I have a 1990 Maxima... for the most part it runs like a champ...the other day the RPM went up to 2500 while is was going 65 in fifth gear (5 speed)... my trusted mechanic found a leak in the air pump, replaced that and changed the air filter... he checked the car out on the smog equipment and everything came back well within the "normal" range.
what else can it be?
Without knowing a little bit more about it, it is difficult to say. Did it only happen that one time? If everything is checking out okay and it hasn*t happened again, it is possible that resolving the air leak took care of it. Intermittent problems are the hardest to track down because if it is not doing it when the mechanic is looking at it, there is often no way to find out what happened.
I would recommend monitoring it for awhile and doing a bit of investigative work; does it happen when the engine is hot/cold?, is it only at higher speeds or at an idle as well?, does giving it more gas or tapping the brake bring rpm back down?, how long does it stay up at the higher RPM?, etc.
The more specifics you have for the technician, the better their chances are of finding the problem (if there still is one). If the problem starts being more consistent, let them keep the car for a day or two and encourage them to drive it till they can experience it for themselves.
〉 Answered on Feb 9th, 2009 by Bogi Lateiner, Owner and Technician at 180 Degrees Automotive