Truck runs poorly after sitting, suspect fuel leak, can't pinpoint

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.

  1. Pressure following overnight rest = 0 PSI.
  2. Pressure “key on engine off” = 20 PSI.
  3. Pressure “key on engine on” = 42 PSI (rear tank); 50 PSI (front tank). Gauge shakes slightly on rear tank.
  4. Revving engine drops pressure ~ 10 PSI, either tank. (Lack ability to measure pressure under road load.)
  5. Surging idle for ~3 minutes then smooths out.
  6. 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?

I would have to check (I’m at home now) but I think your fuel pressure is too low. If I recall it should be in the 55-60psi range. And if you unplug the vacuum hose from the regulator with the engine running the pressure should rise. Does it?

You may have a weak rear and main fuel pump.

Haynes says 45-60. Pulling the vacuum line pops the gauge (on the front pump)…but only 5PSI or less. There’s no fuel leaking out the vacuum port, and I feel good suction from the line.

Tomorrow I’m going to plug off the supply line after shutdown and see if it makes any difference in leakdown. A leaky fuel pump check valve would explain why no visible leak, but I’m hesitant to throw parts at it. May price out pro help.

Offhand, it does sound like a check valve in the fuel pump leaking off due to 0 pressure overnight. This can be a real irritation as that means pump replacement.

One thing I like about the old Bosch pumps on European cars; they have a replaceable check valve on the end of the pump.
Disconnect the fuel line, unscrew the old valve, screw in the new, reconnect the fuel line, done.

The same thing just happened to my Lincoln the other day. It’s sitting outside right now with the fuel pressure test gauge on it to be checked in the morning but I know it’s going to be on 0.

This does cause rough running until the air is burped out of the system. Think of it as a water hose on a hot day that is spitting and sputtering until the air is puked out.

Since it runs better after warming up, coolant temp sensor?

Leaking check valve in the pump or maybe even a leaking injector.

Most people recommend removing the truck bed rather than dropping the tanks to access the fuel pumps . Considering the problem is a bad check valve in one or both pumps { this may be a stupid question } would it be possible to install an inline check valve or valves right at the tanks & maybe solve the problem without dropping tanks or removing the bed .
I’m always looking for an easy work-around , some make sense , some don’t .

Have known people to cut an access panel to get to the fuel pump. My reccolletion of people with a bad check valve was mostly a starting issue, not a driveablity issue, but that may vary from vehicle to vehicle.

From what I can tell, to check the fuel pressure you have to ground something on the Diagnostic Connector first. Then with the key on the fuel pump operates, even though the engine is off. You should see 50 to 60 PSI. Since you are only seeing 20 PSI, I think you must have something wrong with the pump or the check valve. A bad fuel pump, check valve, or FPR, or clogged fuel filter is consistent with what you say about the pressure dropping to 10 PSI when the engine is running and you bump the rpm briefly. When you bump open the throttle, that should increase the rail pressure, not decrease it. The reason is that the FPR attempts to keep the difference in the rail pressure and the intake manifold pressure constant. When you open the throttle the intake manifold pressure increases (the vacuum is less). So the rail pressure should increase. That’s why there’s a vacuum hose from the PFR to the intake manifold.

UPDATE: Rear fuel pump replaced; problem solved! It was getting bad there at the end: knocking (rear tank only) under load, reduced power, etc…all solved with a new pump.

Thanks for posting an update. Glad your new pump got the car back to purring like a kitten :wink: