Slightly rough idle.

I've noticed that in the past few weeks my idle seems to be slightly rough. I can't really see any change in the tach, but I can feel a slight stumble every 30 seconds.

Here is a little background info:

- My xj has a 133k miles on it, had the spark plugs changed back around 110k or something like that at the jeep dealership(I know I should know exactly when but its my fiance's jeep - I always keep my receipts).
- I had my catalytic convertor stolen at 128k miles and had to drive it home about 30 miles but they didn't damage the 02 sensors when they stole it.
- I had the cyclinder head replaced at 130k miles with a new head from clearwater cylinderhead (but my mechanic didn't change the plugs, or he didnt charge me for them). About 200 miles later I had a valve spring fail and had to drive it home about 5 miles. Got the spring replaced and didn't notice any issues with the idle.
- Been driving it since then and put around 2k on the jeep. The only other thing I can think of is the pigtail for the 02 sensor that is after the muffler (towards exhaust exit) came off it's clip and was banging around. I got under it that night when it happened and just ziptied it back. Didn't look like the wires were damaged but the black mesh sleeve was ripped apart.

Any suggestions on where I should start?

Check your vacuum lines maybe? A vacuum line being disconnected can really play hell with the system because the air going into the engine isn't all being controlled by the IAC (speaking of which, clean your TB, IAC valve seat, and IAC valve plunger) and measured by the MAT/MAP sensor(s), so perhaps you have something like that going on. When I seafoamed my engine it nearly stalled out a few times because I was using the brake booster vacuum line to suck up the seafoam and it really, really did not like the extra air inlet. You can trace down cracks in intake manifolds / vacuum leaks by using a propane blowtorch turned on (but not ignited) and running it along the vacuum lines. When the idle suddenly changes, it's because the propane is being sucked in and burned and changing things up.

what year? small differences in the years does make a difference.

check the color and condition of all the spark plugs along with the cap terminals, rotor, and plug wires(unless you have an 00-01). clean the IAC and throttle body and check for a gasket leak where it mates to the intake. retorque the intake and exhaust manifold bolts(should be done after a new head is installed).

if everything checks fine then i would run a diagnostic/testing all the sensors.

rough idle could be as simple as bad gas or a dirty injector where a good fuel system cleaning takes care of the problem.

sorry I thought I had my signature up. will fix that. It's a 2000. I'm wondering if it's an intake leak since I'm getting this noise that sounds like when you blow over the top of a beer bottle but much higher pitched when I'm on the gas. it goes away but I can't tell if its because the noise of the engine or what.

I was thinking of changing out the spark plugs on friday but pretty nervous to do the seafoam thing. It's our daily driver (and only mode of transportation) and I really cant afford to f' it up. Thanks for the help so far!

a new head should not require a seafoam(top end cleaning) treatment unless debrie was in the intake and got logded into a valve seat. btw, seafoam or top end treatment is not hard to do.

vacumn leaks are most noticeable at idle then under load. check for a loose, cracked, or fallen off vacumn line. check where the throttle body mates to the intake for leaks.

so I took my jeep to my buddy's landscape shop ( he wasnt there) and worked n it last night. Replaced the blower resistor motor, rear tailgate support shocks, wired up some new Hella 550s, changed the spark plugs and put a new 02 sensor in. First time ever changing spark plugs but I'm glad I took the chance and did them. Bought a nice micrometer torque wrench and went at it. Jeep runs much better, no rough idle.

The old spark plugs looked allright, the white base was kinda black and there was a little greenish on the metal tip.. what does that mean?

they must be irish plugs

Emerscape, you change the cap, rotor, and plug wires as well? My idle was really rough before I did that plus the spark plugs.

EDIT: also - if you were over recommended mileage on those plugs, I'd say that's pretty normal condition. At the worst, it sounds like you have a clogged air filter, fuel delivery problems, or an ignition system problem. If the terminal in the center on the insulator is really rounded and worn down compared to the new ones you probably just went way over on mileage, some deposits will be normal.

no I didn't since it is a 2000. To be honest I'm not sure when it last had a tune up. I think it was about 3 years ago which would have been between 30k - 36k miles ago.

Oh. I'm an idiot how did I miss that :anon:

Pics of the plugs, if possible? No big deal, I'm just curious. Mine were pretty hideous when I replaced them, I think I may have been the first person to ever touch them after it rolled off the assembly line and I did them at 120k. It was an amazing difference.

unfortunately I threw them away. I was so tired after all the work that I had gotten done. I'm only 29 but crawling around on a concrete floor for a day killed me. I can't wait to make enough money to just pay someone to maintain my jeep for me. I love learning/knowing how to fix everything but getting tired of the time it takes and not getting everything done. The partners at my consulting firm can't understand why I haven't bought a nice car, but I guess they don't understand having a mortgage payment, student loans for undergrad, graduate and law school (fiance's). Oh well, I'm proud of my jeep..even if it does smell like wet dog (Newfoundland)

:cheers: Yeah, I get that a lot... "you make lots of money where you work why are you working on an old car get a new honda or something"

um... no? On BOTH counts? lol

I recommend running a can of BG44k through the fuel to clean the carbon and the injectors. Accept no substitutes.

Emerscape said:

unfortunately I threw them away. I was so tired after all the work that I had gotten done. I'm only 29 but crawling around on a concrete floor for a day killed me. I can't wait to make enough money to just pay someone to maintain my jeep for me. I love learning/knowing how to fix everything but getting tired of the time it takes and not getting everything done. The partners at my consulting firm can't understand why I haven't bought a nice car, but I guess they don't understand having a mortgage payment, student loans for undergrad, graduate and law school (fiance's). Oh well, I'm proud of my jeep..even if it does smell like wet dog (Newfoundland)

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i usually keep a few plugs just in case a new one goes bad(they do go bad sometimes). most times all they need are a good cleaning and re-gapped.