Rev up & down

96 Acura with a weird habit. On cold mornings, after a few minutes it starts this thing where it revs up about 500 or so, then back down. It does this while parked or when driving. When driving, it still revs up, but also revs down - like it’s putting in extra resistance or something; have to give it extra gas when it does that. This usually stops once it warms up. Also, it runs a fair bit higher until warmup, then settles back down to ‘normal.’

Does this sound familiar to anyone?

Yes. There’s something wrong with the idle control system. I’m not sure how that system works on your car, but on many cars of the mid-90’s, they tended to have a separate gadget called an idle air control valve, which did basically what you do when you press on the gas, only it does it either by computer control or just by coolant temperature. These tend to gunk up over time, and need to be cleaned so they work properly.

On some cars of that era there’s also an idle air bleed screw and if that screw gets adjusted out of its proper range, like someone used it to correct for a vacuum leak say, then that confused the engine computer and this symptom can be a result.

There’s other possible causes, but ask your mechanic to check those first.

The two most likely causes–IMHO–are a vacuum leak or a gunked-up IAC valve.

I agree with George and VDC here.