Engine stalls when throttling down

[iceman]

So, I brought my boat to my mechanic for a routine service, and an intermittent problem cropped up that they are having difficulty troubleshooting. I am hoping someone has seen this before.

Here is what is happening:

The engine is a Mercruiser 5.0L MPI. The engine starts fine, and idles fine. If they increase throttle everything works fine, and the boat can get on plane with no issues. Once off of plane, as they throttle down, the engine will stall. It fails as though there is a complete loss of power, that is, as if someone shut the engine off with the ignition key. This is very repeatable, and always seems to happen in the vicinity of 1500 RPM. It seems to run fine at 1400 RPM and below, and 1600 RPM and above.

There are no fault codes. The mechanic has checked out all of the sensors and they seem fine. He suspected the wiring harness and that there was an intermittent disconnect based on engine vibration at 1500 RPM, but a new harness did not fix the problem. Most of the electrical components (distr cap, rotor, cam sensor, spark plugs and wires) have been replaced.

Some recent history that may or may not be relevant: the engine overheated in 2015 due to clogged manifolds. After that the boat seemed to be sluggish getting on plane. In 2017, the engine had a severe loss of power (no ability to get on plane). This tuned out to be a partially melted crank position sensor that was attributed to the overheat condition 2 years prior.

During the troubleshooting process it was discovered that the engine heads had warped and they were machined. After these repairs, the boat seemed to perform better than ever during a trip last February.
The mechanics have ruled out a whole lot of things besides what I mentioned above, so we are heading into checking some of the more expensive parts like the MPI computer. I am hoping someone has run into this and knows what the root cause is. Any advice or suggestions are appreciated. Thanks in advance.

[mmwjr]

Darn Ice not again!

So I have a few questions, when you took it in did this issue exist? Did it just come out of winter storage or been running? Does it restart after shutting down and do you need to do anything special to restart? What work did the shop do?

[iceman]

Hey Mike, yeah I'm getting frustrated after last year's adventure.

After the CPS was replaced in November, I launched it for a short test, and then again for a trip in February. One hour up to the City, one hour back. This issue didn't come up. So the first time it surfaced was in March when the mechanic took it out to test it on the water.

It wasn't in winter storage, I just keep in on a trailer in my driveway. When I'm not actively using it I fire up the engine every 4 to 6 weeks.

It restarts right away after the stall. Nothing special needs to be done.

As far as the work they did, routine maintenance like spark plugs, wires, impeller, fuel filter, zincs, stuff like that. When they noted the problem they tested and/or replaced all of the sensors (IAC, MAP etc). They bypassed the boat harness using a test fixture, so it isn't a faulty ignition switch or kill switch. They inspected/bypassed a bunch of power and ground connections and they weren't the problem. The engine harness was a last (or close to last) resort. They felt that possibly the harness got damaged when the engine heads were removed and replaced last year.

Tom

[Gugi47]

Do the engine have carburetor? or injection?

[mmwjr]

Tom I’m not sure what is going on but I doubt it’s the ECM. If it were me I would instrument up the fuel pump and ignition to see which is loosing power first when it shuts down. A volt meter connected to each and watched by someone as you back down to 1500 rpm is what I would do. This way you’ll know if the fuel or ignition systems are the issue. Based on this you’ll know where to focus from there.

[mmwjr]

Originally Posted by Gugi47

Do the engine have carburetor? or injection?

MPI=fuel injection.

[Gugi47]

Originally Posted by mmwjr

MPI=fuel injection.

Yes, I see that in his signature now...
Maybe losing line psi....but why only at 1500 rpm?

[iceman]

Originally Posted by mmwjr

Tom I’m not sure what is going on but I doubt it’s the ECM. If it were me I would instrument up the fuel pump and ignition to see which is loosing power first when it shuts down. A volt meter connected to each and watched by someone as you back down to 1500 rpm is what I would do. This way you’ll know if the fuel or ignition systems are the issue. Based on this you’ll know where to focus from there.

So it sounds like your theory is a momentary interruption of DC power to the fuel pump or the ignition system? I imagine it can't be the alternator since I would assume the batteries would take over to fill in any gap in power supplied from the alternator. So it must be a connector/wire breaking somewhere? Since the motor wiring harness was replaced, there can't be too many places where this could happen....

[Gugi47]

If you can find from somebody a oscilloscope, that would be the easy way to find the problem.

[mmwjr]

Originally Posted by iceman

So it sounds like your theory is a momentary interruption of DC power to the fuel pump or the ignition system? I imagine it can't be the alternator since I would assume the batteries would take over to fill in any gap in power supplied from the alternator. So it must be a connector/wire breaking somewhere? Since the motor wiring harness was replaced, there can't be too many places where this could happen....

I’ll need to look at the wiring diagram for you engine but the first thing that comes to my mind is the oil pressure switch but I don’t recall how it is connected with the MPI. I’ll take a look when I have time and post my thoughts.

[iceman]

Mike, the mechanic says if the Oil Pressure switch was bad it should have thrown a fault code. No codes were logged.

[mmwjr]

Originally Posted by iceman

Mike, the mechanic says if the Oil Pressure switch was bad it should have thrown a fault code. No codes were logged.

I would agree with the mechanic if it was a hard fault but being intermittent the computer may not have enough time to collect data to register it as a fault.

[iceman]

OK I see what you mean. So, assuming there is an intermittent failure of the oil pressure switch, how would this cause the engine to shut down?

[mmwjr]

Originally Posted by iceman

OK I see what you mean. So, assuming there is an intermittent failure of the oil pressure switch, how would this cause the engine to shut down?

Yes the system shuts down the engine to prevent gas from continuing to be pump when the engine is not running and this also protects the engine from damage by running with no oil pressure. I would spend the $20 on this before $500+ on an ECM. I have read folks posts says the engine was shutting down and spent some time trying to resolve the issue and in the end it was a weak oil pressure switch.

[iceman]

OK it looks like the engine is fixed, so here is the update.


It was not the ECM after all. Turns out the harness was damaged, and it in turn damaged the IAC. The mechanic had replaced the IAC early on, and that didn't seem to fix the problem. After replacing the harness, the problem became more repeatable. Ultimately his troubleshooting led him back to the IAC. He swapped out the IAC again with a used, but known good one, and the engine ran fine after that. He believes that the damaged harness caused the original and the replacement IACs to burn out. A brand new IAC was once again installed and the engine was thoroughly tested on and off the water. He showed me "pinch marks" on the harness wires that could have been caused by mishandling when the heads were removed last year. I suppose its also possible the wire insulation melted some when the boat overheated back in 2015. Either way some sort of intermittent short seems to have caused this. The IAC failed in a non-typical way, that caused problems at 1500 RPM vs 600 RPM, otherwise this might have been a lot easier to diagnose.

I appreciate your suggestions Mike. This one was obviously a bizarre case.

[mmwjr]

Tom I just glad it has been resolved early in the season so you now enjoy time on the water. That’s for posting the findings.

[iceman]

Thanks Mike, yes I'm looking forward to this season after missing most of last year.