Strange idle issues

[DennyDoler]

My truck has started doing some very strange things concerning the idle. It is completely random, but is getting more frequent. Sometimes after starting the engine it will rev around 2k rpm without even having to touch the gas pedal. It is not frozen or stuck and actually moves very freely. I can shut the truck down and restart and it goes right back to 2k rpm, if I put it into gear and put it back into park or neutral after 15 seconds it will go back down to around 800 rpm and then drop again to a normal low idle.
The other symptom is when it is at normal idle and I put it into gear and put it back into park or neutral is races the rpm's back up to around 1k rpm, hold steady for about 20 seconds and then go back to normal idle.
I'm sure it is the stepper motor doing this, but what would make it happen? When it is not acting up it runs and drives fine, has anyone seen the ecm do this before?
Another symptom is at idle waiting to accelerate, when I step on the pedal it stumbles a bit and the gauges act like they lost power for a split second.

My truck is a 92' Classic with a 95' 3.9 utilizing all of the 92' harness with the addition of the 95' DI alternator. I suspected the alternator was the cause at one point. Is the voltage regulator different in any way that may cause weird electrical issues? It doesn't hunt and charges fine.

 

[knewsom]

Replace your ground cable.

 

[Butch]

I had the same problem with mine, replaced the TPS and it still did the same thing. I took it in and had the ECU reset. The voltage to the TPS was way low. Haven't had a problem since.

 

[bmn1965]

Sounds like the stepper motor but maybe look for vacuum leaks also that can make the rpm's spike.

 

[Timmy!!!!!!!]

hmmm not sure what it could be but if it isn't a tps issue then it is probably something else. I am pretty sure the idle is mechanicaly set on 14cux trucks unlike the GEMS trucks where the ecu sets the idle.

 

[knewsom]

Pretty much no matter HOW your idle is set, a bad ground can screw with things - on my 78 VW bus, I had a rough idle, and a miss that nobody could figure out - replacing the ground fixed it. On my D1, I had a similar idle issue as the OP, then failure to start, erratic behavior, and clicking relays... yup, bad ground cable. The guys at British Car Service in Inglewood were scratching their heads at first, then kicking themselves afterwards when I told them what the problem was. Most people don't even think about it, but ground cables go bad very easily - two different kinds of metal with electricity flowing through it constantly, it oxidizes and corrodes inside the connector sometimes, and it doesn't even look bad, but it'll screw with things. Good to replace it now and then.