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2001 XJ stumbling idle, only in gear

F

Fred85

NAXJA Forum User
Location
Orange, VA
I wouldn't make another rough idle thread, but none of them pertain to the latest models........so here it goes:

My jeep has a rough idle, but only while in gear and holding the brakes. In neutral or park it idles wonderfully and without a hiccup, which steers me away from thinking it's vacuum leak related. This has started within perhaps the last 10k miles, so it makes sense to me that something needs to be replaced....but i don't know what all affects the idle.

I just took off the throttle body assembly and cleaned it very well, especially around the IAC ports, but this had no effect at all on the rough idle. Despite this, the motor runs great and without any other issues. Sensors and wiring are factory original, with 102k miles on them.

Help?
 
its not misfiring, the plugs are relatively new NGK's with around 15k on them i think

though, they're cheap enough i suppose i could go ahead and replace them
 
Well, you have a vacuum operated power booster that is employed while your foot is on the brake pedal. There could be a connection.

Is the rough idle there, under the same conditions, when the engine is cold? Does the rough idle only show up, under the same conditions, after a hot restart?
 
Don't replace the plugs blindly, pull one or two random ones and see if they are all gunked up or worn out. If they're fine just put em back in.

Joe's got a good point with the brake booster - pop the hood and have a helper stand on the brakes while you listen, if you hear a hissing or gurgling noise from the booster it's probably got a tear in the diaphragm.
 
I've always wondered about the lifespan of a brake booster. Mine whooshes (technical term) when i apply the brakes, which i understand to be normal???

What else would be worth checking? I'm driving around a 2004 4.0 Wrangler right now and the idle is considerably smoother than my XJ, so i know these motors are capable of a smooth idle....
 
i felt the need to bump this thread.....

i also wanted to add that i have a vibration at idle that shakes the entire car, in addition to the low idle. If i give it a little gas the idle is super smooth and the vibration goes away. Also, these problems are only while holding the brake in drive...if i'm idling and slowly moving forward the problems don't exist......

this is weird....
 
Yeah, that sounds like my XJ back before I replaced cap/rotor/wires/plugs. Since you've got an 00+ I'd bet plugs since cap/rotor/wires is replaced with coil rail, and plugs are cheaper to try first. The fact that it started 5k after you replaced the plugs sorta makes me suspicious. Are you sure the coil rail is seated fully and grounded properly? I have heard tell that it grounds through some of the bolts holding it down, maybe those connections are a little dirty.
 
Fred... sound exactly like the problem I am having except for it still has a low idle and vibration with foot off the brake.

I have another thread on this just haven't updated yet.

Installing NGK V-power plugs tomorrow to see what that will do.
 
If i brake torque, it stops.....it's only when fully warmed up, foot on brake, and idling

if it was misfiring, wouldn't that be a surefire thrown code? The only experience I've had with a motor misfiring was with the heat soak problem a couple years ago, and that was way more violent than what's going on now...
 
Hook up a vacuum gauge, observe normal idle vacuum.

While observing gauge, depress the brake pedal.

What happened?
 
Assuming a rhythmic, almost vehicle shaking miss it's likely an injector or perhaps compression problem. Try this, have someone hold the brake when it's acting up. Go around back and hold your hand just behind the exhaust. If you feel individual pulses hitting your hand that's a misfire, not just a rough idle.

If it doesn't get worse under light load the ignition system is fine, it would act up most at light to moderate throttle.

OBD2 systems are supposed to throw codes for misfires, absolutely, but it doesn't always happen.
 
I am having the same issue with my XJ (2001). It is sparratic though when it idles low. I tried replacing the PCV and it temporarily solved it. a couple weeks later it started again. Today, it started doing it from the start up. It normally idles around 800 rpm and when in gear it goes down to 400 rpm. The brakes aren't always on when it is happening. I do the torque brake thing and its fine. I have also noticed there is a slight hesitation when you are coming off the low rpm after you step on the gas.
 
I tried something yesterday that has so far fixed the issue. I readjusted the tranny kickdown on the throttle. It's still early to see if it has worked, but so far so good. I will probably go through vacuum lines this weekend just to be sure.
 
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