mercruiser 7.4 mpi cold start stalling

jmowood

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will respond after work tomorrow thank you for all the help:joyous::joyous::joyous:. the fuel pressure Gage i borrowed might be off. who knows.
 

alldodge

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could the regulator be causing pressure to drop

Yes it's possible, I just don't think this would be your current issue. Have discussed several folks with problems and pressure being a bit low, but none caused what your seeing. That said, there are over things which can also cause lower fuel pressure.
 

jmowood

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ok so far she started up and stayed running. i need 3 or 4 more cold start mornings to say Yes if it was the ECT.
 

alldodge

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Sounding great, sure hope that was it. least the testing lead that way
 

jmowood

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Well third morning and she stalls 8 times before staying on. Wonder if the 35 degree night is the cause. :(
 

alldodge

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That's a bummer, it does have something to do with the temp. The only other temp sensor is in the plenum but I don't see any testing or trouble shooting in the manual. The sensor does control fuel delivery but not like the ECT. Fun Times have any thoughts?
 

Fun Times

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Is that the one that is very similar to the ECT but in the plenum port side
Yes it called an Intake Air Temp sensor. Item number 27, http://www.mercruiserparts.com/bam/subassembly/31918/2705/130

Merc part number 805223T - Sensor Assembly (iat) or Sierra part number 18-7702
http://www.iboats.com/Mercruiser-Te...5895223--session_id.985038937--view_id.269378
http://www.ebay.com/itm/MerCruiser-...MAT-Sensor-Sending-Unit-805223T-/190970017793

The intake sensor has some control of the fuel mapping system of the ECM but not as much as the engine coolant sensor but the sensor ohms readings should still be investigated for range accuracy. Without looking it up I believe the ohms values would be/are the same as the oil PSI sensor...Pretty sure the manual mentions the IAT and Oil someplace....Can look later if needed.

Have you cleaned the inside of the throttle body and throttle body plates on both sides for carbon fouled buildup? Also try to start it with the flame arrestor removed to see if it helps any.

I haven't read your full topic yet but 35* is kind of cold. So before starting the engine, try turning the key on, wait for the 2 second fuel prime, then key off again then slowly key back on a few seconds later till the fuel prime is done, then try to start the engine to see if that helps too.
 

jmowood

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Seems ok in warmer weather. But now idle at 3000 rpm then quickly sift back to neutral idle she stalls.
 

alldodge

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Seems ok in warmer weather. But now idle at 3000 rpm then quickly sift back to neutral idle she stalls.

Now that's just weird, starts me thinking about corrosion on connections. Could you explain a bit more about the 3K rpm idle then the shift to idle and stall?
 

Mischief Managed

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I have nothing to add for the 3000 RPM idle, but my 7.4 MPI has always started and stalled in cold weather. Usually takes 6 to 8 tries to get it to stay running. Been like that for 11 years and runs perfectly otherwise. I ignore it since I never go boating below 40 degrees.
 

Fun Times

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but my 7.4 MPI has always started and stalled in cold weather. Usually takes 6 to 8 tries to get it to stay running. Been like that for 11 years and runs perfectly otherwise. I ignore it since I never go boating below 40 degrees.
That's sort of one of the reasons I suggested trying the key on, off, back on then crank to start. I faintly recall that being a non official recommended way to have to start some engines back in and around the 90's.
 

jmowood

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As far as the 3000 rpm. If I rev the throttle up to 3000 rpm and bring it back down to neutral position it will go down to idle, putter, then stall.
 

Mischief Managed

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That's sort of one of the reasons I suggested trying the key on, off, back on then crank to start. I faintly recall that being a non official recommended way to have to start some engines back in and around the 90's.

I have tried that too, thinking fuel pressure has bled down and the fuel pump cycles would fix it. Makes no difference.

Here's my theory:

I always figured the fuel injection mapping bottoms out at around 40 degrees F and it just runs too lean below that until there's a little heat built up. After it stalls, it always takes a few seconds of cranking to start again; I assume that the injectors are firing during that time and the fuel is condensing on the intake and head until it reaches the saturation point and starts to evaporate enough to sustain combustion. At the same time, I figure the ECU has been slowly opening up the IAC so that when it does fire up, it goes to a very high idle (but not 3000 RPM) and the velocity of the high RPM airflow absorbs all the condensed fuel, keeping it running. The ECU senses the high RPM and closes the IAC (which happens slowly, I've tested it with my scan tool and it's physically impossible for the IAC to operate quickly) and the engine speed drops. At the same time, the excess fuel is used up/the slower air can't absorb it, and the engine sputters and dies. After this happens 6 to 8 times, the engine has warmed enough to prevent the fuel form condensing on the intake and cylinder head and it runs fine.

If the OP happens to live near southern New Hampshire, I'd be happy to scan the motor with my Tech Mate scan tool.
 
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