My car just recently got a new transmission (used).  Now and then, the
rpm gauge revs up by itself (not upon acceleration or switching
gears). It could just rev up by itself as the car is just "coasting"
without a foot on the gas.  Does anybody know what this problem would
be?  I have a 90 day warranty on the transmission itself, but it may
not be related to that.  Please advise.

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Request for Question Clarification by nenna-ga on 24 Aug 2005 09:57 PDT

Hello smcclafferty-ga,

When the gauge fluctuates, does the car also rev up, or is the gauge
itself just showing it's up with no change in the automobile itself?

Nenna-GA

Do you mean that the gauge just revs without the motor reving up?
Or does the motor rev up by itself, and the gauge shows this?

It sounds like an electrical problem
- I had a rev counter that behaved peculiarly after some swine ripped out the radio

I mean the motor revs up by itself, and the gauge shows this....thank you!

There is something wrong with the engine control system

I had a 1987 Jeep Cherokee that did this exact thing.  Caused an
accident at one point, because I was on ice and the darn thing chose
that moment to rev up on me.  It turned out to be a common problem
with the mid-80's Jeeps -- something about a throttle sensor being
bad.  But it took FOREVER to diagnose, because nobody I took it to at
first was familiar with this problem.  Once I took it to a Jeep
dealer, they fixed it right up.

Thank you so far for all of your comments and suggestions.  I will
refer these possibilities to my mechanic.

It sounds as though your problem is related to the transmission.
Automatic transmissions are prone to "slipping" when the friction
surfaces are worn; momentary loss of friction causes the engine to rev
briefly (without increased accelerator pressure), and then return to
"normal" rpm when friction is regained. Since you installed a used
(not rebuilt) transmission in your car, this seems to be the most
likely explanation; I'd advise you to have the transmission rebuilt or
replaced before the warranty expires, as such problems only get worse.

Thanks for the information...I do have a warranty, but it is only 90
days, and it's been about 2 weeks already, so I will have to take it
back.  Thanks again for the help!

Yes, the car actually revs up, not just the gauge...thanks!

nevermind all the "sciency" stuff posted , this is clearly a case of
demonic possession. Easily enough gotten rid of with a grass hula
skirt and a hand full of chicken bones.

I have had about the same problem, I have a 97 pontiac grand prix gt.
I coast down some of the hills out here in NC so I notice the rpm
guage go from 1 to 1.5 to 2 but it also acclerates when that happens
so the transmission is engaged. What thoughts on this?
Thanks,
Bob