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Discussion Starter #1
I have a 1994 Mercruiser 502 motor (fuel injected) which is having some problems which I have no idea how to resolve. The boat runs great for a good part of the day. I will turn the engine off for 30 or 40 minutes and try to start the engine. The engine will start up with no problems but it will run really bad. The motor sounds like a lean popping with no black smoke. If I feather the throttle for a min or two the motor will pick up and start running normally for the rest of the day. Someone told me that it sounds like a carbureted motor with a bad vacuum leak when experiencing this issue. Someone also told me that I had a bad Idle air motor. I replaced it and am still having the same problem.

I had another issue as well. after running the motor for a good part of the day it wouldn't start. I had to hold the throttle open all the way and crank it to get it to start. After it started it ran perfect.

I'm not sure why it is acting like this because when the motor is running it is running great. If anyone has any ideas that I could try I would really appreciate it. I just bought the boat and want to get this fixed.
 

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probably should go to the dyno section? I would look at the MAP and Throttle sensor and make sure it has correct voltages in and on reference lines? the IAC will not likely cause what you are talking about.
 

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Bass Lake Dream'n
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It's vapor locking. When the engine is shut off, there's no air flow in the compartment and it just heat soaks. The gas in the fuel rails and VST (vapor separator tank) is boiling off.
Either open the hatch a little while parked or check fuel psi. Might have a high pressure pump problem.
The 454 & 502 mags are notorious for this.
 

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It's vapor locking. When the engine is shut off, there's no air flow in the compartment and it just heat soaks. The gas in the fuel rails and VST (vapor separator tank) is boiling off.
Either open the hatch a little while parked or check fuel psi. Might have a high pressure pump problem.
The 454 & 502 mags are notorious for this.
Exactly. Used to happen with my 1995 502. I noticed that if I would let the motor idle for a few minutes to bring down the temp before shutting it off that this would work for me most of the time.
 

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Discussion Starter #5
My 502 is fuel injected

It's vapor locking. When the engine is shut off, there's no air flow in the compartment and it just heat soaks. The gas in the fuel rails and VST (vapor separator tank) is boiling off.
Either open the hatch a little while parked or check fuel psi. Might have a high pressure pump problem.
The 454 & 502 mags are notorious for this.
I thought vapor lock is only for carbureted motors? My motor is fuel injected.
 

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I thought vapor lock is only for carbureted motors? My motor is fuel injected.
My 502 was fuel injected also. IMO having that VST tank on top of the intake manifold was a bad design. When I rebuilt mine, I got rid of the mechanical fuel pump and the VST tank system and mounted a electric fuel pump on the transom. Never had another problem after that with vapor lock.
 

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It's a vapor lock in the VST. To check if it's that. Just take the tool with you and unscrew the release valve on top of the VST when next time it happens. Up you should hear vapors releasing out. Then just start the engine. If it starts. You need to look in the VST and change the bad design. There's a great thread about it.

1997 502 MPI Issues - Vapor Lock?
 

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I had all the same problems. I changed the return from the fuel rail to the gas tank instead of the VST (put a plug in that). Moeller sells a rlacement fuel sending unit that has an NPT fitting on top. Haven't had a problem since. My theory is that when you stop the engine, the fuel vaporizes and when it reaches 30psi it goes to the VST. The incoming fuel at 7psi can't get into the VST tank when you restart. It took me 5 long angonizing years to figure this out. Contrary to Mercruisers bulitin fix, you could leave the hatch open for ever and let it idle for ever and it did nothing to fix the problem.
 
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