My 1990 Chevy S10 4.3 (TBI) has a running problem. The symptoms are as
follows: Shortly after startup (immediately to 3-4 minutes), the idle speed will suddenly climb to 1500-2000 rpm and stay there. This is usually accompanied by a check engine light (code 21 - TPS high voltage) and a return to normal idle speed. Sometimes the code will clear while driving and the high idle will return. At least once I have seen it clear and return to a normal idle with no check engine light. More recently, after a bit of driving with the light on, the motor will start to hesitate and want to stall, requiring some persuasion with the throttle to keep it from dying.
I have found that resetting the ECU will often clear this up for a short period of time. I followed the procedure outlined in Haynes to check the TPS output with the ignition on and engine off. It read 4.8 volts across A and B, way above the 1.0 volt maximum. The manual says to perform the base idle procedure before replacing the TPS, so I did. After doing this, the truck ran great for a spin around the neighborhood, but just as I turned back into the drive the idle kicked up again. I did the base idle procedure again and measured the TPS output before going for a drive: 0.8 volts - right where it should be. Another drive and this time it set code 21 and began to stall. The TPS
output was again around 4.8 volts after this drive.
One thing of note during the base idle procedure: the idle air valve makes an unhealthy winding noise while in operation. Furthermore, the Haynes manual made it sound like this valve should eventually extend and stop, which it did not seem to do even after waiting more than the recommended 30 seconds. Is this normal?
So, where to go from here? It seems like the IAC valve is a possibility. Can this thing be bench tested? I think the TPS is ok since it does output the correct voltage at times, and it looks brand new (guessing the PO installed it in response the the code 21). I also
suspect the ECU, as the PO said a shop had told him the truck needed one (at $1000, he declined). Are the $90 remanufactured ECUs I saw on napa online any good?
Any advice would be appreciated.
I ended up replacing the IAC valve, which fixed the problem described below. It appears that this part can fail in a manner that affects the TPS voltage seen by the ECU. Hope this helps somebody.
I was having the same thing going on it was a 89 S10 4.3 it was the TBI it was sucking in air at the bushings of the butterfly and throw verey thing off it would run great then all at once high idle then die then same times it would be really hard to start. Hope this might help. EARL