Unique Idle Problem...

At idle, in DRIVE only, RPMs go up about 200-250 for about 6 seconds, EVERY 6 seconds. Its pretty strange. It will idle at about 600 RPMs (normal), then I'll hear a "tick" then the RPMs jump to around 800-850 RPMs for about 6 seconds. Then it will go back down to 600 RPMs again for about 6 seconds, then the "tick", then back up to 800-850 RPMs again for 6 seconds. Over and over again. Its almost like when the AC is on and the cooling fan kicks on and off causing the RPMs to change, but the AC is not on when this is happening. In park, it stays at a constant 700 RPMs.

I know its not that big of a deal but WTF is causing this? Its almost annoying. Has anyone seen this before? I tried searching this forum for this particular issue with no luck. I've got a 00' with a 4.0, 159K miles, and it runs fairly cool (185ish-200).

Thanks in advance.

Do you have the heater on defrost? Not sure about xj's, but most vehicles since the 90's use the ac when on defrost. It drives me crazy.

Pchitti said:

Do you have the heater on defrost? Not sure about xj's, but most vehicles since the 90's use the ac when on defrost. It drives me crazy.

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No I don't have anything on. The knob that controls heat/AC has stayed off because its winter time here in Vegas, meaning its a nice 65ish degrees out side. At night, when the temp does drop a little, I'll turn the heat up but never on defrost.

Pop the hood and check to see if the A/C compressor is kicking or out in when the idle jumps.

sounds like AC cycling, disconnect compressor and see if it stops

cruise control maybe

evap control maybe

ABS?

ehall said:

sounds like AC cycling, disconnect compressor and see if it stops

cruise control maybe

evap control maybe

ABS?

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Feels like AC cycling, but how? The AC is completely turned off when this is happening. Also, could you elaborate on the cruise control/evap control as possibilities. Thanks for the suggestions.

Just because its off at the dashboard doesn't mean its disabled. It could be grounding out somewhere.

Cruise control has a throttle actuator cable that pulls on the throttle (actually I'm not even sure about your year). The actuator could be cycling for unknown reasons. Its unlikely but its a possibility.

Just throwing out things to check

Having worked on several 2000-2001 XJs, it sure sound like the A/C cycling rapidly on and off.
As previously suggested, pop the hood and watch the A/C compressor, for engagement, and anything else that is moving, such as the throttle, when the engine is running and in gear.
You are asking for our help but evidently not willing to spend five minutes out in driveway, checking things under the hood.
Get out there and take a look before reporting back to us. We need information to help you.
You can see when the A/C compressor engages/disengages and the same with the cruise control connection moving the throttle lever.

I'd also suggest trying to isolate the "tick" noise. It's probably a relay and probably associated with the AC system. Even if the AC is not on, and the AC compressor is not cycling, the engine management computer might think the AC compressor is cycling. In this case, if I'm not mistaken, it boosts the engine RPM to compensate for the compressor load.

I havent been ignoring the suggestions, my battery died yesterday morning and I havent gotten it swapped/Jeep started since. Ill look at it tonight when I get off work.

I have this exact problem. It's the ac unit for sure, but if I turn off the heater it goes away. (sucks when in 4low and I need my heat). What did you mean about it grounding out? Would it be in the controls? also my fuel mileage has gone down super low lately, like a 1/4 tank on 20 miles(or two round trips to work,5.1 miles each way) could that also affect that?

Defrost setting will run the A/C for dehumidifier effect

Heat (floor) and Vent settings should not cause the A/C to run

Ok, so I'm an idiot. The idle dropping IS my AC compressor kicking on and off. I didn't realize that it does that every 6 seconds or so. Is that normal?

Yes its normal