OK, I thought I had this one fixed, but I was wrong. I think this shows in my signature, but my truck is a 96 F150 5.0 4r70w 4X4, 130K miles

Here are my symptoms:

Truck runs like poo below 1500 rpm. Acts like it's not getting enough fuel- surges and sputters. It also idles down too low (500rpm) Above 1500 rpm, it acts fine. Mileage has not suffered, no black smoke. It has only thrown a code twice, and both times it was gone the next time I started the truck. Tried to have the codes pulled at Autozone, but they said they can only pull active codes.

On occasion, it will actually die when I'm pulling away from a stop. When it does this, it restarts easily, and will run normally (or very near normally) until I shut it off.

Temperature doesn't seem to have any effect on the problem at all- it will happen right after startup, and when the engine is up to temp.

This part is bizarre- the problem started almost immediately after a starter change. I replaced the starter and starter relay (solenoid) and the problem started within 5-6 startups after that. I don't see how they can be connected, but I can't rule out a connection, either.

Here's what I've done so far:
2 cans of Seafoam- one through the fuel, and one through the vacuum line for the brakes (not at the same time, though).

I've checked for vacuum leaks- I found a bad one and fixed it, but the problem didn't change a bit.

It's almost certainly not the pump- the problem acts the same with both tanks. Besides, if it were a pump problem, I would think it would be worse at higher rpm, not go away.

There's no fuel in the vacuum line to the fuel pressure regulator.

Fuel filter has less than 20K miles on it.

I would suspect that a check of fuel pressure would be my next step. This whole thing is screaming regulator at me, but I'd rather be more sure before I start throwing parts at it.

For checking the pressure, do I have to install a gauge inline, or is there something I can screw into the schrader valve on the fuel rail? other than cleaning, and repairing a leaking core plug in my throttle body, I've never had to deal with FI much, so any guidance is appreciated.

btw- coolant leaking out of the core plugs in the TB is not a "replace the TB" fix, no matter what they say at the stealership. If anyone wants, I can outline the ten dollar permamnent fix for it.

 

I have a liquid filled pressure gauge permanently mounted on the shrader valve on my truck, the liquid dampens vibrations so it's easier to read. You can see it in this pic just above the oil fill cap..



Have you cleaned the IAC recently? What codes did you get, and have you pulled them yourself? May be time to get a code scanner.

 

Paul,

No, I have not cleaned the IAC recently, and I have yet to pull the codes myself. I did find a proceedure for pulling codes (that will pull the codes in memory, too) without a scanner. It doesn't say specifically that it works on a factory MAF truck, but I'm hoping that it will.

Is that gauge something you bought ready to screw on to the valve, or did it require any "erector set" work?

 

Your truck will be OBD-2, it will have the diagnostic connector under the dash. If so the procedure on fordfuelinjection won't work you'll need an OBD2 code scanner.

The gauge I have is from Summit, the fittings are from a hardware store, standard NPT stuff, I have a 45deg elbow on it to make it easier to see.

 

OK, a bit more information:

I'm assuming by "clean the IAC", you mean take it off and blast it with carb cleaner. I did that, and while it might have improved the problem a bit, just idling in the driveway, it still has an off idle stumble.

I did a quick check of fuel pressure. KOEO on the rear tank is 37 psi, KOEO on the front tank is 39-40 psi. KOER is 30 psi, and when I pulled the vac line to the regulator, it shot back up. I turned the truck off, and the pressure rose back to about 36 psi (front tank)

From what I know, it seems like the more I learn about this problem, the more confused I get!

 

Off idle stumble says vacuum leak to me... and these can be a bugger to find, you may need to attach a vacuum gauge to monitor what is happening. You may also have a TPS problem, check the whole range of throttle movement for glitches on a multimeter, the voltage should vary smoothly from closed(~0.5-0.9v) to WOT(~4.5v).

 

I just posted this in my thread. If nothing else it's a good read:

Fuel Injection Service, Not Just Cleaning

 

Thanks guys for the help. I went through the vac leak search a couple more times, still didn't find anything.

A faulty TPS just made so much sense with the symptoms I was having that I went ahead and replaced it. I haven't driven the truck much since then, but everything seems fine now.