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Idle keeps reving

I have a 90 nissan p/u and after the engine has been running a while,and I stop at a light,and the gear is in neutral,the rpm will go from 1000 to 2000 rpm every 2 seconds.When im driveing it’s ok,or when im stopped but have the ac on it’s ok.I have had it in at an auto electric shop and they can’t figure it out.They replaced sensors in the radiator that might be telling the computer that the engine is cold,and various other things.They tell me that it must be the computer,but they have no way of determing if the computer is bad or not.They said that if I bought a new computer for $600 and then found out it wasn’t the computer,that I wouldn’t be able to return the computer,and I would still have the problem.So what can I do?

I sounds like a vacuum leak. Considering the truck is 20 years old, have you replaced the vacuum lines yet? This would be a great place to start, and cheap too.

Does the car not rev high in drive? If not,stay in drive. Same thing with the A/C.

Did that,no vacume leaks

Vacuum leak? EGR valve stuck open?

A 1990 Nissan might have an idle adjustment. You can try turning it up. If a small tweak to the idle speed makes the problem go away, you can probably put the computer at the bottom of your list of likely problems.

the truck has a 5 speed manual transmission.When sitting at idle it revs.When the ac comes on it levels out,but when it kicks off it starts reving.It has been doing this for 8 years now.Doesn’t bother me much in the summer because the ac is allways on,but in winter it is annoying.I don’t suppose it will hurt anything because I always take the truck out of gear at a stop light and foot off the clutch.