Yes the PCM controls the idle by varying the amount of air introduced by the AIC. If someone messed with the "idle hard stop" screw ( which should never be touched ) on the throttle body, you will never get the engine to idle steadily and correctly.

 

Ok guys I think I am making headway but not there yet. Put in a new pcm and now the truck shifts as it is supposed to no banging except as follows.
The truck idles nice when I stop and put in reverse and then put in drive most of the time.
Problem is after warming up and driving a bit part of the time the idle is up to high like some time it drops as it should and sometime it sticks up around 800 900 and this causes a bang in shifting operation.
Any suggestions as to why sometimes it is just fine and then it goes and sticks up like that?? I have checked the vacuum and reads at 15 and steady. I check it on the line out of the tb. Is it possible I have a vacuum leak with those numbers maybe somewhere else? I read you can get like an internal vac leak but I have no idea what this would be or where. I have also checked the reading on the tps and the iac . both are ok at least as to what I read.
Also if I flick the throttle foot feet when it is idling to fast it does slow down and the throttle body is a new rebuilt.
thanks much for all the tips and help
Grumpy
one other thing the linkage on the tb is back on the throttle stop screw when it is doing this also. not like the springs aren't returning the linkage.

 

15 inches is too low. You should have 18-22 inches and all my vehicles are always at 20-21. Things that will cause low vacuum are: intake gasket leaks, PCV plumbing and brake booster/ plumbing, items requiring large vacuum demands.Those tiny plastic lines are not sustantial enough to cause your issue.Timing issues are another cause. Worst case is low compression. Another symptom of vacuum issues is a faster than normal idle. The vacuum when increasing engine speed from idle should briefly drop down the rebound up, if it goes low and stays there, look for a restriction in the exhaust. BY ALL MEANS, DO NOT ADJUST THE IDLE HARD STOP SCREW!! If you did, put it back, it should not have anything to do with idle speed, the computer controls idle speed.

 

Thanks Raski; I have no idea as to the tampering with the idle stop screw,. If it is not correct how do you adjust it there has to be some specs somewhere on the item.

So as I assume I have a vacuum leak how do you check for a leak in the pvc plumming? I think you should be able to find a intake leak with carb cleaner on the outside?
Thanks much for the help
Grumpy

 

Any CEL?

 

hayapower; I guess your above my paygrade; what is cel? is that a special vacuum equipment? or ?? also im at 5000 ft elevation if that matters on my 15 reading/??