As Cloudmaster mentioned, just return the fuel to the tank instead of the VST and the problem will be fixed forever.....guarenteed. It is really simple to do. If you can't get to the fill hose to return it, then I can tell you how to make a fitting to put the fuel back into the tank. I have done more than I care to think about, so we know what works.
Give me a call if you have any questions. I can walk you through it.
Eddie

What about on the later 502's without the VST?

Had an issue last summer with both of mine vapor locking, especially after hard run and sitting for a bit.

I also read an article recently that claims ethanol fuel has a lower boiling point and can worsen an already bad vapor lock problem.

thanks Eddie when i get to that point I'll call you!

The later ones return to the fuel filter head. You can simply plug off that port on the filter head and just run the fuel line back to the tank.

Fuel with ethanol is more susceptible to vapor lock.

Eddie

My 502 mpi's will vapor lock with ethanol quite a bit and rarely vapor lock with regular fuel. Like Eddie said, you can run a return to tank or Merc also makes a kit with a booster pump that will cure the problem.

that merc EFI set up was B/S from the start even with all there updates(band aids) re plumb with a aero a1000 pump, a return regulator, good filter, and return to the tank...the way it should have been...

Eddie... my problem is.... the fill line for the fuel tank is right in the middle of the boat and the boat is 40 ft. long. Is there any any other way to return it without drilling, because the tank has about a 100 gallons of gas in it.
thanks,
kevin

There certainly is another way. Give me a call and I will be glad to explain it to you. It's hard to put into words to type here. Otherwise, I would just post it here. I've tried to explain it before and I don't think it made sense.

You can reach me anytime at 615-216-7449.
Eddie

Give Eddie a call, he knows his sh-t has helped me several times. He'll be doing my motors when the money fairy pays me a visit.

you can get a sending unit that fits your tank(depth) for a diesel, they have a return fitting already made in it...pretty simple..