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Old 04-28-2018, 09:52 PM   #1
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Default Engine stalls when throttling down

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.
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Old 04-29-2018, 03:28 AM   #2
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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?
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Old 04-29-2018, 04:17 AM   #3
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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
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Old 04-29-2018, 06:56 AM   #4
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Do the engine have carburetor? or injection?
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Old 04-29-2018, 11:33 AM   #5
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Tom I知 not sure what is going on but I doubt it痴 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値l know if the fuel or ignition systems are the issue. Based on this you値l know where to focus from there.
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Old 04-29-2018, 11:33 AM   #6
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Quote:
Originally Posted by Gugi47 View Post
Do the engine have carburetor? or injection?
MPI=fuel injection.
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Old 04-29-2018, 01:31 PM   #7
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Quote:
Originally Posted by mmwjr View Post
MPI=fuel injection.
Yes, I see that in his signature now...
Maybe losing line psi....but why only at 1500 rpm?
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Old 04-29-2018, 05:18 PM   #8
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Quote:
Originally Posted by mmwjr View Post
Tom I知 not sure what is going on but I doubt it痴 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値l know if the fuel or ignition systems are the issue. Based on this you値l 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....
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Old 04-29-2018, 07:21 PM   #9
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If you can find from somebody a oscilloscope, that would be the easy way to find the problem.
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Old 04-29-2018, 08:59 PM   #10
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Quote:
Originally Posted by iceman View Post
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値l 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稚 recall how it is connected with the MPI. I値l take a look when I have time and post my thoughts.
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