View Full Version : Running hot

sweet 16 1966
06-09-2006, 02:57 PM
Over the last couple years, I have replaced the bellows and shift cable rubber. Gone through the outdrive and replaced any bearings, gears, waterpump and impeller. The engine is new. Replaced its waterpump very recently. 302 enging with Mercruser outdrive- pre alpha. Temp has shot up to 220 and likes 190. No thermostat. No cranking issues at any temp range. Engine feels like it may not be as hot as indicated although I do n ot want to count on that.
My thinking is the water hose which connects to the outdrove and is connected to the thru fitting in the transum is getting crimped and keeping enough water from cooling adequately. Pics do not show it as well as expected.

Appreciate any comments on this matter as to a fix for the " weak rubber waterhose" ??

Morgan's Cloud
06-09-2006, 03:07 PM
The only time I ever had a 'hot run' situation like that (with the same outdrive/trans assy) it was in fact the hose you described as being the culprit. We used a non Merc straight pce of hose and it did'nt 'conform' to the curve the way it should have.
Definitely caused a kink that was hard to see and then up went the temp gauge !

sweet 16 1966
06-09-2006, 03:27 PM
How did you correct? Its a merc hose I am using. I try to store with outdrive down when possible so it lowers the "memory issue" but to no avail.

06-09-2006, 07:52 PM
I am not going to ask why no thermostat. But the hose you mentioned goes to the cooler and directly to the water pump and thru the engine and out the exhaust. If you have thru transom exh., how much is exiting from the pipes? Do you have stock exh.? I suspect exh. riser manifold. Give more imfo. I see you are local. Has the boat been in salt?

06-09-2006, 09:05 PM
You know a simple, fairly inexpensive solution for this and the silting problem discussed elsewhere is a through transom water pickup. First, it's very hard to pick up silt from that high, and second, you do have a simple straight hose to furnish water.

06-09-2006, 09:13 PM
I'm a big believer in an offshore type water pickup. I have them on my 302, and on my 22 with the TRS. Keeps the water from being full of sand, silt, and such that is worked up by the props.

06-09-2006, 09:57 PM
Only problem it is an Alpha with a drive pump, but you are 100% on transom pickups and inboard pumps!

06-09-2006, 10:36 PM
The intake hose you mention ususlly colapses on the inside. and can't be seen (in most cases) from the outside. this is an early boat which makes it real possible. First make sure it is really overheating and not the guage or the sending unit. A lot of times if a sending unit is overheated it may read high constantly especially if it is the thermowax variety which was stock back in the early boats. Un like Jim I want to know why the thermostat was removed. In that motor (depending which setup you have,there are at least 3 that I know of) you may need a t-stat for proper colling. I am local and live on Lainer and I have inferred temp guage we can test the real temp of the block. My Corsican runs at 190 on purpose I set it up to run this temp. under no circumstances should it run less than 160F and I don't care who thinks this is wrong, it is correct!!! I will explaine it over lunch or dinner. I won't do it here again because I have done it here 4 times since 1998. We can fix it providing it is broken.

06-10-2006, 07:46 AM
The intake hose you mention ususlly colapses on the inside. and can't be seen (in most cases) from the outside. this is an early boat which makes it real possible. First make sure it is really overheating and not the guage or the sending unit. A lot of times if a sending unit is overheated it may read high constantly especially if it is the thermowax variety which was stock back in the early boats. Un like Jim I want to know why the thermostat was removed. In that motor (depending which setup you have,there are at least 3 that I know of) you may need a t-stat for proper colling. I am local and live on Lainer and I have inferred temp guage we can test the real temp of the block. My Corsican runs at 190 on purpose I set it up to run this temp. under no circumstances should it run less than 160F and I don't care who thinks this is wrong, it is correct!!! I will explaine it over lunch or dinner. I won't do it here again because I have done it here 4 times since 1998. We can fix it providing it is broken.
Running those high temps are fine for freshwater usage, but if you ran those temp in salt water, you'd have trouble. Salt water forms crystals VERY quickly at temps over 140.

