drain plugs. And everyone else had great ideas that seemed in some fashion to also be part of the problem. Most likely it was an air leak, perhaps in more than one place.
The previous mechanic sort of walked away on the project a couple months ago, and left the motor partially disassembled. So after waiting patiently for his return, I was fortuitously given a referral to an different mechanic through a most unlikely source (the guy who services our community water and sewage system knew a guy...LOL!) So this guy comes out promptly, surveys the motor and disassembled parts and begins to dig in. 3 hours later, he has completely removed and inspected and reattached every hose, fitting, and attachment on the motor and drive and put them all neatly and tightly back together. He says the first guy appears to have actually done the thermostat replacement, although with one that is 10 deg too high. He also says he is taking the risers home with him to clean up the mating surfaces with a grinder before reapplying the new gaskets that he will put there. The risers are completely clear as are the manifolds. No blockage of any kind and he says they look perfectly good for years to come. So he takes the risers home for the weekend to prep them for installation.
Monday he is back with all the right stuff, completes the reassembly in about an hour, takes a lunch break to let the gaskets dry, comes back and fires up the motor. He makes adjustments to all the controls, zeros out the trim gauge on the drive, and we watch the motor warm up.
It is on the hose with earmuffs and nose fob.
It starts to over heat again.
Water is flowing through the system, the risers stay cool, no hot engine parts or smells, but the actual flow out the exhaust tips is weak compared to my previous experiences on the hose in the yard.
We shut down the motor.
He climbs in and notices water dripping slowly from the lower blue drain plug. He tightens it up again. We switch from the hose to a deep bucket of water under the drive to ensure the motor and drive are sucking water on their own without being pressure fed by a hose. Yes, techincally we changed two things at once so that sullies the scientific method of troubleshooting, but we do it anyway.
Boat fires right up, water starts sucking through the drive and motor and out the exhaust tips at the normal rate of flow, and the engine warms up BUT DOES NOT OVERHEAT! TA-DAHHHHHHH! Probably was an air pocket, and probably from more than one point of origin.
He is going to replace the thermostat back to the correct 160 deg vs the 170 deg part the previous mech put in. Then we are going to give it the ultimate litmus test by lauching, with him along for the ride just to make sure.
I'll report back here when we have completed the test ride.

06-17-2011, 10:36 AM
The new blue drain thing is nice in winter but seems very sensitive. My engines have always run a bit on the warm side, upwards of 180 on the analog gauges.

Well, I developed a small drip on the port engine at that lower blue drain and now that engine runs 10 degrees cooler.

I'm wondering if I should try Ranman's idea this weekend... purge the water while in the water on the Starboard side to see if that one drops.

On a side note I developed a habit many years ago of slowing down well short of the slow speed zones and keeping the engine(s) up to 1200 rpm or so for a bit, then dropping to 1000, then dropping all the way down. it seems to really help the temp flux as well as oil PSI flux.

Also... I noticed my oil pressure stays roughly 10 psi higher after going to the Mobil 1 double sized filters this season. They used to drop to 20 psi after a run, now they stay around 30 psi.

Last Tango
06-17-2011, 03:27 PM
I agree with you. Since my first powerboat many years ago (Classic 16, then Classic 18, then Z3250, then this 22ZX) I learned right away to always ease down on the throttles when coming off plane and going to idle, in order to minimize the rise in heat or lack of water flow when the block is still hot from running. Exactly like a race car ar the track - take a cool down lap after running hard so as not to overheat anything. I always ease the speed down slowly, and then try to keep the idle revs at about 900-1200 when still underway. I very rarely let the engine idle at 650-ish. Just not enough water flow in my opinion. Even on the trailer when flushing the system I usually run the engine at about 1300-1500.

And Ranman's suggestion is good. I believe we accidentally did just that after the mech saw the drip from the blue drain plug, because the water was still running into the engine from the hose (just like sitting in the water) when he readjusted the plug.

