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Matt Hardy
Matt Hardy, ASE Certified Technician
Category: Jeep
Satisfied Customers: 920
Experience:  ASE Master Auto Tech, ASE L1, Jeep/Chrysler Dealer Tech, 18 yrs exp
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my engine revs up and down when the heater is on. It only does

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my engine revs up and down when the heater is on. It only does it when the heat or defrost is on. help

Hello and welcome to JA.

How much is it much does the RPM change?

Does your Jeep have A/C?

Customer: replied 7 years ago.

When you turn on the heat or defrost the engine revs to about 2000 rpms. If it is in gear driving it feels like cruise control is pushing the gas. If it is in neutral, or the clutch pushed in it revs up to about 2000 rpms for a couple of seconds then falls back down to below 1000 where it sounds normal. If the heat is off it is ok, it stays around 1000 rpms.


Does your Jeep have A/C?

If so, does it work properly? Does it make cold air?

Customer: replied 7 years ago.
yes it does have ac and it works well, but it too causes the engine to rev up. the only time it does not rev is when the heat or ac is not on.

For now, I am going to re-open the question so the other experts can view it.

Customer: replied 7 years ago.

Is the check engine light on?

Have you had the fault codes checked? (Most auto parts stores will do this for free.)

Have any recent repairs been performed, that might be related to the idle?


Here is something to check:

Inspect the large wiring harness that runs next to the silver A/C accumulator/drier, which is located near the battery. I have seen several cases where the wiring rubs through on the edge of the drier, causing a short-circuit in the wiring.


See picture below for more detail:



Edited by Matt Hardy on 1/20/2010 at 12:49 AM EST
Customer: replied 7 years ago.
the check engine light is not on, we have had no recent repairs, i will have the codes checked tomorrow. the harness you show in the picture looks great, no wear in the plastic around the wiring harness.

Sorry, I am running out of ideas on this very unusual problem. Having the fault codes scanned might shed some light...but finding the answer might require a scan tool to check the target idle and sensor readings.

I am re-opening the question...maybe another expert has an idea or has seen this exact problem before.

I will check back with you periodically.


Edited by Matt Hardy on 1/22/2010 at 12:56 AM EST
Matt Hardy, ASE Certified Technician
Category: Jeep
Satisfied Customers: 920
Experience: ASE Master Auto Tech, ASE L1, Jeep/Chrysler Dealer Tech, 18 yrs exp
Matt Hardy and 2 other Jeep Specialists are ready to help you

Hello my name is XXXXX XXXXX I will be more than happy to assist you.


From what it sounds like you have a vacuum leak.


Go to the check valve #2 under the hood, disconnect the vacuum line that goes though the fire wall. Turn the vehicle on and AC or heater on, the idle should be high, Block the check valve so there is no more vacuum leak. Does the engine idle correctly?


The check valve is #2 at the top of the picture. Not #2 at the bottom right.


Customer: replied 7 years ago.
That did it, when I took apart the hose it lost pressure, then I put it back together and drove it. It has not reved up since, maybe it has a clog or something. This may just fix it temporarily, but never the less it seems to be fixed for now. Thanks, XXXXX XXXXX

It is just fixed temporally. That was just a test to see if I was on the right track about a vacuum leak. you could have moved the hose just enough to seal the leak or the AC/Heater control head is now on a different setting. Try different setting on the AC/Heater Controller and see if the problem come back.


If it does then you have a vacuum line broke or loose under the dash and that is why it only does it with the AC/Heater on.