Diagnosing Rough Idle Hell

[Schurkey]

No. It's the programmed idle speed that the computer is trying to achieve by adjusting the IAC.

So, for example, the computer wants 650 rpm, but the engine is actually running at 725; you can expect problems with the IAC, the IAC wire harness, a faulty ECM, or there's an air leak that's beyond the ability of the IAC to compensate for. You'd be looking at the IAC counts to see if the computer is trying to adjust the speed.

But if the computer wants 650, and the engine is actually running at 650, and the IAC counts are reasonable, it's likely that the whole speed-control system is Okee-Dokee.

 

[DylChrFla]

No. It's the programmed idle speed that the computer is trying to achieve by adjusting the IAC.

So, for example, the computer wants 650 rpm, but the engine is actually running at 725; you can expect problems with the IAC, the IAC wire harness, a faulty ECM, or there's an air leak that's beyond the ability of the IAC to compensate for. You'd be looking at the IAC counts to see if the computer is trying to adjust the speed.

But if the computer wants 650, and the engine is actually running at 650, and the IAC counts are reasonable, it's likely that the whole speed-control system is Okee-Dokee.

Thanks for the clarification. How could I figure out the commanded speed? I believe my IAC counts are high while sitting in park. Over 20 all of the time.

 

[Schurkey]

Some vehicles will display commanded idle speed on the scan tool. I don't know if yours is one of them.

 

[DerekTheGreat]

Mine doesn't for either our '92 or '89 trucks, I use the MT2500 brick. Commanded idle speed would be cool to see. Won't these also throw a code if they cannot reach commanded idle within 30 seconds? Thought I read something in a GM fuel infection book from 1989 or 1990 about that.

 

[PlayingWithTBI]

Won't these also throw a code if they cannot reach commanded idle within 30 seconds? Thought I read something in a GM fuel infection book from 1989 or 1990 about that.

I haven't seen any "Flags" in earlier calibrations (1227747, 8747, etc) for that. Maybe newer ones had it?

I believe my IAC counts are high while sitting in park. Over 20 all of the time.

You can adjust the base idle screw in a little to get your IAC counts down closer to 10. Just don't go too far where your idle goes up and the IAC counts go down to zero. YMMV

 

[DylChrFla]

I haven't seen any "Flags" in earlier calibrations (1227747, 8747, etc) for that. Maybe newer ones had it?

You can adjust the base idle screw in a little to get your IAC counts down closer to 10. Just don't go too far where your idle goes up and the IAC counts go down to zero. YMMV

So I need to tighten my screw? Would that adversely affect my idle speed?

 

[PlayingWithTBI]

So I need to tighten my screw? Would that adversely affect my idle speed?

There's a fine line. If done right, the IAC will close accordingly to maintain your idle. What's happening is your throttle blades are too far closed so, the IAC is allowing more air through than it should have to.

Which, BTW, may cause a rough idle with the fuel puddling on the throttle blades and then dripping into the intake manifold instead of staying atomized and flowing smoothly, evenly into the combustion chamber.

 

[DylChrFla]

There's a fine line. If done right, the IAC will close accordingly to maintain your idle. What's happening is your throttle blades are too far closed so, the IAC is allowing more air through than it should have to.

Which, BTW, may cause a rough idle with the fuel puddling on the throttle blades and then dripping into the intake manifold instead of staying atomized and flowing smoothly, evenly into the combustion chamber.

Okay, I will try to mess with it tonight if I get the time. Thanks again.

 

[redfishsc]

If you're blowing smoke when it's stumbling sometimes you may want to pull compression numbers. You may have some worn or stuck rings.

The other thing I'd add is to check everywhere for vacuum leaks while she's hot. I've had other vehicles have a lousy rough idle when hot b/c expansion of hoses, gaskets, etc.

 

[DylChrFla]

I messed with the IAC for an hour or so and had no luck. Went through the reset minimum idle procedure a few times. I found that my truck was having a lot of trouble idling below 600 rpm with the IAC unplugged. No matter where I set the screw, it would not idle lower than 850ish rpm with everything plugged back in. I also checked my rotor and firing order just to make sure.