I’ve been having a mysterious issue where every time I start my ‘97 SD in the morning it idles very high at 2000 rpm. As it warms up it drops down to about 1600 rpm and stays there, but only in park. Shifting to Reverse or Drive drops it to 900. But the shifting is very hard, making the truck jerk. After driving for 10 minutes and putting it back in Park, it’s back up to 1600 rpm.

The mystery of this is that if I turn the ignition off and re-start once the truck is warmed up, it idles at a perfect 700 rpm, and shifts nicely with no jerking.

I’ve installed a new Coolant temp. sensor but the problem remains the same.

It’s a kind of Doctor Jekyll and Mr. Hyde situation. The problem only happens when the truck has been sitting for about 5 hours (happens when I start it after work).

 

You need to reset your adaptive values when cold that should correct it. Reset Adaptive Values Thread.

 

My 96 high idle episodes where solved by rotating the TPS CW a bit. The % TPS value reduction can be read at the OBD port. It might only take a percent or so to fix. I had to hog out the holes. High idle occurs when TPS signal is higher than baseline when the engine is started. It could be the TPS is wearing out. Could also occur if the throttle plate is not fully closing or stuck on some crud, or the TPS loosened up.

Among other things, resetting adaptive values resets the TPS baseline to a new value (records the position of the TPS at resting position). When the TPS is replaced with a new unit that does not exactly match, a reset will be needed. Until you can get a reset done, you could try the TPS rotate hack, just to get by. Good luck!

 

Aha! I have messed with the TPS a few times, but mostly just adjusted the idle screw above it to get idle down to 700 rpm, but I did it when the engine was warm. I’ll have to look more in to “reseting the adaptive values”. Since I only have an Ultra-Gauge and not serious computer I’ll have to hit up P.T. Schram for help.

 

Resetting the adaptive values worked on mine for awhile, but the root cause ended up being a failed TPS.