1994 F150 XL, 4.9L I6, 191k mi.
PROBLEM: Truck has problems starting, that began a few months ago, that have gradually gotten worse. Especially after sitting for several hours, requires repeated cranking to start, then runs rough/stumbles, and may die if throttle not goosed occasionally. Recently, problem persists a few miles after driving, mostly surging idle and stumbling under load, like it isn’t getting fuel. It should be noted this happens on either of the twin tanks. It runs smoothly thereafter, and when started after sitting less than 30 min. No CEL (OBDI) illuminated.
DIAGNOSTICS TO DATE: Plugs pulled immediately after a rough start: no carbon or raw gas noted, though cyls# 4-6 seemed to have more ashy combustion deposits than 1-3 (still not abnormal IMO.) FPR replaced; symptoms, if anything, got worse. Fuel pressure gauge attached to Schrader valve on fuel rail; the following data recorded.
- Pressure following overnight rest = 0 PSI.
- Pressure “key on engine off” = 20 PSI.
- Pressure “key on engine on” = 42 PSI (rear tank); 50 PSI (front tank). Gauge shakes slightly on rear tank.
- Revving engine drops pressure ~ 10 PSI, either tank. (Lack ability to measure pressure under road load.)
- Surging idle for ~3 minutes then smooths out.
- Upon shutoff, pressure dropped from 42 to 22 PSI over the course of 90 minutes.
HYPOTHESIS: I suspect there is a fuel line leak that is NOT the FPR or an injector. I also suspect it is pretty far upstream–before the filter?–which is why it runs like crud until the air in the line is dealt with. I also realize I have a weak rear fuel pump, but it does not seem to be causative.
I cannot, however, smell or see the leak. (My wife says she sometimes smells gas, but–as I usually have landscaping machinery and jerry cans on board–that doesn’t necessarily mean anything.) Would a shop be able to “sniff” a HC leak? Any thoughts or alternative theories?