502 EFI Vapor Lock

[duffster]

What is the 502 EFI vapor lock fix??? It gets to about half tank and it quits. Full tank it not a issue.

[OkieDave]

Is the tank vented?

[centerhill condor]

you may have a leak in your "in tank pick up" tubing? 'bout 1/2 way to the top causing air infiltration and vapor lock systems.
If it is true vapor lock, the high liquid level in your tank corresponds to a higher pressure at the bottom of the vessel and fuel line. but how much pressure would 6" difference of fuel level be? 0.020lb/in2. not enough to trigger vapor lock. Besides, doesn't that efi run hign enough fuel pressure to prevent vapor lock?
If the tank vent system is not venting, a half full tank of vapor will contract/expand with the temperature causing less than atmospheric pressure and no fuel flow when cool.
An investigative technique would be to lightly pressurize the tank with less than 1/2 tank and see if liquid or vapor exits the tank discharge tubing.
I'm sure you'll get some good answers from this group.
Best I can do for a Tuesday! good luck

[duffster]

The tank is venting correctly. And in regards to the tank pick up it's also happening on my second tank when it gets to half. This issue only started happening when I switched from a carb 7.4 to the 502EFI.
I did move the water separator from the original location next to the exhaust away to a cooler location. And their was no change the engine basically runs out of gas after it starts getting warm. I also did blow compressed air in to the tank and heard bubbles so I know there is a half a tank of gas left. I'm very stumped on this one.
you may have a leak in your "in tank pick up" tubing? 'bout 1/2 way to the top causing air infiltration and vapor lock systems.
If it is true vapor lock, the high liquid level in your tank corresponds to a higher pressure at the bottom of the vessel and fuel line. but how much pressure would 6" difference of fuel level be? 0.020lb/in2. not enough to trigger vapor lock. Besides, doesn't that efi run hign enough fuel pressure to prevent vapor lock?
If the tank vent system is not venting, a half full tank of vapor will contract/expand with the temperature causing less than atmospheric pressure and no fuel flow when cool.
An investigative technique would be to lightly pressurize the tank with less than 1/2 tank and see if liquid or vapor exits the tank discharge tubing.
I'm sure you'll get some good answers from this group.
Best I can do for a Tuesday! good luck

[centerhill condor]

you may need a pump in the tank to increase the pressure to your primary pump. as an alternative, consider mounting the existing pump closer to the switch over valve or lower relative to the liquid level to increase the NPSH at the pump suction.
that's it for me...

[Dave_N]

What year 502EFI? Vapor seperator or Cool-Fuel?
Dave

[duffster]

It's a 98 502EFI Mag I just put it in the boat it had 0-hours. I don't think it has a Vapor seperator or Cool-Fuel. If so how can I tell?
What year 502EFI? Vapor seperator or Cool-Fuel?
Dave

[duffster]

Do you think it might be the VST screen on the second pump? Dose anyone have a screenshot or any information on how about cleaning it?

[Beer-30]

It's a 98 502EFI Mag I just put it in the boat it had 0-hours. I don't think it has a Vapor seperator or Cool-Fuel. If so how can I tell?
Does the fuel go through a cooler before getting to the motor?

[duffster]

I was wrong it's a 96 502EFI with a VST tank no cooler.
Does the fuel go through a cooler before getting to the motor?

[SUPERDUTYRHINO]

My old 502EFI had a couple problems with the VST; vapor seperator tank? If I recall correctly I had to have it changed a couple times. I believe they did away with this system on the newer models, mine was an 1994.

[SUPERDUTYRHINO]

Put your money on the VST tank, also, check the little yellowish colored line that goes from the back of the fuel pump to the intake manifold, if there is fuel in it you may have a leaking diaphram.

[duffster]

It's not the VST tank. I think it might be that my fuel pickip lines in the tank are rusted or broken. :mad: