My car, just recently, started having a problem with idling. Every couple of minutes, when I come to a stop at a light, or in traffic, or whatever, my car's revs drop below 1000 (which is what they normally are) and can even drop as low as 500. I'm sure other people have already asked this question before, but I haven't been able to find any posts about it. Thanks.

 

It should idle at more like 750-800 rpm.

Flaky idle can be caused by a number of things...
Air in the cooling system (really!)
Throttle cable too tight - it MUST go slack.
Dirty throttlebody bore & throttle plate.

Some more specific stuff depends on V-6 vs 4-cyl engine? You gotta tell us what you have.

 

Oh. Sorry. It's a 4 cylinder. For all the years I've had it, it generally idled around 1000-1250. All of a sudden, it just started acting differently when talking about the idle speed.

 

There's an IACV (Idle-Air Control Valve) on the back side of the intake manifold. Look in the DIY forum for a thread about idle issues, with some nice pictures from someone who took it apart & cleaned it.

Then there's an idle-air adjustment screw (little needle valve) just above the throttle bore. After everything else is cleaned up & checked out, here's how to set it.

1) Get the engine all warmed up completely. Turn off EVERYTHING. Ventilation, headlights, AC, defogger, EVERYTHING turned off.

2) Unplug the wire from the IACV. This should make it almost stall. You might have to nurse the throttle to keep it running.

3) Adjust the needle valve for 550 RPM or so. If the radiator fan comes on, keep the engine running, wait till the fan shuts back off, & finish adjusting while the fan is off.

4) Turn off the engine, plug the IACV back in, let it cool down.

5) Reset the ECU by pulling fuse #13 in the right-end of the dashboard - near the right-front door hinges. After 15 seconds or so put the fuse back in.

6) Start the engine WITHOUT TOUCHING the gas pedal.

7) Let it idle until it completely warms up. DON'T TOUCH the gas pedal.

DONE.

 

Wow. Thanks. But just a quick question, what will that do?

 

That forces the ECU to re-learn the behavior of the idle-control system. It's a good idea to do that procedure if you've done much cleaning / fixing / whatever, in the areas that affect idle.

If you've had a vacuum leak (for example) for awhile, the ECU will try to compensate. So when you fix the leak you want to erase all that from its memory.

If some misguided mechanic has used the throttle cable to adjust the idle RPM, that can get the system all confused. Just another example of junk you want to erase from its memory.

 

Ok. I get it. There is one more thing I see wrong (or at least different). My revs are now much more sensitive. Like when i take my foot off the gas, the revs will hover for about 1/2 second, then quickly drops to like 1000.

 

That's probably part of the idle control system too. But it might not go away. Tremendous amount of emmissions happen right when you suddenly close the throttle, so some of that might be programmed in (to meet EPA regulations).

Back in the carburator days that was done with a dashpot installed on the throttle shaft. Now it's probably programmed into the actions of the IACV.

 

Ok. Thanks everybody for the help.