Engine/Transmission (1998.5 - 2002) Erratic engine speed at 1200-1800

[LR Smith]

Alright fellows I need some direction. Have not posted much, but have been around for awhile. 2000 model with 178K miles. The engine has a stutter from slightly off idle to about 1800 RPM. If the throttle is held constant at say, 1200rpm, the engine speed jumps around about 50-100 rpm and seems to have a stumble to it like the VP44 is hunting very quickly. This is also indicated by the tach. If you tromp down on it, it smooths right out and runs out good. It seems to run fine at RPMs over approx 1800. This problem started all of a sudden with no warning and has not worsened in the last 1000 mi. . First, the VP44 is original, the lift pump will supply 14 psi at idle and drop to no less than 11psi at wot and the truck is completely stock. Fuel filter replaced with no change. IAT cleaned also. I am inclined to suspect the APPS, but have read posts about problems similar to this being fixed by the crank and/or cam position sensor. Other than the slight 25-50 rpm "hunt" the tach has always done, it is acting normal and never drops to zero, so maybe not crank/cam sensor? Read one post about MAP sensor fixing similar problem also? I have not had a chance to "recalibrate" the APPS yet, but will try that. Although it could be the VP44, it almost acts like it is getting an erratic reference signal for fueling. Before I get out the shotgun and throw a bunch of parts at it, I would like some opinions.

Thanks in advance,

Lain

 

[emjay]

Your APPS may be worn out. Some folks clean 'em and they work for awhile longer. You can check it with the key on and engine off with a DVM (Voltmeter) connected between the TPS signal wire and ground. Slowly press the pedal down, the voltage should gradually increase. If it jumps around, probably the APPS should be replaced.

 

[LR Smith]

Thanks Mike, I will try that first.

Lain

 

[Newf Limo]

I would have guessed a bad TPS. Might atleast try a Re-training of the TPS. If I remember right, Neg Batt term disconnect 30+ seconds. Re-install. Ignition to run (no start) depress throttle slowly to floor and return. I believe it is done after that. Just a thought.