Mercruiser MPI Won’t Stay Running

[alldodge]

Backfiring is to much fuel or incorrect timing.

When the key is turned ON and fuel pump runs, after it shuts off does FP hold or leak down?

 

[BradsBoat]

It holds.

 

[alldodge]

Sorry, the threads are blurring together for me
Has compression and leak down test been done?

 

[BradsBoat]

Yes.
Any chance the new CPS isn’t working properly? Seems this could present observed behavior. Maybe the ECM gets a weak signal during startup, then as it speeds up, the signal fades?

 

[alldodge]

I doubt it, because if it was voltages would not be correct and there would be a code

 

[Stinnett21]

Not a tech but haven't seen IAC muffler mentioned. Please excuse if being too elementary.

 

[BradsBoat]

I just replaced the IAC muffler.

 

[BradsBoat]

Assuming the timing is set correctly, per the Bulletin describing the LED test light, and assuming the fuel delivery is correct, the only sensor I can think would affect the timing would be the CPS since the ECM relies on that signal to determine the position of the crankshaft. It does pass the Mercruiser/Rinda tests, as does the IAC, coil, and injectors. Yet, I just can’t think of anything else it could be.

 

[BradsBoat]

Just in case water in the fuel was the issue, I removed the fuel separator and examined the contents in a clear jar - all fuel.

 

[BradsBoat]

This issue turned out to be several bad items - the IAC, the injectors, and the Crankshaft position sensor. All were aftermarket via Amazon. Only using OEM going forward.

 

[QBhoy]

I’m going to be unadmired for this. But is the fuel ok ? Classic symptoms of bad fuel to be fair.
Although it’s not your issue..I’d also encourage you to use the iridium plugs too. But I think you have a fuel issue for sure, most likely.

 

[achris]

I’m going to be unadmired for this. But is the fuel ok ? Classic symptoms of bad fuel to be fair.
Although it’s not your issue..I’d also encourage you to use the iridium plugs too. But I think you have a fuel issue for sure, most likely.

I think the answer is....

All were aftermarket...

I'm not sure how or why, but aftermarket are extremely cheap (compared to OEM) for a reason...

Chris...