I have a 95 GMC Jimmy 4.3 that has a random high idle. It will idle fine at about 700 rpm in drive, but randomly the idle will start bouncing up and down between 700 and 900 rpm and then rev up to about 1200 rpm and stay there. I’ve had the upper plenum off checking for vacuum and fuel leaks, all look good. I was able to get a scanner to see data on the OBD 1.5 and when the idle is okay the tps % is 0-0.4%. When it’s high it’s 3-7%. I threw a new tps on it and the car was fine for about a day and a half and now it’s doing the same thing. Anyone have any ideas? Should I try another tps?

 

Since the new TPS did not solve the problem it’s time to check the quality of the 5v ref and lo ref feeds as well as the tps output wire back to the computer. As a first check does the tps value jump around with key on engine off when you move the cable and connector around? Then measure the net 5v ref level with the meter leads back probed into the connector on the gray and black wires also while moving everything around. If all that is ok, we need to check the dk blue signal wire back to the computer. Which engine VIN do you have (8th digit)?

A guy who can read live data in an OBD 1.5 engine, holy crap. You should buy a lottery ticket. What scanner are you using?


George

 

I am using an Actron CP9690 Elite scanner. I had a tech II clone, but it crapped out on me. Vin is W code. I’ll check the voltages later today. TPS % seemed fine with engine off key on. 0-100% TPS % operates smoothly when adding throttle. I’ll try messing with the connector more.

 

Reference voltage is 4.98v across grey and black wire with key on engine off. I unplugged the TPS sensor wire about 6 times while watching the scanner. The TPS percentage would jump up about .5% every consecutive time I plugged it in. I got it up to about 2% with no throttle input, blipped the throttle to 100% which showed on the scanner, released the throttle and it stayed at 2%. I got it back to zero by shutting the key off for about a minute. Could I be losing continuity in the connector? I may try to take it apart and spread the tabs to make better contact. Stupid question, where does the blue wire go into the pcm. It’s hard to see the wire color with 27 years of dirt and grease on them. lol

 

Plugging and unplugging the connector may confuse the experiment because of the PCM recovering from the mini load dump if you will. I would stick with moving things around but sure, getting the contacts clean and tight to start is a good idea. Did you monitor the 5v reference voltage across gray and black while the idle rpm/tps value floats around? We need to rule out a wonky reference voltage, which can be from wiring, the pcm or another sensor playing havoc on that shared ref line. If that is stable then it’s on to that dk blue signal wire. It lands on A15 at the PCM. You can unplug the tps, measure resistance on that wire while moving things around, truck completely off.


George