My analog gauge normally reads just a needle-width under 180 deg under normal cruising conditions. The digital gauge reads 159 deg normally. I attribute that to three possible things - all of which could be happening:
(1) Accuracy of the analog gauge, including it's small size (therefore lack of good detail), and paralax error (reading the gauge from different angles).
(2) Different locations on the engine for the temperature sensors for each type of water temperature gauge. Analog is up near the thermostat - digital is somewhere else internal.
(3) Basic accuracy of the analog gauge to begin with. I consider all analog gauge readouts to be "suggestions" rather than facts. My analog tachometer reads a couple hunder RPM higher than the digital tach in the SmartCraft display. Which would correllate with all analog gauges. Analog speedo's "always" read whatever that want - high/low/not-at-all, compared to GPS speedo's or RADAR gun.

Off Topic: I consider RADAR guns to be less accurate than GPS readouts because the RADAR gun reflects off the surface of the water for its info. That requires at least some ripples to get the reflection. Ripples and waves are driven by the prevailing wind which adds a speed vector that the RADAR gun does not calculate out. And if the reading is taken on a lake filled with wakes or crossing wakes, then it has all of that to deal with, as well, and it does not. GPS is a pure speed over the bottom reading, not speed through the water which is what an analog gauge reads - the biggest factor being current on the river or lake. Analog is great for speed through the water but is not an accurate system since it does not factor for water temperature or density and relies solely on the water pressure coming into the pick-up tube. And analog gauges rarely have the depth of detail available from GPS or RADAR - thus not the best instrument when doing prop testing and such.

Last Tango
06-20-2011, 10:24 PM
So the mechanic took home the "old" thermostat, and plunked it in the water with the new thermostat that he ordered. The new one opened right spot on at 160 as advertised. The "old" themostat started to open at 170-ish but never fully opened

He installed the new thermostat and we ran it on the trailer for over half an hour. Both temperature gauges recorded perfect warm-up and temperatures at all running RPM's. Excellent water flow in and out.

So we will still do an actual sea trial in a few days, but it looks like we have resolved the issues.

To review the solutions: obviously the thermostat, and also the blue drain plug seems to have been the problems.

Phil S
06-21-2011, 12:55 AM
Was hoping I was about to hear, "your wish is hereby granted ! " ;)

"Problem solved"...is equally nice to hear though. ;) Thanks for the feedback on the solution.


Phil S.

Last Tango
06-24-2011, 09:29 PM
Zoltan did in fact grant the wish I prayed the hardest for... and that was that the Repair Bill would NOT be BIG. LOL!

Last Tango
07-20-2011, 09:33 PM
So today we finally got the chance to do the sea trial.

Problem with overheating is still as bad as ever.

Naturally I am disappointed since we had such a good trailer run a few weeks ago.

Mech was in the boat with me and saw clearly what happens. Now he is going to remove, inspect, and replace as necessary two flow valves in the lower part of the motor. The motor was too hot to try that today, so he is coming back tomorrow afternoon. Perhaps, if the clear weather holds, we'll get another sea trial late tomorrow evening.

Water is flowing well from the drive to the motor and out the tail pipes.
There is plenty of water pressure showing at all speeds.
The risers are relatively cool to the touch, even after running.
But the block heats up as though the water is not flowing through it.

I'll update you when we run the boat again.

07-21-2011, 03:46 AM
Ask you mech to install clear hoses in place if the riser feeds, run it up and watch for a stream of air. If you see air even the tiniest amount it indicates either air entering the intake water flow or a head gasket leak, that is an old very simple test to check for that condition without tearing into it. If you have air entering the system it ends up filling cylinder heads allowing the block to heat up while everything else stays cool. I have seen something as simple as a loose hose clamp on the intake side of a sea pump cause a condition like you have. Another test is to use a heat gun check the heads and low on the block there should only be about 15 degrees difference, with the top being hotter any major variation means the air is forcing the water out of the heads!