sweet 16 1966
06-10-2006, 08:21 AM
Well, I have to give much thanks for your responses. I have not run a thermostat for years. If this is wrong, advise and I will correct. The first thing on my list today is to go to Holiday and get a new impeller just to be safe. Then I was instructed by a mechanic friend to pull the hose off the waterpump and while running, it should emit a 6" water spout if properly pumping. That would eliminate the hose from being the culprit if all is OK.
He also said Merc had hot issues on the 470's w/o a thermostat for it to cool properly-why, I don't know. I could then replace the sending unit, all are SW gauges so maybe the sending unit does not match up? Tune in around lunch time and I will advise progress. Big Griz, lunch or dinner anytime on me!
Tom, I owned since 1980, I am from here but lived in Va and Texas. This boat has seen the battleships at Norfolk Va. , rode in the Patomic river. Been up the James river to Jamestown. ( also , the San Jacinto river was fun)- 1988-1991. It has seen and smelled the strange odors of the Houston ship channel and I believe it saw Galviston in its days but that was long ago. Recently us Seadogs ran the St. Johns River, then Lake Cumberland. The manifolds seem to allow enough water to pass but who knows. If all seems OK, Manifolds may be next. BTW, It does not feel hot when the gauge shows 220, cranks well and seems happy but I wonder. Off to the shop.

sweet 16 1966
06-10-2006, 08:34 AM
Engine shot.


From 2004 when I re-corred the deck. Like I have nothing else to do!
Not a fun job. This is a labor of love which I hope to one day put into a 22.
In the mean time , work continues on the 16.

sweet 16 1966
06-10-2006, 08:37 AM
BTW, this is a very old boat. No clam shells, came with an eaton outdrive which I tossed in 85 . Not a fan of them.

06-10-2006, 10:06 AM
Ok you8 have a 1965 16 and the origional drive was the Eaton probably the one with the shift cable on the top. You switdhes to the Merc pre Alpha probably a 120. Your friend said they had trouble with the 470 cooling without the t-stat. 470 refers to the mer4c 470 engine which is half a 460 ford and is no longer in production exceot in car racing. all thoes were fresh water cooled eith the cooler in the exhaust mamifold. I don't remember this engine ever in a Donzi. I have always though it would be a good match and 2anted to build one. Power and fuel econony are great with that engine. As I said we can fix that problem. Come-on down. Give me a buzz, I live on the lake(Lainer) around War Hill Park cell 770-318-1136 home 706-216-8194.
Cudaou are a little conservitive 180 is the christalization point of salt in your area it attacts the exhaust at the riser-hottest point. if the engine temp is 180 the water on the raw warter system would be about 150 heat transfer etc stinks. However it is a small concern if unit is flushed out. Me I only have closed cooling systems and the exit water is never that high and has never been a problem since the early 1960's which is when I switched to closed cooling. Reason for switching was other issues in a cruiser, open system and salt related

06-10-2006, 11:13 AM
Grizz, my father was a chief engineman in the Navy for 23 years. He said they had to run all things at 140 or less for cooling. The salt crystals would form and block the small cooling passages.

06-10-2006, 11:34 PM
Cuda . not to say anything about the Navy or what your Dad says but the Navy teaches what they think they have learnrd on the old ships with big engins and hot running with big deisels or steam engines that run hot and long. All the boats I have run have had 160 T-state, I wasn't going to get into it but I will. Factis if your engine runs at140 F or less your engine wear increases over 40% and extensive slug build up. These experiments were done by GM in the mid 1960"s and again in the late 1970's. In 1982 Honda had us do the same tests with the same answers. like I said the water temp in the exhaust of a 160 degree T-stat ed engine is not a 160 degree. The T-stst works by monitering the temp in the engine and mixing cold raw water with it to matain a nominal 160 degree in the engine. inorder to crystalize water also has to be moving slowly with steam pockets. After working with some engineers at Honda you would be surprised what misconceptions science used to preach. Another false hood is a supercharger in a 500HP big block boat can't be closed cooling "it will run too hot". My Criterion runs at 170 (safty margin for bad gas and clogged sea strainer) and has since new. We have built several with over 800HP the same way with no cooling system related problems. Granted these are modern full flow systems. Theories that were gosphel yesterday are not true to day. When I was a kid, household voltage was110 volts and 220volts, then in the 1960 it was 115v and 230 volts now it is said to be 120v and 240v. Fact is it has always been 118v and 236v average depending on line drop etc. I am not going to get into another discussion on these subjects. Do what you feel is right and I will do what I feel is correct. We can talk more at Eufala.


