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Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 16109
Experience:  Degree in Marine Technology. Gas and diesel marine mechanic.
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Mercruiser 350 mpi with alpha outdrive. Runs fine above 1200

Customer Question

Mercruiser 350 mpi with alpha outdrive. Runs fine above 1200 rpm, but after idling for extended period, the engine vapor locks. Let it sit for 20-30 minutes to cool down, it will run fine again above 1200 rpm. Cause??
Submitted: 1 year ago.
Category: Boat
Expert:  Jason replied 1 year ago.

Hi Steve, my name is Jason.

I don't thinking you have the correct idea of what vapor lock is. A running engine can not vapor lock. What exactly is the engine doing, is stalling when you idle it for a long time?

Customer: replied 1 year ago.
After idling for an extended period, the engine sputters and then quits. Can't restart. When I check the schrader valve, there is air and low fuel pressure. Let the engine sit for 20-30 minutes, it will restart.
Expert:  Jason replied 1 year ago.

These engines do not like to idle for long periods of time, they can flood out. How long are you idling the engine for exactly?

Customer: replied 1 year ago.
20-25 minutes
Customer: replied 1 year ago.
Will running the blower correct the problem when I'm idling for extended period in hot weather?
Expert:  Jason replied 1 year ago.

No. The blower won't have anything to do with this. What's happening is the engine is flooding out. These engines are not designed for excessive idling. All marine engines run rich by design, and excessive idling can cause them to flood out. Why are you idling the engine for so long, are you trolling with it?

Customer: replied 1 year ago.
When I say idling, I'm talking about 1000 RPM in extended no wake area.
Expert:  Jason replied 1 year ago.

Is your engine under warranty? If yes, have you spoken with the dealer yet?

Customer: replied 1 year ago.
I believe it has a 5 year warranty. I can give them a call. Thanks.
Expert:  Jason replied 1 year ago.

You are very welcome. Specifically what you want to have them check is fuel pressure, and then check and see if any of the fuel injectors are leaking down. If the engine is running any richer than it should that will cause the stalling. In the mean time, see if you can get your rpms up a little bit, that will help keep it from stalling as well. In a nutshell what is happening is the engine is loading up with fuel and flooding out, that is what is causing the stalling.