I have a 1986 S-10 Blazer that has an erratic idle. It gets so bad that you have to pretty much stand on the brakes to keep it stopped at an intersection. I started looking at possible problems and stumbled across one that baffles me. When it's in park or neutral, everything is fine. Once it's in reverse or drive it surges to a real high idle. The weird thing is that when it's in neutral, I can ease the shifter each way without letting it engage into a gear and the idle raises and lowers. Is the neutral safety switch somehow tied into the idle circuitry? I need some help here, what else can I check?

 

The idle speed is controlled by the ECU via the IAC sensor in the side of the throttle body, but it is only controlled when in park, neutral, or the throttle is closed. What may be happening in your case is that the TPS sensor could be not returning to what the ECU is expecting for closed throttle and causing the condition.

 

Maybe try cleaning the throttle body/TPS.

 

it's definitely the TPS or too much Carbon in the throttle body