I have a 99 Malibu LS that the idle seems to slip when I'm sitting still. It idles at about 750 and every few seconds, it will drop to below 500 and the headlights will dim and then it goes right back to 750. Every so often when it does this, I can hear the fans or something kick in under the hood. I don't know if this has anything to do with it or not. The check engine light has never come on so I can't get a code but it's starting to annoy me. It doesn't seem to matter whether the car is warm or cold, in park or drive, or the radio is on or off or anything else. Can someone please help me figure out what's wrong? Thanks in advance.

J

Is it possible that the fans coming on may be causing the sag? If so, what if there is something switching them on as often as you say. It is not uncommon for a high amperage surge to pull the idle down, such as the fans coming on.

Not every time that it does it do the fans come on. And when the car is cold why would the fans come on anyway? If it is the fans causing it, could it be the switch or relay going bad or what else could be causing the fans to be going on for a few seconds, shutting off and then coming back on?

J

I had a similar experience with my '98, only difference being that the tach would dive and then surge a bit above normal.

The alternator was in its last days. Fortunately, I figured it out before it stranded me on the side of the road.

Hey thanks for the reply. I'll get the alt. tested asap and see if that may be the culprit.

J

My thought is that something is wrong that is making the fans come on. The fans are controlled by the computer, which may be thinking the engine is suddenly hot(bad sending unit) or the AC compressor clutch just applied(bad signal from HVAC head or other). My first step would be to watch the PCM data when the lag occurs, whatever is happening should appear in the data.
Otherwise, you may have a power source intermittently cutting out.
Hard to diagnose from a distance, but everything is fixable eventually.