Last Tango
07-21-2011, 08:59 PM
My mech and I discussed your suggestion this morning when he came to take another look at things. Turns out he is a member here and read your suggestion last night. Currently, he does not believe we have an air leak before the sea strainer since he sealed that up during his first inspection. But we are absoluterly NOT totally dismissing your suggestion.

Today, he removed the two T-fittings that allow water to drain or circulate water from the exhaust manifolds to the block. These T-fittings are fitted to the newer (2005-ish and up) 350 exhaust manifolds. They allow water to circulate through the heads and the block. There is a valve in these fittings that is not on earlier versions of the 350. The valve opens to allow water to be drained from the manifolds for ease in winterizing. The valves remain open by pressure from the water pumps.

One T-fitting was completely blocked. The other was almost totally blocked.
Although the valves were opening, they were partially obstructed and did not open fully.
This means that cold water was flowing through the exhaust manifolds, but it was not then going to the block.

We are replacing both fittings with new ones (about $30 each).

Important to this discussion is that the material blocking the valves is metal flakes from the decomposing manifolds. Since the manifolds are made of cast iron, and were not ceramic coated inside in those days, this decomposition is normal. The manifiolds were made in 2005, installed for running in 2006, and have just a tad over 200 hours in brakish water. Five years is about normal for manifolds to begin the decomposing process.

The Mech is going clean out the passageways to and from the fittings. We are not at this time going to replace the manifolds. I will probably squeak out another year or two, and then need to replace the manifolds.

This is both good and bad news. I have been waiting to replace the existing manifolds and risers with EMI Thunder polished and ceramic manifolds and risers. The stock manifolds aren't quite dead yet and I want them to hang in there just a little bit longer.

We'll see if this works.

Pictures attached below:

07-21-2011, 09:14 PM
If you wait too long to replace those cast-iron manifolds any internal waterjacket failure could do severe engine damage.

I would not tempt fate too long ~~~If I were you.

07-21-2011, 09:22 PM
Just my o2,,from what your describing is sounds like the engine circulation pump may have no fins left to it,,remove the water hose from the thermostat housing that goes to the circulation pump..start the engine for a split second and give it a quick rev to see if water comes out of the hose..if not remove the circulation pump on the front of the engine and remove the back cover to see if the fins are rotted off..the engine will idle all day long with no fins but once you put a strain on the engine it will start to get hot rite away.

Last Tango
07-21-2011, 09:34 PM
I read you loud and clear on that Silverghost. And I agree completely. This is just a "patch" to tide me over until, next summer. I do not expect to put more than 10 hours on the boat between now and then. I have a logistical issue right now. So the the "patch" is to tide me through until I either get a tow vehicle, or go back to dry storage. Mostly, the boat is going to get run on the trailer for the next year, to keep things lubricated internally. And the new exhaust system will get put on, as well as the new custom Bimini.

But I still want it "right", even if I'm not using it much. Ya never know when opportunity will strike to run it for a couple hours on the water. Near term, I'm just interested in burning up the gas in the tank so it doesn't get too old in there. I use only certified ethanol free gas, and I treat it anyway for water condensation. No need to have 40 plus gallons wasting away in there for a year (52 gallon tank).

Last Tango
07-21-2011, 09:46 PM
I like your idea, too. I'll have the mech do this. Maybe the flakes are from the internal circulation pump impeller rather than the manifolds.

07-21-2011, 10:30 PM
I like your idea, too. I'll have the mech do this. Maybe the flakes are from the internal circulation pump impeller rather than the manifolds.

in the past i have also seen that where the impeller is pressed on the shaft will spin,if and when he takes it apart hold the shaft and se if the impeller will move..

07-21-2011, 10:43 PM
that impeller and backing plate should be SS

keyword > should

07-24-2011, 11:32 AM
Makes you wonder what was on Mercury's mind when they made that design change huh?
Seems to me to be a perfect area for malfunction.
I'll bet you cured the overheat.