06-11-2006, 08:15 AM
So Grizzly, you are saying the laws of physics has changed since the 60's?:rolleyes:

It wasn't an issue of how much wear at lower temps, because I can assure you that any excess wear is better than an overheated, siezed up engine.

06-11-2006, 10:52 AM
Living in S. Fl. for 30yrs. I had many boats used in the salt. One thing I always did was flush after use. In the 70's my son and I picked up an abused 69 18C. We replaced the engine with a worked up SBC and a 140 stat. It started good and ran decent, but not good. We swapped out the 140 for a 180 stat and did it liven the boat up. Performance we never expected. Boating was year around and many hrs. on for the 6-7 yrs we owned it. Ran with a aluminum intake and logs. Never overheated or any major issues. BUT, I flushed after EVERY use. I lean toward what Grizz says. Age and experience equals WISDOM. Jim

PS- Engine was raw water cooled.

06-11-2006, 11:08 AM
There are a few other guys that have quite a bit of experience with high performance engines also.

Here's a quote from Dean Gellner.

I run our N/A engines that are 9.5:1 - 10.0 compression @ 140 degree. I would recommend running Hardin Marine oil Thermostats as well with a the proper size oil cooler. Running the water temp past 160 you will lose actually lose horsepower.

Dean Gellner
Gellner Engineering Marine Power

Another from Dean

Depending on what compression ratio the engine is the loss of horsepower will vary. We have played around with this alot on the dyno in many different types of engines. On A 9.5:1 "B" class race marine engine from 140 to 160 we lost 4-6 horsepower on average without changing anything. In a pleasure boat application - will you tell the difference? Most likely not. But for those who are always looking for a little bit more, which is what we are after, thats 8-12 HP between 2 engines. Every little bit adds up. When I ran IHRA Pro Stock we would run the water temp at 100-110 degrees leaving the starting line. The engines are very high compression and the higher water temp would create higher combustion chamber temps thus losing HP.


Tyler Crocket also concurs.

We also run our engines 110- 130 also it keeps the cylinder temps down and we make more power also. We also use a oil thermostat on all our engines to keep the oil temp above 190 and the condensation out.

Another respected opinion

I like the 120-140 deg range on a saltwater cooled motor.
Salt drops out and starts to crystalize and accumulate at higher temps. Especially at flanges, Ie. water ports, exh. water passages. Even if the motor doesn't overheat, corrosion is extremely increased by heat. Also when you shut down a hotter engine ,if not flushed , the salt crystallizes in the motor and also some water boils off allowing further corrosion, due to allowing oxygen to the salt and metal.
I feel that a raw water cooled motor should be cooled enough to prevent condensation in the crankcase. Oil temps should be managed high enough to get rid of any water vapor.

06-11-2006, 12:00 PM
OMC for one used 160 Tsats in all there raw cooled engines they ran at about 170 they did not salt up, lower temps like mentioned tend to muck up the inards they need to be hot enough to cook off the moisture. Also I believe Griz said he made more power with the higher temps. The statements by the engine builders I think applies more to radical HP engine, look at the automotive industry they run engines pretty hot for efficancy. I run a 180 in mine and would not go down, when Gcarter got his cooling system it came with a 190, I think hotter is better until you start pushing the compression up.


06-11-2006, 12:54 PM
I think the point being missed here, is that you can run the oil temps higher, which is really a better indicator of the heat you are making, while keeping the water temps down.

sweet 16 1966
06-11-2006, 12:57 PM
OK, its Sunday, I lost Saturday with other work. I am not an expert as was attested by my inability to replace the lower unit in a short period of time. Its always a line up thing! I took a break this am and when I returned, it just slid right in, even in forward! I replaced impeller, sending unit. The inside of the old impeller was gouged some which may have indicated the lock shim may have moved but I don't see how that is possible. Ran at 150 in driveway which is really no indication of anything except no water or oil leaks! Did find a short in the gauge ground. Water flow out the exhaust is good. Hey Griz. I found you on the map, looks like it could be shallow running up the cove to your place?
BTW, the grass is not cut, deck not painted etc...something has to give way for this kind of progress ! Still no thermostat. I will replace but not until I locate root of this issue.

sweet 16 1966
06-12-2006, 09:48 AM
Update, Found Old Grizzle's place on North Lanier Sunday. Actually he heard me as I was passing looking for his home. We determined it must be the gauge not matching up with the sending unit and giving a wrong reading.
Will sinc up the SW gauge with correct sending unit and put in a thermostat.
Good to see Randy and thanks for the advise!

