I have a 1997 Toyota Camry with over 200,000 miles. It is a manual: 5 speed. In the past few months it has started to idle at around 2000 RPMS, which is way too high. I took the car into a mechanic where I am currently living (Austin, TX), and they couldn’t diagnose the problem, because, according to them, it didn’t do it the whole time they had it. I admit, it doesn’t do it all the time, but I know something is not right. It is beginning to idle high more frequently. I would love to have some insight as to how to diagnose this problem and to solve it!!
Is there any check engine light on?
If not, then the first thing I’d say to do with an idle problem is the clean the idle air control valve (IACV) and throttle body and check it for vacuum leaks. None of that is very time consuming or expensive and it’s surprising to me that a shop would not have suggested this and done it - with the caveat that it might not solve the problem. If you are at all handy, you could probably do this yourself.
I had this problem on my early 90’s Corolla recently. I checked for vacuum leaks, faulty power steering pump idle-up valve, egr valves, PCV on the fritz, intake manifold gasket leaks, etc, but it turned out to be the IACV was faulty, just like @cigroller is suggesting in your case. If your car sports a replaceable IACV, if that is what it turns out to be, should be a simple and fairly inexpensive repair.
There are other potential causes of course. The throttle valve is sticking, etc. Even if the problem turns out to be the IACV, I think it would be a good idea to ask to get the throttle body internals cleaned at the same time. The gunk in the throttle body could contribute to fouling the IACV possibly. And definitely make sure the PCV is working correctly while the mechanic is at it, as a non-working PCV can foul the throttle body sometimes.
I (or “we”) should have added a cleaning of the MAF sensor while the other stuff is being checked and cleaned. This is also easy and not very expensive.
If the gas pedal feels like its stuck when you first step on it, then you have a dirty throttle body. If the gas pedal works smoothly, then it is more likely the IAC or a vacuum leak.
You think it is random, but it may not be. Note everything when this happens. Car warmed up or cold, outside weather conditions (rain, temp, etc), HVAC controls (heat, AC on or off, defrost, vent position). Have you just come off the freeway or are you stuck in traffic. All these things and more are clues. You may find a common cause that way and it will go a long way to finding the source of this problem.