Morgan's Cloud
07-24-2011, 01:16 PM
One of the best things about the internet (at least for me ..) is when people who have had strange things happen are joined by other knowledgeable pros and can get together to hammer out a problem.

Although it doesn't come into play here , I figured it might be pertinant to tell a short story of what happened to my brother and I a few years back with the Magnum. (502mpi)

He had taken it a few hundred yards from the slip to the yard to be hauled ..everything was fine .. I had even used the boat a day or two before and everything was ok.
The run to haul it was all at idle.

Later on in the yard when we went to flush the engine it wouldn't pump water.

A subsequent rebuild of the raw water pump showed that it had only just let about 4 of the vanes fly with more about to fail. When we examined the impeller the rubber had the consistency of plasticene or modelling clay. the mechanic had never seen one like it before. You could effortlessly peel the vanes off without even trying.

I can only say that it was damn lucky that it happened the way and when it did , because I bet if my brother had gone out and gotten on a plane he would have cooked the engine.

Last Tango
07-24-2011, 04:55 PM
Originally that part was just an elbow, not a T, and thus it was far less likely to be an issue. One could let the manifolds decay a lot longer before problems surfaced- particularly there. The Mech offered to replace the T with an Elbow and eliminate that problem. However, it would then prevent the engine from being easily drained for winterizing.

I said "No" for two reasons:

1: Although I have never had to worry about winterizing here in Florida, you just never know what the future brings and I didn't want to eliminate that choice completely.

2: The Smartcraft sensing systems in the newer engines are run by computers and neither of us knew if bypassing the drain feature would ultimately throw a fault somewhere else in the system. The temperature sensing in the Smartcraft showed the symptoms of the overheating problem far earlier than the analog gauge.

The parts are cheap and easy to acquire for this change. He also has a tool to "Roto-Router" out the passageways before installing the new parts. If the weather co-operates we will be doing the sea trial again on Wednesday.

07-24-2011, 09:34 PM
+2 on the circ pump not being an issue, they wear out long before they rot. In over 40 years as a tech never had one rot and only had one 318 Chrysler spin the shaft, they used an S/S shaft with a pressed on bronze impeller. It happened on one of my favored customs boats, you can bet I cross pinned the new pump so it would never happen again.

Last Tango
07-27-2011, 08:40 PM
So the mech installed the new T-valves today. Ran the boat on the hose and it overheated almost immediately...

We pulled the output hose off the circulating pump and it gushes water even at low RPM's.

So... The next step is to completely remove the exhaust manifolds and have a look inside them.

A side note on the new T-Valves is that they have a large drain hole than the originals. But the holes are just small, not the full diameter of the tube. Keeps the water from gushing out when the valves are open. Not sure of the reason for this, but the newer ones are obviously larger and would not block up quite as fast as the older design.

Everything done so far had to be done anyway so I don't feel that all this trouble shooting and parts replacement was a waste of time or money.

The mech and I are taking a two week break as we are both out of town for a week each, just not the same week. LOL!

Last Tango
04-17-2013, 11:27 PM
So, here I am, two years later, and the immediate overheating problem has persisted through two more mechanics. I haven't had the boat off the trailer for over 18 months. I did get the fuel tank drained last Fall since the gas in the tank, even though heavily treated, was still pushing two years old. So he popped the tank cover seal and drained everything out and cleaned the tank and added a couple gallons of fresh and heavily treated Ethanol-free gas, so we could continue our on-again-off-again trouble shooting.

Tonight, we put two brand spanking new batteries in the boat. It was still running the factory installed originals, now easily close to 8 years old, and they were showing signs of near death.

