My 92 f-150 runs fine for 30 minutes or so and then the voltage indicator on the dash starts to vibrate. If I shut down and try to re-start immediately, it doesn't want to ignite. After waiting it starts fine. It almost seems like it is vapor locked. The battery and alternator have both been repalced and seem to be working OK. Connections at battery are OK. Could be electrical or ignition. Help.

 

A truck with EFI really can't vapor lock, since the fuel pump is in the gas tank. But it could be the fuel pump is not working. Can you hear it come on if you turn the key, but not crank the motor?

 

I will check this. The problem occurs about the same place each day on the way to work. I have a stretch of 10 miles or so of wide open raod. Once I get into town in the stop and go I seem to have the problems. Also I have been having problems starting in the rain and fog, but this could be a result of a loose plug wire that I found. Could be a clogged fuel filter.

 

Actually F.I. doesn't totally eliminate the possiblity of vapor lock. The voltage gauge fluctuating might indicate cavitation. I'd do the fuel filter.

 

Changed the fuel filter. Problem is still occurring. Also I am hearing a whistling sound that I can't localize that seems to occur simultaneously with the erratic voltage. If I shut the engine down and am able to re-start, the voltage indicator acts OK.

 

Pull the codes and test the fuel pressure, especially at the moment you notice the fluctuation & whistle. Pull the vacuum line off the fuel pressure regulator (FPR) and see if it has gas or a crack in it.