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jeep revs up then down?? ideas??


NAXJA Forum User
i have a 96 cherokee , 4.0 aw4 , banks headers are the only mod , 180,000 kilometers on the engine. what happens is the engine will rev to like 1400 rpm and stay for a while and the drop back down to normal , its totally random , some days it doesent even do it . anyone have a similar problem?? it sounds like a vacume leak but everything seems to be workin good in that department

thx carl
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hmm my 88 did that until it turned into constant hunting once warm. it happened over some time. I went through everything I could think of for a while even throwing money at new sensors. well I guess I missed one cause it was the CPS that was my problem. I know that they are different ECU's but how long has it been since the CPS was replaced on yours? They do demagnatize over time.
ok if and when you do go for one yours make sure that they do not give you the one for 87-90 it looks similar but is a different sensor. Oh just to clarify I am speaking of the Crankshaft Posistion Sensor located on the back of the bellhousing opposite side of the engine. I am sorry but I do not have an available test as the type in my rigs have two wires with a resistance measurement.
Hallo Jeeper.
Not only the CPS is important, but the TPS (Throttle Position Sensor) is also needed for a proper shifting.
Check the wire from the sensor to the firewall for a good ground. :wave1:
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I had a similar problem and it was the o2 sensor. It had a bad dead spot in the pedal when it was cold till it warmed up. Also my gas mileage was down 1-2 mpg, and also it had an oscillating idle like yours.
My second guess would be the tps, I honestly bet that it is most likely not the cps.
Look closer for a vacuum leak. Especially if these symptoms happen only at idle, when vacuum problems are most evident. Take a can of carb spray or a spray bottle of water and lightly mist around every hose and throttle body, etc and see if that changes your idle speed/quality. If you have a check engine light, read the codes.

This does not sound like a CPS in my opinion. When the crank sensor goes, you lose that base reference signal to the computer which results in a miss or the engine dying, not a fluctuating idle.