During normal driving conditions my truck has recently started revving up several hundred RPMs and then going back down. One up and down cycle of the RPMs takes about .5 seconds and it just keeps going and the truck lunges forward and slows with the RPMs. It occurs when I try to maintain a steady speed, on a flat roadway and not accelerating or pulling a load. Basically, when the engine is under very little load it happens. It seems to continue doing this until I let off the throddle or accelerate. It's happening about 1400RPMs, around 30 to 50MPH. If I accelerate or put any load on the engine, it builds RPMs without a problem normally. My fuel pressure is about 10 to 13psi after the lift pump.

I replaced my lift pump about 2.5yrs and the injector pump about 6mos ago. I have after market gauges installed. The fuel pressure use to alway be 14psi and would drop to about 11 during heavy acceleration. Now at an idol, it's still 14psi but drops down to about 10psi during normal driving and drops to about 8 or 7 during heavy acceleration.

The truck is completely stock.

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I paid a little more attention to the above described behavior and noticed it seems to only happen when the truck is in 5th gear at about 47 to 50mph. I'm wondering if it's not a tranny issue and what I'm describing is the torque converter locking and unlocking?

Any help would be great!

 

Auto or Manual 5 speed? If it is an auto the lunges forword is the TQ locking and unlocking. This is caused by interference in the APPS or TPS. First check all the grounds on the truck.

 

Its an auto trans. I agree it seems to be the TQ. Thanks for the input!

 

It's a common symptom, and can be caused by poor grounds, electrical interference, or poor connections. There are a few cheap fixes one can try to eliminate it.

 

Suncoast doesn't have the link no more to the APPS rewire diagram.

 

I've noticed that as well, rotten buggers.


I know it's small, but if I enlarge it all the text gets blurry. Essentially, Cut wire #4, and add wire to each side of the wire you cut to a good ground. Do NOT put it to your battery though. Frame is better.