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Mercruiser 5,0 mpi- rough idle/stalling ecm 555

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Mercruiser 5,0 mpi- rough idle/stalling ecm 555

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Old 05-14-2019, 02:32 PM
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Default Mercruiser 5,0 mpi- rough idle/stalling ecm 555

I rebuild a 2004 mercruiser 5,0 MPI and I'm having problems with bad idle (too high or stalling).Compression checked; perfect on all 8 cilinders Engine fires up right away, runs for a couple of seconds @ +- 1000 rpm and returns to (bad) idle. Accelerates as it should be, no hesitation, seems to be powerfull. Diacom shows NO fault codesReplaced IAC (2 good working taken from running engines for testing, 1 brand new) 12V present on the IAC It depends which IAC and the engine idles @ +- 1400 rpm (and alot of surging noise) or idles @ 300 400 rpm and dies because too low rpmIf I undo the vacuum line from the IAC engine runs 2000 rpmif I close manually (with my thumb) partially this vacuum line, engine will idle at 500 or 550 or 600 rpm if closing more or less the vac line .Perfect as it should be Diacom shows IAC duty cycle between 22,25% and 100%Checked PCV valve and valve is not blocked, seems to be in good working orderWhen engine running, diacom shows spark advance until -22 ??? Also replaced ECU temporally for testing.. still no better or worse.TPS & MAP sensor also replaced, no change ECM 555 experts wanted

I'm out of options at this moment
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Old 05-14-2019, 03:10 PM
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Diacom movie, engine running 30 s

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Old 05-14-2019, 04:39 PM
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100% distributor is installed correctly and no vac leaks ?
What method did you use for installing/referencing dist ?

Any iac learn methods on these motors?
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Old 05-14-2019, 05:32 PM
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Originally Posted by evaluex View Post
I rebuild a 2004 mercruiser 5,0 MPI and I'm having problems with bad idle (too high or stalling).Compression checked; perfect on all 8 cilinders Engine fires up right away, runs for a couple of seconds @ +- 1000 rpm and returns to (bad) idle. Accelerates as it should be, no hesitation, seems to be powerfull. Diacom shows NO fault codesReplaced IAC (2 good working taken from running engines for testing, 1 brand new) 12V present on the IAC It depends which IAC and the engine idles @ +- 1400 rpm (and alot of surging noise) or idles @ 300 400 rpm and dies because too low rpmIf I undo the vacuum line from the IAC engine runs 2000 rpmif I close manually (with my thumb) partially this vacuum line, engine will idle at 500 or 550 or 600 rpm if closing more or less the vac line .Perfect as it should be Diacom shows IAC duty cycle between 22,25% and 100%Checked PCV valve and valve is not blocked, seems to be in good working orderWhen engine running, diacom shows spark advance until -22 ??? Also replaced ECU temporally for testing.. still no better or worse.TPS & MAP sensor also replaced, no change ECM 555 experts wanted

I'm out of options at this moment
Surging like that is classic vacuum leak. I'd start there first. Check all hoses, gaskets, fittings etc.
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Old 05-14-2019, 08:51 PM
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Most likely it is the shift interrupt that is activating as the motor comes down to an idle which is retarding the ignition timing to reduce engine torque during a shift. However, in most Alpha drive applications this is supposed to be a momentary interruption of the torque by retarding the ignition timing, you most likely have the wrong shift interrupt switch that is constantly reducing the torque whenever the motor drops below a certain rpm (which will cause the engine to stall).

The link below is a video showing the same symptoms in a 350 MAG MPI. Please follow our youtube channel (speedmastermarine) as we have many informative videos like the one below.


I hope this helps!

-Speedmaster Marine

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Old 05-14-2019, 08:52 PM
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Old 05-14-2019, 08:55 PM
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