I posted a few weeks ago asking if I should replace my injectors on my 1994 F150 5.8. Its got a surge/miss at around 1500 rpm. This issue is now evident when trying to pull a hill. The transmission does Not want to downshift when the truck is missing (transmission was rebuilt about 5000 miles ago). Replaced the Transmission Neutral Lever Position Switch recently and properly lined up the shaft to the sensor. I have checked All the engine sensors with volt/ohm meter (they were all within specifications), Compression is around 175 psi on all except 1 cylinder (it's at 155). Fuel pressure checked good according to book. Engine vacuum is good (tested with gauge). I checked for vacuum leaks, throttle body is clean. Replaced spark plug wires, & cap. The spark plugs look Very clean like the truck is running lean on All 8 cylinders. The computer was replaced last year. Pulled no engine codes except system pass (111). Truck runs better when colder the warmer it gets the worse it runs. It has never stalled. What else should I look for? Could the upper Plenum be gummed up causing this issue? Could the throttle body need replaced due to age? Truck has 330,000 miles. Any advise would be appreciated..

 

When it starts to miss under load, if you press the gas pedal more, does it get worse?

 

Yes the engine miss gets worse and the transmission (when it does finally downshift) is really harsh.

 

This is the key info I think. The problem is temperature related and is linked to fueling, when the engine is first started it runs in open loop which uses a richer fuel map, once it reaches operating temp it goes into closed loop which runs on a leaner fuel map. The engine temperature sensors and the MAP are the key sensors affecting fueling, if those have not been replaced I'd give it a try despite the fact they test OK... maybe start with the MAP as it has the biggest impact. Another possibility is the O2 sensor, these can drift off target and still function.

 

How did you test the fuel pressure? In your drive way or while you were driving and the engine was surging?

 

Parked with engine running. At idle then when I hit the throttle. Pressure was well within the specs in the book. Its got the dual tanks so I didn't think both could be weak. I forgot to mention but when I plug the truck into my computer scan tool it runs perfect. No missing, Engine is smooth not shaking. (Of course the scanner shows no codes except brake pedal not pushed and overdrive switch not pushed. LOL)

 

You may try disconnecting the vacuum line from your EGR valve and drive around to see if it starts surging. It's easy and free.