we have a 92 with 3.9L & auto trans and the engine loses power and stumbles at high rpm the longer the truck runs. After 20 mins on the highway, it can't maintain 60mph and the engine needs to have the throttle feathered (usually less than 1/2 throttle) to keep it from stumbling. If you go WOT, it falls on it's face.
We drove it with the windows open and there is a very strong gas odor when it is running rough (WOT), but this disappears when the throttle is 1/2 open.
The odd part is that when cold and first on the road, it will do WOT through all gears and pull to 75mph (it does get a little weak at 4200rpm, but will pull up to shift point about 4800rpm).
It logs no fault codes.
so far:
we theorized the ign coil was failing as it warmed up, so replaced the coil with a spare. no change.
we have checked the coolant temp sensor resistance at cold and hot (after it starts acting up) and it is within the resistance range listed in the FSM manual.
We have checked the inlet air temp sensor and it is within spec
we have checked the MAP sensor, and the voltage signal is within range listed in FSM, both at the sensor and at the PCM connector.
we have checked the TPS, also within range listed in the FSM.
We have pulled every ground we can find in the engine compartment and cleaned to bare metal.
we have replaced the cap/rotor, cleaned/gapped the plugs (they were 10 months old), The secondary leads are about 2 yrs old.
We have changed the cam sensor in the distributor body, and checked the rotor shaft for play (seems to have normal drive gear lash, and the shaft is solid and the shutter wheel is not loose.

we were starting to think the manifold plate was leaking (but had no sign of oil inside the manifold looking from the throttle bores) so we attached a vacuum gauge to the manifold and ran it till the problem surfaced, and found vacuum was stable, with no change from cold to warm. idle vacuum is ~16" and stable.
We have two spare PCM, and both show the same pattern of operation.
The CAT was removed a couple of years ago and it has an "open' muffler, so we don't think the exhaust is restricted.
Even when this engine is at the point where it can barely move the truck from a stoplight, if you shut it off and try a restart, it fires right up and idles smooth. Does not stall when you put it in gear, but if you give it more than half throttle, it falls on it's face.

We are looking for any ideas or suggestions the group can offer.
Thanks in advance.

 

A few things I can think of is the fuel pressure/volume is low due to clogged filter or bad pump. The transmission is slipping which is what you feel like the engine has no power. Or you have a bad vacuum leak or a bad EGR valve.

 

Need to check your compression also

 

Thanks for the input,
Checked fuel pressure today, 40psi with key on, drops to ~36 at idle.
We plan to take it out on the road and run till it starts acting up and then check fuel pressure again to look for a difference. (probably tomorrow eve).

the trans shifts seem solid when the engine is cold. When it is cold or warm, the converter clutch seems to be holding and we find no rpm flare which would be in line with a trans slip condition.

We had found & fixed one loose vacuum line connection at the manifold during initial checks. but have not found much else amiss. I think we will try plugging all the lines we don't absolutely need and retest.

Didn't think about the EGR valve. any ideas on how to disable this or troubleshoot it?

We had done compression checks some time ago, but didnt' keep notes. Will repeat and post the results.

 

Update,
Took the truck on the highway till it started to act up, (about 30 mins) then checked the fuel pressure.
Found it was running about 30-32psi at idle, and dropped a couple pounds if we revv'd the engine a bit.
The FSM lists approximately 35-45psi at idle speed as the spec. Looks like we are near 5psi under after running for a while.

 

Update,
We pulled the truck box and changed the fuel pump module.
Testing over the last few days have proven that fuel starvation was affecting the performance of the engine.