89 S-10 Blazer Crazy RPM Problems

rublazn · December 6, 2005

Have 1989 S-10 Blazer 4.3 217K. I have been having trouble finding what is causing the variations in RPM while driving. I have checked the codes and nothing. Sometimes while driving the RPM will elevate to 2500 to 3500 once I come to a stop. The park rpm should be around 1000 but it's reveing on up. If I cut the truck off and leave it for a while and crank it up once the engine is cool again the truck runs fine. I have noticed really poor gas mileage also. I have changed the map sensor and several other ones but nothing seems to make a difference. Not sure what to try next.

GM-Tech · December 6, 2005

Possibly the idle air control valve, but without much information, that's just a wild guess.

rublazn · December 7, 2005

Possibly the idle air control valve, but without much information, that's just a wild guess.

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Is there any way to check that part. I have replaced that about a year or so ago. Should it send some codes or something.

GM-Tech · December 7, 2005

There are no codes for the IAC that I recall. Having a scanner that can display IAC counts would be the first step in seeing if this is what's wrong. If the idle is high and the counts are zero, that means the ECM is trying to extend the IAC fully to slow the idle down and if it's not, it is either a bad IAC or there's a wiring problem. If the counts are high, say 40 or 50 (rough guess), then the ECM is commanding the IAC to open which is causing the high idle. Now you need to find out why the ECM is commanding it open.

We have a little device we can plug into an IAC and operate it ourselves. With that we can see if we can change the idle up and down. That's not to say it can't stick at times and not operate.

So there's where you start.