502Mag MPI re-starting issue
 

One of my 1997 502's is giving me some warm re-start issues. Engine cranks and fires right up cold. To restart warm, if fires and dies. I Have to advance throttle a bit for it to start and then its fine and idles when I return the throttle back. I have replaced the IAC and it still does it. This is not my occasional Vapor Lock condition, but something else. TPS maybe? Any hints or diagnosis would be appreciated.

Could be a few things, MAP, IAT or injectors. Injectors may be leaking and need to be cleaned and flow tested. If they haven't ever been done it would be a good thing to do

Check fuel pressure, if its to high this could be causing it to be to rich 37 psi

For the suspect motor, there should be yellow tygon hose from the top of the VST to the intake manifold. See if there is any fluid and or residue of fluid in the line. If so you likely have fuel percolating out of the VST into the manifold, effectively flooding the engine. There is a diaphragm inside the VST that deteriorates with age and fuel from the VST will seep through and escape via that yellow tube.

The fact that adding throttle helps start the motor during this warm condition does two things. First it increases air flow via opened throttle blades helping to clear extra fuel, and second, advancing the throttle will force the MEFI controller into a 'flood clear mode" by shutting off the injectors. Depending on the MEFI that is usually between 25-75% TPS.

I've had this condition happen on my motor always thinking it was vapor lock, when in fact it was not vapor lock but this ruptured diaphragm causing the situation you just described.

I first noticed this problem on my last run last year. Funny you mention those diaphragms. The engine in question has always been vaporlock prone. Over the winter I read here on OSO that some early 1997 VST engines as mine had the spring installed on the wrong side of the diaphragm. I switched them per the bulletin. When running the boat on Saturday, I noticed both engines ran a bit rich with springs reversed. I put them back to OE config on Sunday and replaced both diaphragms, as the trouble engine had a torn rubber seat on the diaphragm. Maybe this had some thing to do with it. Have not had a chance to run since due to rain.

Oh, I replaced IAC over the winter too. No difference. With a little luck, the diaphragm did it.

Did it fix it