Fired up the motor to run some oil over the bearings. And after a couple minutes it still overheated. When the thermostat opened and the heat continued to build, the mech hand-squeezed the two flow lines on either side of the T-fitting that takes incoming water directly to the exhaust manifolds, supposedly only until the thermostat opens and allows the water to then course through the engine block. The moment he squeezed those two lines, the engine temperature (now running very hot) almost immediately dropped from the 190's well down into the 150's over the course of less than a minute.

Those two hoses come directly from the plastic T-fitting that directs water to the risers and exhaust manifolds. That water comes directly from the WATER DISTRIBUTION HOUSING sitting about 18 inches of rubber tubing below the T-fitting. When he released his squeeze, the temperature immediately started rising again.

The Water Distribution Housing in older model 350's has a rod, ball, and spring attachment inside that determines from pressure how much cooling water goes to the engine, and how much just goes to the risers, similar to a spring controlled turbo waste gate. The pressure is a function of water temp.
The Mag MPi series I have does not have this "mechanical" pressure relief valve. It is just a bare and bulbous housing with hoses in and hoses out.

We pulled all of the hoses and T-fitting off and checked for any blockage. None.

He removed the water distribution housing to see if there appeared to be a broken piece inside like a valve of some sort. There is NOTHING inside the housing, and there is no appearance from any angle that anything was ever in there to begin with. So what in this housing determines when the pressure is too high and diverts the water to the block rather than straight out the risers? Right now, and on the Mercury Design Drawings, there doesn't seem to be anything there or inside the housing to do that. Looking at the housing drawings, it does not indicate anything mechanical is inside that housing. Yet, whatever made that descision before is no longer doing it. Right now, the water just takes the path of least resistance and continues to go straight up the hose from the Water Distribution Housing to the T-fitting and out the exhaust, rather than going to the engine block. Unless he hand-restricts the hoses at the T-fitting (not safe to try to squeeze the hose from the distribution housing when the engine is running since it is behind the belt) the engine overheats. If he gives the two hoses a bit of a squeeze the engine cools down right away. He removed and inspected all of the pieces and everything flows. There is just nothing at or after the Distribution Housing to encourage the water to go to the block. The water is simply taking the path of least resistance. Yet a light squeeze is all it takes to get the water to flow like it should to the block.

Does anyone here have a detailed working knowlege of the inside of the Water Distribution Housing? Has something that was originally there dissolved and gone away?

04-18-2013, 03:52 AM
You keep talking about a water distribution housing. Is this the thermostat housing you're talking about? I'd like to see a pic of what it is.
The multiple drain system on the Mercruisers of late have me a bit irritated. It's hard to know what the hell is going on!
I looked through my microfische and couldn't find one single Mag MPI motor that had a thermostat housing with a T fitting on the top, like I think you're describing.
If you have this T fitting housing, with the 2 hoses going to the risers, and 2 going to the bottom of the manifolds...I have to wonder if this is the correct housing.
This is a pic of what I found:
Now I realize this doesn't show the multiple drain, but it does show the thermostat housing. Note it has no hoses going to the risers.
If you have a T fitting at the top of the housing, with hoses going to the risers, it needs the check ball fittings inside the T.
2 ways of dealing with this. Remove the T, plug up...and put caps on the riser inlets. Send all the water through the bottom of the risers.
Or...put the restrictor check balls in the T.
You've found the problem...now all you have to do is fix it!!!!

04-18-2013, 07:04 AM
Mark, two years ago I chased an overheating problem on my 22. I melted an exhaust hose and all the fingers pointed to clogged risers. For multiple weekends I rooted around inside the risers with a piece of wire and found nothing, still had a overheating problem. The impeller was new earlier in the season, but I changed it again. No luck.

My neighbor who just happened to be in the boat house while I was changing the impeller said that the "plastic" housing for the pump was worn.

Short answer, I changed it and no more over heating issues.

Pump was pumping enough water at an idle, but under way the pump couldn't keep up.

I haven't read the whole thread, so if you have already changed your pump housing.............


