502 mag mpi spontaneous stalling at extended idle times
 

My 502 is in a 1997 formula fastec with 280 hrs, that is the one with the high pressure pump on top of the engine in the aluminum canister. My issue is half a dozen or more times now the engine has slowly lost rpm to the point of dying. Try to restart and it immediately re fires and dies again. Up until the last time, after a few minutes of trying the engine starts and runs fine. This has always happened at idle until the last time I spoke of and I had just left a cove and was on plane at 3500 rpm. The engine would fire but never stay running. After being towed to the nearest dock, then getting a ride to my truck and trailer, two hours later I was back at my boat, tried to re start and it really wouldn't even try to fire. The next day i took it to an auto mechanic(I'm an hour away from closest marine mechanic) he thought it sounded like fuel starvation, put a pressure gauge on the fuel rail turned key on and 38 psi. He thought that that was low, but after reading for several hours on net it seems to be right. He had me crank it and it fired right up. Took it home and started at the tank removed tank outlet and anti siphon valve. It takes 2 in. Of vac. To open valve. Removed fuel line from tank to filter, looked good but will replace. Cut open fuel filter didn't see much just a little rust. Took top off elec. fuel pump housing and checked screen in bottom of it and it was clean as well. Checked float and needle seat in same housing, moves freely as it should. Is there still something that could be fuel related with this or do I ignore majority and start looking at ignition? Any input will be greatly appreciated. Thank you Adam

Went through a similar issue- the mechanical lift pump had failed. I'd pull it and inspect it.

To make a long story short- The front seal on the fuel/sea pump housing failed dropping all the oil out, this cause the foot on the mechanical pump to wear to the point that it would not pump enough fuel to keep the VST full.

Ended up replacing the pump with Airtex model #60932, it's a perfect drop-in fit. http://www.amazon.com/gp/product/B00...?ie=UTF8&psc=1

Thanks I'll take a look tonight.

It could b vapor lock. Or the mech pump like cole said.

+1 on Coles comment. I think the gasket on the mechanical pump (sea pump) doesn't like ethanol fuel. Mine didn't fail (stop running) but we took it off for service and the oil res was full of fuel...on its last leg.

I'd also check the looks of the ICM and the cap/rotor. Mine looks burnt up after 100 hours.

Mine has acted up like that before, swearing it was fuel related...cap rotor and ICM fixed it right up.

ICM expensive? Did you ever replace the coil when you did the cap and rotor? I'll look at the sea/fuel pump. Thanks guys this is very frustrating I have owned this boat for almost 6 weeks and have already had to replace the entire outdrive, and now this. Thought I did good by spending extra money on a low hr fuel injected boat but now kicking myself in the.......

vst tank engines are very very prone to vapor lock,first step is to put the new mechanical pump on,and not the airtex brand,mercury only,anti siphon out and I bet problem mostly gone.

There are many many threads on this issue.. 454 and 502 MPI's are prone to vapor lock.. I chased for two seasons.. Talked to Eddie Young of Young's performance. EFI guru.
I ended up running return lines from both vst's to the tank. Also found mech pump that feeds vst running low and the pump arm was worn. Replaced.. Now good.. Will still vapor lock on a hot day but turn the key couple times and she's all good. It cycles the air out quickly... Not hard to do just took me a full day and about $400 but I used s/s braided fuel lines an AN fittings. CP performance had a fitting you can splice into your fuel fill hose, it has one bung in it.. I run twins,, so I had another welded in..

All good information above. One more thing to note the next time it happens, so you are not stranded. On the top of the VST tank there is a inspection plug on top of it. If you pull the plug when the problem sneeks up, it will relieve the vapor lock pressure in the fuel system. You might have to crank it a bit if the tank if empty, but the motor will start right up if it is a vapor lock issue. If the manual pump is bad or marginal, it will cause this problem, but this procedure might get you home. If it is the manual pump problem, you won't see it with the fuel pressure because the is on the high pressure side of the system.
I would not drive the boat with the plug off, unless just idling and make sure you have good ventalation and the blower on.

Thanks a bunch, I was wondering if relieving the pressure could be done there or at the fuel line coming into the VST. And pro boat do you mean remove the anti siphon permanently?