06-12-2006, 09:53 PM

Hopefully I'll make it to Eufala, and I hope that we can have some discussion on this topic, but, for right now, I'll leave everyone with this stuff I found while trying to gain a little knowledge.


Understand completely about engine temp / performance, but, isn't a concern in a boat (due to the poor air circulation), also the loss in performance due to higher inlet air temps (according to MercuryMarine, as much as %14)? Would it be beneficial to run the air intake out of the engine compartment? is this Coast-Guard legal?

Just questions, as just like most people, I want to know how to milk out every last little HP from an engine.

Here is what I found, just some more on the topic being discussed


From the Mercury website - some other stuff regarding temp and HP

Why does my boat perform differently on a hot day verses a cool evening?

It is a known fact that weather conditions exert a profound effect on power output of internal combustion engines. Therefore, established horsepower ratings refer to the power that the engine will produce at its rated rpm under a specific combination of weather conditions.

Corporations internationally have settled on adoption of I.S.O. (International Standards Organization) engine test standards, as set forth in I.S.O. 3046 standardizing the computation of horsepower from data obtained on the dynamometer, correcting all values to the power that the engine will produce at sea level, at 30% relative humidity at 70 deg.F (20 deg.C) temperature and a barometric pressure of 29.61 inches of mercury.

Summer Conditions of high temperature, low barometric pressure and high humidity all combine to reduce the engine power. This, in turn, is reflected in decreased boat speeds--as much as 2 or 3 miles-per-hour (3 or 5 Km per-hour) in some cases. Nothing will regain this speed for the boater, but the coming of cool, dry weather.

In pointing out the practical consequences of weather effects, an engine--running on a hot, humid summer day--may encounter a loss of as much as 14% of the horsepower it would produce on a dry, brisk spring or fall day. The horsepower, that any internal combustion engine produces, depends upon the density of the air that it consumes and, in turn, this density is dependent upon the temperature of the air, its barometric pressure and water vapor (or humidity) content.

Accompanying this weather-inspired loss of power is a second but more subtle loss. At rigging time in early spring, the engine was equipped with a propeller that allowed the engine to turn within its recommended rpm range at full throttle. With the coming of the summer weather and the consequent drop in available horsepower, this propeller will, in effect, become too large. Consequently, the engine operates at less than its recommended rpm.

Due to the horsepower/rpm characteristics of an engine, this will result in further loss of horsepower at the propeller with another decrease in boat speed. This secondary loss, however, can be regained by switching to a smaller pitch propeller that allows the engine to again run at recommended rpm.

For boaters to realize optimum engine performance under changing weather conditions, it is essential that the engine have the proper propeller to allow it to operate at or near the top end of the recommended maximum rpm range at wide-open-throttle with a normal boat load.

Not only does this allow the engine to develop full power, but equally important is the fact that the engine also will be operating in an rpm range that discourages damaging detonation. This, of course, enhances overall reliability and durability of the engine.

A fairly comprehensive test of the salt issue (although on diesel) engines
but, talks about salt build up throug out the engine, super / turbo chargers / exhaust - probably a great read for Rootsy / etc.


Thanks Y'all

06-12-2006, 10:43 PM
Wayne you got all the numbers but did I miss the water temp numbers. Any way you are correct The dyno sheet numbers on my Criterion range from 657 to 698HP depemding on temp, Bp etc. to save number on a good day vs a bad day. Infact the Honda BF 225 which is an ECU and O2 sensor controled unit is greatly affected by these parameters due to the precise controles the ECU has on the engines

sweet 16 1966
06-19-2006, 07:46 AM
Sounds like you guys can discuss HP at Eufaula at length!
Meantine, I solved my problem FINALLY!. After replacing the Impeller,
waterpump, hoses, sending unit only to finally determine its was the sending unit. (I think)
That is unless the persons who asked "Why no thermostat"!! Well, I don't have an answer on that one except years ago I was having troubles and it got tossed. I did also replace the thermostsat so technically, it coule be both issues which solved my heating issue and now it runs a cool 160!
Ran fine Sunday!

Thanks once again to the board for your fine sugestions!