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502 MPI stumbles/dies at idle

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502 MPI stumbles/dies at idle

Old 07-14-2016, 12:44 AM
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Default 502 MPI stumbles/dies at idle

I have a 1999 502 MPI with 400 hours, recently it I started having problems with it dying while idling around the dock. especially if I give a little throttle and then come off it quickly (verrroom verroom and it dies). It restarts fine and runs out fine on top end. It is idling at 800 (same as always).

I have checked/changed the quart size water separating fuel filter. Is there another (smaller) fuel filter on my engine?

Anyone have any thoughts on where I should be looking?

Last edited by boating4me; 07-14-2016 at 12:46 AM. Reason: gramer
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Old 07-14-2016, 10:49 AM
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IAC . Idle Air control?
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Old 07-14-2016, 11:03 AM
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Before blaming the IAC, there are two components that let a set small amt of air in the engine also, when idling.

1) Throttle blades. Clean these along with inside of t body housing where the blades almost meet the housing when at idle.
2) PCV valve. Better to replace than clean.
3) Check all vacuum lines for condition and that they arfe hooked up. Also make sure the vac line connection nipples on intake are not plugged.


#24 here:
Note: 27 is just an open elbow. So, this will have no effect since it's hose goes to Flame Arrestor. This can stay.

Last edited by SB; 07-14-2016 at 11:07 AM.
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Old 07-14-2016, 11:46 AM
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these IAC are not the ford IAC that fail all the time in the ECM / PCM 555. These are the GM IAC's. Fail yes but not that often. The TPS fails more often with these than the IAC just saying.

What parts have you changed ? If you changed IAC possible wrong one from auto store ??? very possible - what else have you changed ????.

What is scan reporting IAC counts at ? What is the values of the TPS,

what is your fuel pressure at idle and is it steady. that needs to be the correct pressure and steady eddie ?

Sparks plugs, plug wires and CAP and ROTOR changed ??? I would start there as well. If not fixed I would scan engine and fuel pressure testing.

When was the last time any fuel filters got changed and fuel samples taken ??? how old is your fuel ???

How dirty is your flame arrestor ??? when was the last time you cleaned it. Never met a boater who's flame arrestor was 100 %spotless when I looked at their boat / engine in person. NEVER.

Last edited by BUP; 07-14-2016 at 11:51 AM.
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