The Story:
1 year ago my truck (1991 F150 X-Cab 4x4 SWB) was gettin good mileage (18-20) and was running great. Then one day it started running crappy. I've replaced every sensor, the EGR Valve itself, air filter, plugs, wires, cap, rotor, and finally the PCM. The cat is fine, it has a new mullfer, and a new alternator and battery. When I changed the PCM, it started running better (almost as good as it did last year), however it misses BAD when it idles for more than 15 seconds. When you pull up to a light its OK at first, then steadily it runs worse. It still doesn't have the power it did last year, but its close. It doesn't burn oil and the compression is good. I'm out of ideas except for maybe having the injectors cleaned.
I've thrown 4 mechanics at it and they all said "it runs great" even when you couldn't do 65 in overdrive on the freeway empty. I think the truck is running lean. When I disconnect some sensors it runs better in some ways.
Disconnected TPS: Idles great, but runs irratic (expected)
Disconnected ECT: Idles better, runs a little smoother, but seems less responsive.
Disconnected O2: made it run better until I got a new PCM. Now has little effect.
Disconnected IAC: will idle/run better

If I unplug the computer and let it sit (clearing out all the "learned" data), it runs good for a day. Also the it misses/flutters just off idle (1/16 throttle to 1/8 throttle); It will jump between 1000 and 1700 RPM.

I'm looking for any suggestions (well not ANY suggestions but ones you feel may help me out..). Your comments are appreciated.
Please help me before I have to check into a mental hospital... ;-)

Thanks,
- Aaron Neal

= Aaron K. Neal - 1991 F150 4X4 4.9L I6 =

 

1. Why do you think its lean? Is it running hot?

2. Get the injectors cleaned blueprinted and balanced. Should be $25 each from a place out of any car magazine.

3. Removing the O2 sensor and no change tells me that it may be running in open loop. This may repeat your analysis, except it should be super rich.

4. Run the truck at WOT then turn off the ignition. Pull the plugs and see if they are indicating lean, rich, or missing.

More diagnostics is required.

Randy

 

>1. Why do you think its
>lean? Is it running
>hot?

No but hot weather causes a loss of power. Its not running rich or I would see soot on the O2 sens, the plugs, or the exhaust. Maybe this is a bad assumption? Before I changed the PCM, I checked the reading on the O2 and it indicated that it was lean.

>2. Get the injectors cleaned blueprinted
>and balanced. Should be
>$25 each from a place
>out of any car magazine.

This is the only thing I feel may be left to do.

>3. Removing the O2 sensor and
>no change tells me that
>it may be running in
>open loop. This may
>repeat your analysis, except it
>should be super rich.

I think its running in closed loop. There is "little effect" when I removed the O2 sens, but there is an effect. The computer knows the O2 is removed. Its a new PCM too, so I would imagine that that's not the problem. The leads to the ECT seem OK and I get a good signal voltage. If not then I should get a code if the lead is broken somewhere.


>4. Run the truck at WOT
>then turn off the ignition.
>Pull the plugs and see
>if they are indicating lean,
>rich, or missing.

Plugs always look "clean" (no soot, no white, nothing).

I appreciate your time. Keep the questions coming...
Thanks,
- Nealio

= Aaron K. Neal - 1991 F150 4X4 4.9L I6 =

 

Maybe this is too obvious, but how about swapping in a new ignition coil?

 

Did you test the fuel system? Change the fuel filter? Make sure there is no fuel pump pressure drop at different rpm's and its to spec.

 

Fues system checked out OK both for pressure and volume. The
fuel filter was the first thing I changed.
- AKN

= Aaron K. Neal - 1991 F150 4X4 4.9L I6 =