Engine Revs up every few seconds

1996 Sable wagon 130K miles, has run great and still does except every 5 seconds or so it revs up. Goes up about 2500 RPM and drops back down immediately.

doesn’t matter if moving or not, speed, gear, in park, hot or cold, whatever, it does it all the time. Otherwise everything seems normal. Has been doing it for about 6 weeks now, not changing in any way I can discern. Is this dangerous? Will it do further damage? I mean to get it looked at and fixed but money is really tight for a few months.

check engine light on?

Nope, no check engine light or any idiot light at all.

You’re saying it just blips up to 2500 as if you’d stabbed the throttle, then returns to normal idle?

Just taking a shot in the dark here, try driving without the defroster on (use Vent or the Heater-lower registers- mode) and see if it does it. I am wondering if its related to the AC system cycling.

Thanks but thats not it. Your right that it just blips up and right back down but it does it if its idling or going down the highway. Of course it’s less noticable the faster I’m going but I have tried watching the rpm’s while moving like 40 to 70 on the tollway while holding a steady speed and can usually see it happen though I cant really feel it at those speeds. Under 10 mph its very noticable of course and idling it lurches a bit. It happens whether the defroster is on or not, it happens whatever the heating setting, vent, auto, feet, whatever. I dont know that I have had the temp control off since it started though. Will have to see on the way home tonight.

Auto or manual transmission. Are you just feeling it or are you watching a tach? Does the car speed up or slow down?

I would pull the electrical connector on the IAC and see if it stops. I had a 97 that the battery connector cracked on. That randomly went up to 2500. With your fool lightly on the brake that is pretty scary to have the vehicle lurch forward!

Yep, most likely an IAC (idle air controller) problem, or TPS (throttle position sensor) problem. I suppose a faulty EGR transducer might also be the culprit.

Thanks to everyone!!! Didnt realize any heat setting other than only on the floor would have the same effect as the defroster. Yep, with the heat off or only on the floor it does not happen. I feel a little more comfortable now driving it a couple weeks until I can get it looked at. So, it sounds like worst case isnt so bad. I’ll check the battery connector in a bit. If thats not it I’ll get it in the shop. This isnt as funny as the show but you guys are great! I’ll have to start visiting now and then.

So with that in mind, I think that it IS the IAC on the throttle body.

Here’s what goes on: The air conditioning system has pressure sensors in it. When those pressure sensors signal that the A/C system needs more pressure, the compressor comes on. Since the compressor puts a fair load on the engine, the A/C system sends a signal back to the computer that the compressor is on. The computer, in turn, uses the IAC to raise the car’s idle.

I think your IAC valve is sticking. When it finally unsticks, it over-shoots and ends up idling too fast. That’s my theory, anyway.