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.