Updated! Help! Engine Problems (350 MAG MPI)

[Ranman]

I’m having an intermittent problem with my engine that is driving me nuts. Maybe someone here can give me some guidance. The engine is a 1997 MerCruiser MAG 350MPI Gen+ and is a multi-port fuel injected engine. It currently has about 190 hours on it.

I noticed while at AOTH III my boat had developed a very slight miss that was present while idling. The boat still ran pretty well though and had not lost any speed or power.

Last weekend out on the lake, the boat ran fine pretty much all day until we came off plane in front of an island. We had been running hard all day and while coming off plane we took a couple of good waves over the bow. When I went to power up the boat sputtered and acted like it was only running on 6 or 7 cylinders. I backed off the throttle let it idle for a moment and tried to power up again. On the second try, all was fine and the boat ran well for the ride back to the ramp.

Last Friday, we launched the boat, idled out to the lake and when I went to power up, she sputtered and did not want to go. I quickly turned around and put her back on the trailer. I looked over as much as I could in the parking lot and cannot find anything that is obvious. I went and bought a new cap and rotor and installed them at the ramp. The boat seemed to be running OK again so we launched again. This time on power up, the boat sputtered again and then it caught on and ran on all 8. We didn’t go far, but did head out to a nearby island. On the way back it was doing it again. One minute it sputters and runs like crap and the next second it will catch and run fine, then sputter, then be fine, etc.

Yesterday, I replaced the water separating fuel filter and pulled the plugs for inspection. Although nothing looks really wrong with the plugs, they do all look a tad rich as they are covered with a layer of black carbon. The plugs have always been a cardboard color when she ran well. All the plugs had the black carbon on them and no one plug looked any worse than the others. It seemed to be running better on the trailer with the new fuel filter and after running for several minutes in the driveway, I pulled a plug and it looked like it was starting to clean up and go back to tan.

We went out to the lake where the boat ran fine on the way out to the sandbar, but on the way back began missing and sputtering again. I’m lost. The plug wires look fine, but I will be buying a new set just in case.

Does anyone have a clue as to what is going on? Can a Merc mechanic plug into this engine for diagnostics? Does anyone know of a mechanic in MI that could look at the boat? I’m sure if I call off the street I’ll be in a six week waiting line. Any help anyone could provide would be much appreciated. Our season is just now getting good and my boat needs help! Thanks

[avanti]

Sounds like ignition. I would try a set of new spark plugs and see what happens. You might have a plug that is bad.

[GeneD]

The engine needs to be put on a computer.
Sounds to me as if you have a bad sensor.
Not to worry to much about the diagnostic cost, most Merc certified shops will have the computer which plugs in and will show malfunction codes.
The cost will be minimal or free.
Also, you might want to check the sensors. One of them may have unplugged itself.

[Tidbart]

Randy,
I agree with Gene. Sounds like a sensor,
I will look at the manuals tonight and see what I can find.
If all else fails with a local shop, you can buy a diagnostic reader from Rinda for about $45.
If I remember correctly, Bryan Tuvell bought one a couple of months ago. Not a bad tool to have around to read the codes.
I'll let you know what I find.

Bob

[Tidbart]

Randy,
I took a look at the books. The first thing I would check is all my wiring connections to make sure they are sound. Next, I would take a good look at the ignition switch, jiggle the wires in the back while it is running. If it is making intermittent connections, it will drive the ECM nuts.
Then I would try to find a mechanic with the diagnostic reader, or buy one myself. If there are codes there they should show up instantly. This will point to a problem, not cure it. Depending on the code is where you go.
Another way is to replace a few of the sensors yourself. It could be the IAC motor on the intake, they go south after a few years. I would also look at the Throttle Position Sensor and the Manifold Absolute Pressure sensor.
There is too much to the manual to list here.

Don't let me get too far ahead, I would spend some time looking for a problem before I brought it to a mechanic. Wiring, connections, grounds, etc.

The Merc Service manual is #17 for your setup if you feel like buying it or can borrow one. Section 5E has all the stuff you will need to know about the MPI.

Let me know if you need anything specific from the manuals.

Bob :D

Good luck. I hope it is an easy find. These things can sometimes be a pain in the ass.

[Jamesbon]

I would verify that the risers are not leaking water back into the engine, and also swap out the ign. coil (if it has one).

...chased many of my own "hard to detect" gremlins...

Good luck!

[Ranman]

Thank you everyone for your suggestions. I did check the boat out on most of the points listed. She's in the hands of a qualified Merc mechanic as of this evening. The Merc mechanic seems to think there may be a fuel issue. I will let everyone know what comes back as the problem.

[BabyDonzi]

Ranman, just had the exact same thing happen to me. After replacing the plugs, new wires, cap, distributor and coil, and fuel filter, I gave up. Fuel or sensor problem for sure. Turned her over to Donzi/merc. 200 hours on the engine, we'll se if they claim injectors,------- or sensors.

$$$$$$$$$$$$

[BabyDonzi]

Verdict is in---------

Fuel pump and injectors $1300

[Donzi Matt]

This may not be much help, as I am a Chevrolet dealer mechanic, not a merc mechanic, but from what I have seen the 350 mpi motors seem very similar to the 5.7 liter vortec motors used in the trucks. We have had some problems with injectors on the trucks and they are _very_ susceptible to fuel contamination. The injectors are a pop off style and will only fire off when the proper fuel pressure is available. We have had some luck running fuel system cleaner through them but eventually they will plug again. If the ignition system is similar to the chevy system they are nearly bulletproof as well as most all of our sensors. The only problems we have had are some injector issues, lots of leaking intake gaskets, and some bad fuel pressure regulators leaking down causing a long crank or flooding condition. Of course, if these motors do use a totally different intake system, them I guess I just wasted everybodys time. :)

Matt

87 22 classic

[Tidbart]

How did you make out, Randy?

[Ranman]

Took the boat back to the mechanic after returning from my honeymoon. THis time I was able to replicate the problem to the mechanic. (The first time to the mechanic, the boat ran fine). Of all things, it turns out that I had a couple of bad plug wires. The wires cracked inside the boot elbow right at the spark plug. They were arcing against the exhaust manifolds. The problem was intermittent because sometimes the wire would fire the plug and sometimes it would arc to the manifold. Over time, the arcing burnt a white ring around the insulation which is what the mechanic caught.

Three months ago, when I first started working on this issue I was at the local Merc parts counter with a set of wires ready to go home. The guy behind counter asked "why do you need those?" I said I was replacing all the ignition components. The guy says "the wires last 10 years, don't waste your money". Unfortunatly, I listened to him and put them back. THat's the last time I make that mistake.

Anyway, I go to pick up the boat in a day or so and will water test to be sure I'm back in business. Hope to see you on the water.


Hey, $250 is a bargain if she runs again...

[MOP]

Randy good to hear its solved, hope you finish up the season troulbe free!