Help! 502 efi keeps stalling
 

I have twin 95 gen 5 502 mag efi. Only 1 of the motors after 20 min of running will stall out. I'll turn key and it will start then quickly stall out for about a half hour then will usually start up and run strong. I swapped the vst pumps with other motor thinking it was a vapor lock. But still does it on same motor. I've also now replaced the mechanical pump, had all the injectors cleaned, replaced the thermostat, sea water pump and water pump. I don't know what else to do! Now every time I go out its getting worse! Any ideas? I even set up another fan in engine compartment to move air

When it stalls does it simply shut off or sputter for a bit then quit?

It just stalls. No sputter

Have a scan done for trouble codes and save yourself headaches and money. JMO. BTDT.

Unfortunately I had it scanned and no codes came up. So I'm just chasing my tail

Did they look at live data like IAC counts, TPS voltage,and etc ?

Swapping parts and pumps won't fix vapor lock if it's on the port motor. Next time it does it take cold water and pour it on the fuel/water seperator. If it starts right up its vapor locking. Quick easy check.

Probably a dumb idea, what about the fuel filter h2o seperator? Doesn't it take a great deal of heat to form a vapor lock?

I had a situation years ago with a bad ignition harness where one motor would just turn off. Scary as hell when running and one motor would just shut off. Replaced the harness and never had an issue again.

I tried that also and didn't help.

Only at the dock. And it won't do anything just idling.

Replace the ICM in the distributor cap. Had the same issue, swore it was fuel related. That is about a 100 hour part, or mine looks burnt up around then. Good luck.

Doesn't that engine have thunderbolt ignition...Hook a timing light to engine and point it at you while running the boat ...See if the light goes out as motor dies ..Those ignition sensors ,especially if original are prone to fail...They added a ground wire coming out of sensor on replacements

Stock app should have EST ign set up. Always wise to post your engine serial for the exact set up.

Scanning the engine with no fault codes means nothing MEFI 1 , 2, and 3 are very limited on fault codes anyways. MEFI 1 only will set 7 fault codes - FYI.

You need to seperate the systems to figure out is it fuel related, Ignition related, Voltage related, sensor related to pin point your problem. Its about testing to rule out whats not your problem(s), Sounds like alot of parts buying and changing for not. Might have been cheaper and less time lost to have taken it to a real marine cert repair shop. Just saying.

Have you ran the engine on seperate fuel supply taking all your fuel side out of the mix ? Portable fuel tank with fresh gas and new fuel line with a primer bulb,

Fuel pressure readings for the engine thru out all rpms and what is the psi when the engine is stalling and stalled ?

Have you taken the kill switch out of the system for testing ?

Have you tried a new key switch ?

Do you still have power (voltage) at the electric fuel pump when it is shutting down and from a killed engine ? If so what are your voltage readings ????

You changed your wiring Harness ? or switched them ? is this correct ?

I'm going to suggest it is electrical in nature. Fuel issues typically involve motor lean popping, stumbling, running crappy etc. Rarely do they just shut off for fuel related issues.

Having said that electrical issues can be a bear to troubleshoot. Take a good look at all the ground wires, ignition harness, kill switch and ignition switch etc.

I have ran it off another tank. The fuel pressure is a constant 40 pounds. When it stalls it does not stutter, and fuel pressure just drops to zero. I'm starting to think more electrical. I'm not familiar with the kill switch. I'm going to try a voltage meter tomorrow. And I've called all the certified marine mechanics in the area that have some credibility but all are at a 3 week out unfortunately. Do you know what the voltage reading I'm looking for? The battery's are new in May so if it's low that would mean the alternator correct?

Unfortunately looks like the ecm. I swapped some of the parts 1 at a time to the other motor and when I swapped the ecm the other motor acted up

Good find! Might have solved your problem now you know that!

The joy of owning twins...your own parts store.

Ken