RPM's dropping on the highway.

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It first happened on Friday. I was on the highway and the RPMs dropped to what looked like zero. It came back for a few seconds then did it again. I did some tinkering that night and saw the negative battery cable was loose so I tightened it up. The weekend through Monday went by worry free then it did it again today on my way home. 2 times a few seconds apart at highways speeds. The RPMs just dropped.. no shifting in the transmission. It was fine the whole way home after that.

What do you guys think it is? I would have to guess something electrical. IAC, TPS?Maybe O2 sensors or a clogged cat?
 
tach/sensor going bad?

does it sound like the rpm went to zero(hint they cant in gear)
 
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It could be a speed sensor. My old 95 Accord had the tachometer jumping around and the speed sensor (transmission) was bad.
 
I'm guessing you don't have a check engine light. I don't think the IAC would cause that, or the O2 sensor on a 4.0L.

I've heard of another Jeep doing this same thing, I remember reading about it on one of the Jeep forums, I forget the part that caused the problem though. You might want to do a search over there.

It might be worth testing your fuel pressure (there is a port on the fuel rail) and go for a drive and see what the gauge says. It might be a fuel pump that is on its way out or even a crank position sensor. The TPS is a possibility, but from my experience those seem to cause shifting and transmission issues when they go out, not engine stalling.
 
No check engine light sadly. It isn't throwing any codes according to the dash either.
 
How old is your battery? Jeeps are pretty sensitive to battery voltage. My dad's '99 WJ had a dead cell in the battery and it did all kinds of crazy things.

I'll see what threads I can dig up tomorrow on this issue. I know I've seen it somewhere.
 
Even without a check engine light the OP may have a
pending code stored, get the car scanned to check.

Sounds like a sensor perhaps a knock sensor retarding ignition advance for some reason.
 
The 4.0's don't have a knock sensor. Almost sounds like the CPS (Crank Position Sensor) starting to go on your Jeep. It's a very common failure on the 4.0's. Have you checked for any stored codes ?
You can do so if you don't have a code scanner by doing the key trick. It'll show any stored code in the odometer screen.

Here's how:

Retrieving codes:

Turn the ignition key until the digital odometer displays, repeat three times in succession and then back to "On" (On/Off, On/Off, On/Off, On). At the fourth "On" the odometer will be replaced with codes. If no codes are present then "Done" will appear.
 
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If the tach is dropping to zero then it doesn't see the actual engine speed. The engine has to be turning if you're still in gear. It's very likely a crank position sensor or associated wiring.
On an older car it could be the ignition system, but on newer cars I believe the tach signal comes from the crank sensor.
The fact that you are also losing power means it's not just the gauge. Without knowing the RPM the ECM can't run the engine properly.
 
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Start simple. Make sure a steady electrical supply to the engine computer. It could be a poor ground or bad ignitions switch(see if you can replicate by fiddling with it).

I ran into this on my VW Jetta the tip off for me were the normal dash lights were not illuminating when it happened. It turned out to be a broken ground.

If your check engine light is not coming on it may be a tip off the ECU has no power also.
 
The battery is less than a year old. I did the ignition on off thing last weekend and found nothing. Would an actual code scanner be able to find something possibly?

Thanks for all the help guys. Much appreciated.
 
Originally Posted By: armos
If the tach is dropping to zero then it doesn't see the actual engine speed. The engine has to be turning if you're still in gear.


Maybe on a car with a manual trans, but not on an automatic.
 
Originally Posted By: dlundblad
The battery is less than a year old. I did the ignition on off thing last weekend and found nothing. Would an actual code scanner be able to find something possibly?

Thanks for all the help guys. Much appreciated.


The only scanner that may help is a dealer level scan tool that shows live data from all the sensors. If the sensor isn't acting abnormal enough to trip a code, then it won't show up as a problem on a normal scanner. With the higher level scan tool that monitors live data, you can watch for changes in the data while driving, and see what happens when it briefly stalls.
 
Originally Posted By: jeepman3071
Originally Posted By: dlundblad
The battery is less than a year old. I did the ignition on off thing last weekend and found nothing. Would an actual code scanner be able to find something possibly?

Thanks for all the help guys. Much appreciated.


The only scanner that may help is a dealer level scan tool that shows live data from all the sensors. If the sensor isn't acting abnormal enough to trip a code, then it won't show up as a problem on a normal scanner. With the higher level scan tool that monitors live data, you can watch for changes in the data while driving, and see what happens when it briefly stalls.


And that's the issue with issue.. (or whatever..lol) It is very intermittent. I can just see taking it to a dealer and them not finding anything with a $100 bill for their time. Lol. But if it does look for live data, maybe it will find the issue even though it isn't acting up. Maybe it could sense what was wrong? Im new to codes etc. Lol

** Is it safe to drive like this? Like is the sudden RPM drop and back to life going to kill my transmission or clutch?
 
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