I have a 96 F250 4WD 5.8 liter automatic with 130k that is running lean. This truck will throw a 172 code (OBD1) only when out on the highway at cruising speed, never while puttering around town.

The truck will throw this code every time when running off the front tank and on occasion when running from the rear tank but not always.

I will also say the truck feels powerless when running off the front tank and while not a "powerhouse" better when running from the rear tank.

Tonight I hooked up my pressure gauges, jumped the contacts for the fuel pump "on" and turned on the key. The front pump would max at 23lbs (no matter how long you let the pump run) and the rear pump maxed out at 32lbs. Are both of my pumps weak. what should they be able to achieve? I think they are original.

While the Fuel pressure regulator and the fuel filter have been replaced new recently, I don't think the pumps ever have.

Another thing.....my fuel pressure at idle from both pumps is around 28-32 lbs. (28 on the front....32lbs on the rear). Upon throttle (2500 rpms hold) the front pump actually decreases to about 25lbs and the rear pump is at least able to maintain at around 32 lbs but does not INCREASE at all really.

The truck runs great....no bucking, stumbling, idle issue just gutless and running lean when under a load or at highway speeds.

Any help would be greatly appreciated.

 

You should have 30-34psi at idle and 40psi at WOT or with the vacuum hose on the nipple of the FPR disconnected.

Sounds like either you do not have the adapter for the fuel pressure gauge on good or you have bad pumps or low voltage to the pumps or a plugged up filter.

 

Subford,

How much pressure should these pumps be able to build by simply jumping the fuel pump pins, turning the key on and letting them run and build pressure? I thought I had read somewhere 40-45 psi?

 

Try to run the truck as normally as possible when testing to get as close to the real world situation as possible. Remove the vacuum line from the FPR. Do you see or smell gas in the line? With the line removed, start the motor. What fuel pressure do you have? Removing the line will make the FPR provide max pressure it is capable of.

If that checks out then start working backwards. Fuel filters, like you mentioned. After that the next, more difficult, thing is the fuel pumps. Like SB said, your pump(s) may be bad. Check wiring connections to them first and make sure your ground is good and the power wires look good and healthy (no cracking or corroded wires). From there you might want to look at the pumps themselves. They can get clogged with bad fuel just like anything else or they could just be old and need to be replaced.

Basically start easy then go to the harder to get to parts.

 

You should get 40psi. The book says 40-45 but that is wrong unless you put air pressure on the tipple with a hand pump.

Basically the fuel pressure regulator has a 39.5psi +-0.2psi spring in it to maintain the 39.5psi +- 0.2psi across the fuel injectors at all times.
The vacuum from the engine pulls against the spring to allow more fuel to return to the tank and lower the fuel pressure and to allow it to drop to about 30-32psi at full vacuum at idle.

When I say you may have bad pumps what I am saying is the valves inside the FDM that is inside the tank may be bad allowing the fuel pressure to drop down. The fuel pump is also inside the FDM.

The fuel pump itself will pump about 95psi fuel pressure into a blocked head.

 

That makes sense.

I tend to think it is my pumps. The FPR is new as well as the fuel filter and I cannot achieve anywhere near 40psi even at WOT. (infact it's worse at WOT) I purchased a new pump. I am going to drop the front tank this weekend and replace it. Can't hurt. I will post my findings if any. Thanks for the help guys.

 

Note if one of your tanks is filling while running on the other then the tank that is filling has the bad pump (FDM).

 

Update:

I replaced the pump in the front tank and it fixed many problems. My truck now starts easier, hold it's prime, no longer crossflows fuel and pressure actually INCREASES upon WOT. All good things.

However: my CEL and 172 (lean condition) came back while out on the highway test driving. I am pretty sure now it is not a "not enough fuel" but more of a "too much air" problem.

You wouldn't know anything was wrong with the truck but I am determined to fixed this CEL. But I need help.

What I have done is:

new fuel pump
new FPR
new fuel filter
new 02 sensor
new plugs
new wires
new cap
new rotor

This 5.8 runs great but does have a slight exhaust leak on the passenger side when you first start it but goes away when it's warm. Could this really cause it or should I look more toward air metering or vacuum/intake leaks?

How does my engine meter air?

As always, any help would be greatly appreciated.

Ask questions if you need to.

Thanks,
Mike