Last Tango
04-18-2013, 07:38 AM
http://www.boats.net/parts/search/Merc2/Mercruiser/350%20Mag%20Alpha%20Bravo%20MPI%20-%20%28350CID%205.7L%20V8%29/0W310000%20THRU%200W649999/Standard%20Cooling%20System%20%28BRAVO%29%28Single %20and%203%20Point%20Drain%29/parts.html

The link above is my cooling system.

The part I am referring to is at the very bottom of the diagram. Strangely it does not have a number attached to it, only the hose clamps (25 & 26) and the three hoses (7, 11, 19). However, further research determined this part is called the Water Distribution Housing.

Hose 19 goes up to T-fitting 20. That T is a simple plastic T with no internal parts. It feeds hoses 22 and 23 which in turn feed the exhaust manifolds and risers. Referring to the "squeezing" mentioned in my post above, we squeezed hoses 22 & 23 just a little, and the engine immediately started to cool down. It is unsafe to squeeze hose 19 when the engine is running because it is immediately behind the spinning belt. But there is nothing inside the T-fitting 20, anyway. So the water is being diverted up hose 19 from the Water Distribution Housing when the engine is cold, and then (supposedly) when the thermostat opens, the water is supposed to then divert to the block. via hose 11. We have completely removed all of those hoses and fittings and inspected them. No blockage.

To me, there should be some mechanism inside the Water Distribution Housing (again, bottom of the diagram) to determine when the pressure is right for water to move through hose 11. When you squeeze the two upper hoses, water flows and the engine immediately cools. Yet, the housing is completly empty of any baffles, or parts that would divert flow upon expansion or increase of pressure from anywhere. If that housing is indeed hollow, why is is not just a T-fitting? Why the fancy engineering and shape? There is nothing to resist the water going up hose 19 (unless we squeeze the upper hoses) and right out the engine rather than going into the block.

So, obviously, we are missing something here. The circulating pump has been removed and inspected by 3 different mechs and blessed as perfectly fine, as is the water pump impeller, and the thermostat and its housing. Plenty of water pressure at evey hose and fitting. Huh.

The mech ordered a new Water Distribution Housing so we can compare the two side-by-side. We are also going to remove fitting 9 and the two hoses 10 to ensure they are clear as well. Also pulling off the hoses 4 and the associated Y-fitting (again) to inspect for problems there.

Greg K
04-18-2013, 03:02 PM

Did you check the fitting #8 below the exhaust manifold for any restrictions.
Diagram link: http://www.boats.net/parts/search/Merc2/Mercruiser/350%20Mag%20Alpha%20Bravo%20MPI%20-%20%28350CID%205.7L%20V8%29/0W310000%20THRU%200W649999/Exhaust%20Manifold%20and%20Elbow/parts.html

Last Tango
04-18-2013, 03:13 PM
A worn impeller housing has been previously suggested. We have installed and removed the impellor blades a few times in this process and no one before thinks the housing is scored or worn inside. And the water pressure readings on the outflow side of the pump are normal for each RPM. (Update: See post below! The water pump housing surfaces are indeed scored and we are replacing it).

FYI to all - The engine has barely 200 hours since new, less than a total of 15 minutes of that time has been at WOT (ECM history printout). Over 60% of the run time has been between 2850 and 3550 RPM, the rest of the big chunks of run time are below that. And the impeller has been changed at each 50 hour service, which in the first 4 years was annually. I ran the boat all year around because here it Florida we can. So no winter lay up time. The 5th year I ran the boat very little due to my schedule. And the last two years only a couple hours total due to the overheat problem (most of that at about 1500 RPM +/-) for troubleshooting.

The boat tends to fall off plane below 3000 RPM anyway. Seems to cruise happy and efficiently as possible at about 3300 RPM. I have a Smart Craft gauge that includes among other readouts GPH (Gallons Per Hour) so it has been easy to find its sweet spot for hull speed and engine RPM.

Last Tango
04-18-2013, 03:19 PM

Did you check the fitting #8 below the exhaust manifold for any restrictions.
Diagram link: http://www.boats.net/parts/search/Merc2/Mercruiser/350%20Mag%20Alpha%20Bravo%20MPI%20-%20%28350CID%205.7L%20V8%29/0W310000%20THRU%200W649999/Exhaust%20Manifold%20and%20Elbow/parts.html

Hey, Greg! Yeah, in the summer of 2011, mech number two replaced both of those fittings on each manifold with brand new ones. He even cleaned and inspected the exhaust ports and cooling passages while he was doing that. We had high hopes that the problem was solved since the boat initially water tested good... for about 6 minutes... then same old problem.

Last Tango
04-25-2013, 06:01 PM
So, last night we removed and inspected every external assembly, part, and hose associated with the cooling system. No hose, or part is blocked. Here is what we also found:

We removed and inspected the block water circulating pump off the front of the motor. We used a dental mirror to try to inspect the vanes but could not see into the assembly far enough to verify that all of the vanes are still there. So for that part, we are ordering a cheap regular automotive recirc pump. When we reassemble everything, we will use that automotive pump temporarily to see if the overheat continues. If the motor does not overheat, we will try the marine pump again to see if that causes the motor to overheat. If the motor overheats, we will replace that pump with a proper new marine recirc pump. No point in disassembling the original pump, because once you break the seal with the original shaft, you cannot easily remount the vain impeller (they are press fittings). The cost of the automotive pump is cheaper than the labor and machinery to remove and re-press the shaft. And the automotive pump can be used on another car project later.

We removed and inspected the raw sea water pump on the engine. The rubber impeller was OK but we will still put a new impeller back in because the bronze/brass? faces top and bottom inside the pump were scored. Not terminal, but I have elected to get a new pump because it would otherwise need replacing soon anyway.

The fuel cooler was removed and inspected. No issues there. We just need to order some new water hose fittings because the original ones are plastic and it looked like their sealing faces were distorted by heat, though still water tight

The thermostat housing and thermostat were removed and inspected. No issues there. Thermostat is virtually new from the previous mech and the housing has no marks or leaks.

Water was pressure flushed through the block from several locations and it all ran out just as fast as it could be forced in, so no apparent blockage inside the block.

Next is to completely remove and inspect the exhaust manifolds and risers. Risers are clear, but we are going to make certain nothing is blocked at the heads and manifolds interface.

If everything looks good there, then we are removing the intake manifold and heads to visually inspect the coolant passages in the block.

04-25-2013, 07:16 PM
my MPI has been running 5* hotter the last 4 times out > +20* now..

I am picking mine up tomorrow and i'll let ya know what we found, I have a 300/Alpha 1...

my mech says it's your sea water pump housing, since it cools down when you pinch the 2 riser outlet hoses ...

Budmann just lost his plastic pump housing 2 weeks ago too... he went to a brass pump

or your thermo housing is retaining air and needs to be bled...

Last Tango
04-25-2013, 07:54 PM
Yep, and I hope your mech is absolutely correct. New sea water pump inbound.

And we will use the blue plastic draining valves to bleed the entire system after reassembly.

04-25-2013, 08:11 PM
also, he said the inlet hose from the transom to the pump could delaminate internally since it's multi ply and collapses under vacuum conditions...

he was going to add a bleeder to the TOP of my thermo housing, since the air gets trapped in that housing being the highest water chamber in the sytem, it can't trap air in the manifolds, only debris..

my manifolds and risers are p e r f e c t, and they are 12 years old..

oh, and I changed the thermostat to a 140* this time as an experiment..

if you use an automotive pump, MAKE SURE IT IS THE CORRECT ROTATION PUMP

send me a pic of your engine looking down towards the front..

joseph m. hahnl
04-26-2013, 08:05 AM
also, he said the inlet hose from the transom to the pump could delaminate internally since it's multi ply and collapses under vacuum conditions...

he was going to add a bleeder to the TOP of my thermo housing, since the air gets trapped in that housing being the highest water chamber in the sytem, it can't trap air in the manifolds, only debris..

my manifolds and risers are p e r f e c t, and they are 12 years old..

oh, and I changed the thermostat to a 140* this time as an experiment..

if you use an automotive pump, MAKE SURE IT IS THE CORRECT ROTATION PUMP

send me a pic of your engine looking down towards the front.. ...

