502 heat soak problem hard to start

brent72132 · 06-26-2006, 11:14 PM

I have a 94 efi 502 an after I run it for 30mins or so if I shut it off sometimes it will not start back up about 90% of the time. i will have to hold the throttle w.o and it will start up after spinning it over for 10-15 sec. I will run it at 2500 rpms but then pull it back and it will not idle. it does not smoke black like it was flooded out or any thing .
never has died on me at any speed. az speed and marine said it was vapor locking and I need about 2000.worth of new fuel supply can any one help

Ted G · 06-26-2006, 11:32 PM

As you probably know, Merc changed to a "Cool Fuel" system in 96 on the EFI motors. This was because the mechanical fuel pump that feeds the VST had a tendency to vapor lock. This sounds like what is happening to your engine. You can switch your system over with some new lines to and from the fuel rail, a fuel cooler, and a new electric fuel pump. Another option would be a low pressure electric fuel primer pump before the mechanical pump. When the system seems to lock up, switch on the primer pump and reprime the system. Or you could plumb in the electric pump to feed the VST and eliminate the mechanical pump from the system completely-just make sure you add a relay and some type of engine-off shut off so the pump won't run if the engine shuts down from a mishap.

brent72132 · 06-29-2006, 08:17 PM

thats alot is there any thing else I could do. dont like the ideal of a pump tha I have to shut off

brent72132 · 06-29-2006, 08:18 PM

where do you get a fuel cooler and can I tap it in to the old vst system

Ted G · 06-29-2006, 08:29 PM

You could use a standard cooler like an oil or trans cooler, but tha is not your problem. The fuel is cool coming into the engine. But when it sits in the mechanical fuel pump chamber it starts to vaporize from the heat. This causes pressure to go up in the pump and collapses the diphragm against the spring. Now the pump can no longer pump until the pressurized vapor is displaced by liquid fuel, but since the pump doesn't work, the vapor just sits in there till the engine sits and cools. That's why an electric pump is needed to re-prime the system. If you eliminate the mechanical pump you could power the electric from the ignition switch and then it would turn on and off like a regular car fuel pump.

brent72132 · 06-29-2006, 08:36 PM

why would az speed tell me I need a cooler??????

brent72132 · 06-29-2006, 08:46 PM

so I would need to remove the low psi pump that is hooked in to the belt drive that is bolted on to the water pump and add a low psi elc. pump up to the line there what type of pump should I use a holley blue????

Ted G · 06-29-2006, 09:14 PM

I'm sure they were telling you that you needed a whole cool fuel style system, that would eliminate the mech pump. You could use a Holley pump, it just needs to be regulated to 5-6 psi since the VST is not designed for high pressure coming in.