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'93 F-150 - "Vapor lock"?

My '93 F-150 runs like a champ for ten months out of the year. During the hottest part of the summer though it tends to stall out due to what some of my friends describe as “vapor lock”. What happens is that on long trips at highway speeds it will work fine for a while and then after an hour it will start to run rough, coughing and sputtering, and generally acting as though it’s not getting enough fuel. If I pull over and park with the engine off for several minutes then it will start right up and act as though nothing is wrong - for a few miles. If I let it sit several hours to completely cool then it will take off just fine. It just doesn’t like long runs at above 90 degrees F.

Some relevant facts:

If I switch from one fuel tank to the other then sometimes it will seem to correct itself, sometimes it will not.

It has a new fuel filter and two different mechanics swear the fuel pressure is good from both tanks when they’re cold. I can’t get it to act up when they are around. Can I or my mechanic install a permanent fuel pressure gauge so that I can track the pressure during these episodes?

Both mechanics have suggested replacing both fuel pumps in the theory that they are old and failing but admit that they are not sure that’s the problem and offer no promises that new pumps will fix the issue. Both fuel pumps have been in there since I bought the truck more than ten years ago.

If it’s not a fuel delivery issue then what is it? Bad spark at high temps? When the truck acts up I don’t smell any unburned fuel in the air at it doesn’t act like it’s flooding. How can I or my mechanic test for this?

I’ve had several people suggest to me that I pour a couple of pints of motor oil into the fuel tank at my next fillup but they seem rather vague about what this is supposed to accomplish towards correcting this problem. Does this make any sense? How, if at all, does this work?

All polite suggestions welcomed.

I’d skip the motor oil in the gas tank. Best thing to do is, next time it won’t start, immediately check for spark. Could be ignition module or crank sensor acting up after they’re hot. Could even be a bad computer, or wiring. Trouble with those era cars, is that even with a scanner, you don’t get much information. Troubleshooting usually involves trying somethingm til it works.

“Best thing to do is, next time it won’t start, immediately check for spark.”

I would do this how?

I doubt that spark checking will help you much since you don’t really report any kind of significant no-start issue. If there are times when it dies out & then there is an extended period of cranking with no start then you’d want to check for spark.

Given the symptoms my first two suspects would be a partially clogged exhaust or hot fuel pump issues.

An easy way to check for exhaust blockage is with a vacuum gauge. You might need a mechanic with at least some gray hair to use one effectively, though they are easy to use & not very expensive to buy.

For the fuel, the system does have to be checked hot & under load. I would be thinking like you are - to have a gauge rigged up. You can buy add-on fuel pressure gauge kits that are meant to be installed as dash gauges. Other than that, a test gauge can be hooked up with an extension & temporarily taped to the outside of the windshield or something.

You might also worry about fuel tank venting. The fuel tank needs to be able to suck in air to displace the burned gas. You could try purposely leaving your gas cap loose to ensure air entry & see if that makes a difference. Or loosen the cap as soon as it starts to run rough to see if that helps. This will probably turn on your check engine light as it is an evaporation system leak.

I would worry about those things first, but ignition breakdown, probably from heat, is a possibility. I’d guess though that it would take longer than several minutes for this to correct itself.

sounds like the in-tank lift pump isn;t working…exactly what happens…beware is you’ve got dual tanks. The fuel switchover valve /reservoir says “NOT A FILTERR” on the bowl but often there is a filter in it from the factory…check it…if there is one, throw it away…you’ll need a new O-ring for the bowl.

Your truck is a TFI-IV model and these are prone to heat-related ignition module failures. The symptoms can vary a bit from missing, random stalling, and sometimes flat dying and never allowing a restart.

Testing them is pointless unless it’s a case of the never allowing a restart scenario because they may test fine and still be bad.

This is a very common problem and if you do a net search for “TFI Settlement” this will explain it.
I am surprised that none of the mechanics you visited are aware of this.

Many thanks for the kind replies. There are some good suggestions here but not as much of a consensus as I hoped.

On my next highway run, I’ll try to run with the gas cap removed to see what effect that has.

Ignition modules are not that expensive. I’ll swap in a new one and see what happens.

My front fuel tank has a small leak from rust and needs to be replaced anyway. Installing a new front fuel pump at that time is a trivial matter. If that solves the problem from that tank then I may replace the rear pump or I may live with it until it fails completely.

I should be able to find one of my mechanically minded friends to show me how to check for low fuel pressure and for poor spark so I’ll be ready the next time it happens.

One way or another, I need to track this down. I like this truck and I’m tired of not being able use it during the summer.