A collapsing hose could be a strong contender. From hose 11 up to the the blocks water pump .Back in the old days,you could put a coil wire in to prevent the hose from doing that. There should be a couple of plugged off water ports on the intake manifold. These are used for a heater core and bypass hoses and such. you could put a clear hose from there, back to the water pump port on the lower passenger side /starboard port, to the rear port of the intake manifold . In any case you should have a by pass hose from the top water pump"engine" port back to the engine below the thermostate. This allows water to circulate while the thermostate is closed.

Last Tango
04-28-2013, 09:12 PM
So I will now post a couple pictures I took today after the risers were removed, and then the exhaust manifolds.

Spoiler alert: The risers were perfectly clear and were flowing quite well. However, the exhaust manifolds looked like they were filled with fresh asphalt.

Last Tango
04-28-2013, 09:20 PM
And a couple more:
1. Blocked exhaust manifold
2. Clear risers
3. The scored housing face in the seawater pump and why we are replacing that as well.

Last Tango
04-28-2013, 09:36 PM
The exhaust manifolds were not letting the water out to the risers. Water flowed through the drive, the seawater strainer, the sea water pump, some to the fuel cooler, the rest to the thermostat. When the thermostat was closed the water flowed past the block and out the risers. When the thermostat opens the water flows through the circulating pump which pushes it all around the engine and then out the exhaust manifolds to the risers and out the tail pipes.

Since the exhaust manifolds were blocked internally, the water just ran around inside the motor getting hotter and hotter. Water appeared to be flowing everywhere in and out, but was actually skipping the block, which is exactly what the symptoms were.

So the next step is pictured below: Eddie Marine EMI Thunder for SSB - but either polished or blue.

04-28-2013, 10:08 PM
Wow! That took a lot of troubleshooting to isolate. Frustrating for sure. Glad you can finally put this overheating deal to bed. New manifolds and risers will be great. Just in time for boating season!

Last Tango
07-21-2013, 07:18 PM
So, I finally saved up enough money for the exhaust system I wanted. Special order, custom polish and color. Here they are installed: EMI Thunder exhaust manifold and risers, Mr Gasket aluminum valve covers and oil cap (all coated) and stainless steel hardware all around.

On the trailer, the engine stays well inside its cooling temperature parameters.
Later this week is the litmus test - on the water...

07-22-2013, 05:37 PM
I hope it runs good for you Mark

Last Tango
09-01-2013, 07:54 PM
And so this saga comes to a close. After weeks of rainy afternoons and weekends and busy schedules, we finally put the boat in the water and ran it around for...
... over an hour. All speeds. No problems with overheat, not even close to the limits. The new EMI Thunder exhaust did its job in beautiful Technicolor. And with richer sound.

Since the motor had not been run hard for over two years, there were some initial complaints from the fuel injectors being awakened from their hibernation. But within a few minutes of 3000 rpm running, they cleared their throats and then we ran around for another half hour before we tried WOT. Even in the summer heat and with two large riders on board (me and the tech) the boat and motor pushed up to 65 without drama. Both the temp gauge on the dash, and the Smartcraft multi-gauge showed normal temps that held under all situations - long idle (20 minutes plus), long cruise at 3500 rpm (20 minutes plus), and WOT for several minutes.

I hope this helps folks with similar problems.

Now, off to get those new trailer tires...

09-01-2013, 08:11 PM
It is nice when problems have a positive finish.



09-01-2013, 08:32 PM
Congratulations Mark!