My inline 300 six runs great at 75°+ outdoor temperature, but when temperature drops or humidity goes up it runs like crap. I replaced spark plugs, wires, cap and rotor no change.

Asked by RayB1964 Dec 04, 2018 at 10:58 AM about the 1999 Ford F-150

Question type: Maintenance & Repair

At 75°+ outdoors temperature my truck runs great,
but when humidity goes up or temperature goes
down the truck idles fine but has no power. When it
sets for 2 or more days the gas drains back in to
the tank. So I have to turn the ignition on several
times until I hear the fuel pump change pitch then
it'll start.

1 Answer

101,335

If I was a betting man, I'd wager your fuel pump is getting weak, and those temp fluctuations, which can affect the performance of the entire EVAP system, are causing a loss of fuel pressure at the lower ambient temps or high humidity levels. To start with, since you've already done most of a tune up, you should clean the MAF sensor, PCV valve and hose/elbow, EGR, and triple check for vacuum leaks. Then check the performance of the EVAP solenoids, and clean the charcoal filter. It would be nice if you can hook up a fuel pressure gauge and mount it to where you can read it while driving, I bet you would see it drop or fluctuate when those conditions you describe happen.

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