(Post Edited for addition of video)Hey guys. Love this forum. Always so informative and has saved me thousands. I've searched a lot of the threads about low-idle problems but they don't seem to fit mine.


Princess: 2001 XJ 4.0L


Problem: Low idle. 5-600 RPM under load. Increases to 750-900 with A/C turned on, even though A/c is not working properly (Not blowing cold air, aux fan does not kick on but works because when I unplug the coolant sensor, the fan kicks on.). Stays running but seems to bog way down at stop lights. Rides and accelerates fine. No check engine light.


Side notes. Replaced coil rail, plugs, IAC, TPS, thermostat.


Can't quite seem to chase down the low idle and it's starting to get expensive. Any ideas?


Thanks for the help


In drive at normal operating temp. Rumble is loose exhaust hanger. Rumble only happens at this low idle:
http://www.youtube.com/watch?v=w1-BL...ature=youtu.be


In park:
http://www.youtube.com/watch?v=UyIoJ...ature=youtu.be


Could it be resistance from a weakened water pump? I hear a howling noise from the water pump area. Sort of hard to hear with the sound on my camera.
Two different videos, both at normal temp:
http://www.youtube.com/watch?v=C2sCE...ature=youtu.be


Can hear the howl slightly better in this one:
http://www.youtube.com/watch?v=IeJcl...ature=youtu.be

I've got no business butting in on a 2000, but have you gotten the codes read? It's free at allot of parts stores.

Btw I believe on yours, the AC needs to be charged, reach some pressure to turn on the E-fan. Might should go on defrost and over 218* or so.

Might want to do a fuel pressure reading.
also cleaned out the throttle body/valve? And TPS, what brand (to tick that box)?

One of those ****ty TPS brands from Autozone. I may have answered my own question on that part

You might just have
Well, you can try the other things first, before you swap the TPS again...

I forgot to mention it isn't present at start-up but only 5 minutes into driving at normal operating temp

I still have the 331 head, if that helps anyone

No, the head is not making a difference regarding this issue. Although your '01 different coil rail setup etc might.

How did you go so far with the things from post #2 and #3?

Thanks. Still have to check fuel pressure. The store nearest me doesn't have a check gauge so I have to find time to hit another store. Will keep you posted

No one mentioned the Idle Air Control Valve? That little gismo (technical term?) on the IACV needs to be clean and move freely otherwise your idle will fluctuate randomly.
Takes 15 mins to do a complete R&R.

I mentioned in my opening. Cleaned it. No fix, so I replaced it. I read on here that it's the cause of a bunch of problems so $60 for a nice one wasn't worth diagnosing the old one's usefulness. Never been changed in the 18 months I've owned it so I figured what the hell.


I'm going to replace the serpentine, check fuel pressure and take a look at the clutch pulley on the a/c. I noticed last night that while accelerating, the illumination on my radio pulsed every so slightly. so I think something isn't getting the power it's supposed to. I know I must be over-looking the obvious here somewhere..

Pulsing in line with your rpm's?

I can't tell because I'm accelerating but most likely.

Sorry, I missed that. Any slack in the throttle cable? Looks like you've checked/replaced most of the parts that could cause low idle.

Actually, was looking at that yesterday. It was a little loose. Will research how to adjust it on here and keep you posted