TheKingdomofShenan

Hi all,
I just recently purchased a 2000 XJ sport with 130k miles, a new 3" lift and 31s, all else is stock. I bought it knowing it had a rough idle but after a good bit of research on here and other forums it seemed like it wasn't too much to be worried about as it seemed to be predominantly caused by maintenance items. It is only rough at idle, goes away with throttle application (although it gets very slightly rough again around 1250 RPM), it is noticeably worse when cold. While playing with it today I noticed that if I roll forward from a stop, apply the brakes and stop, set the parking brake, the rough idle is gone. As soon as I disengage the parking brake and let the weight of the Jeep be held by the transmission then it starts idling rough again.
Maintenance done since I bought the truck:

• Ran seafoam with a top off of good gas (half tank has been in it since September), about to finish that tank off, so bad gas still hasn't been ruled out
• Changed oil for Rotella T6 5w40 (it was probably about 1qt overfilled, so it calmed down a little bit with the proper amount of oil in there)
• new spark plugs
• new air filter
• new coil rail
• both motor mounts and transmission mount
• Inspected and clean IAC, TPS, and throttle body

My best guesses right now are torque converter bolts need to be tightened, cracked or warped flexplate, or general transmission shenanigans.
Anybody have any ideas to point me in the right direction?

Thanks for the help guys!

Radi

Is it idling rough when your foot is on the brake? Might be developing a vacuum leak in the brake booster.
How's it idle in Park?

The plugs you removed- see anything unusual? Any of them look different from the others?

Any noise from the torque converter area?

TheKingdomofShenan

It doesn't idle rough when the brakes are holding the weight of the car and it doesn't idle rough when brakes are released in Park on idle ground, thus not "leaning on" the transmission.
I personally did not remove the plugs, it was done by the PO troubleshooting the truck immediately prior to sale.
The noise does seem to be coming from the flexplate/torque converter area which has me 90% convinced that it is either the torque converter bolts are loose or the flexplate is cracked. I'll hopefully be able to check it out this weekend.

TheKingdomofShenan

I'm at a loss now. All of the torque converter bolts were sufficiently tight but I pulled them, added red lock tite, and reinstalled and the Jeep feels about the same. Couldn't see any cracks in the flexplate either. Any ideas?

Radi

The rough idle....is it a noise at idle, or a vibration/shaking? Flexplate bolts won't cause a rough idle, but they'll sure make noise.

TheKingdomofShenan

It's a vibration/shaking. I thought maybe it was a vibration due to the 3" lift so I installed a 1.25" TC drop and it got noticeably worse. It now vibrates quite aggressively from acceleration to 20mph or so and when stopped in Drive with your foot on the brake. As soon as you shift into neutral it stops. Could it be U joints? The fact that it goes away when in neutral has me concerned it my be the torque converter.

CCKen

Check engine mount cushions.

TheKingdomofShenan

Both engine mounts and the transmission mount are new.

madmanmarty

wires and cap?

TheKingdomofShenan

It's a 2000, so coil rail, which is new. And just to clarify, I no longer believe it to be a motor or idle issue. It seems to be more driveline. I thought it could have been ujoint issues or vibration from lift but it continues when the jeep is stopped and in drive with foot on the brake.

madmanmarty

Sorry,I did not notice the year.
The more I think about post 1, the symptoms lean toward driveline,not engine.
A few thoughts,it's my understanding flexplate cracks are hard to see, u joints are possible- I would look at them,I wonder if the new lift was tightened on the ground-loosen and tighten to spec all bolts, check to see if your exhaust is close to the trans/tc-mine was rubbing/hitting under acceleration.
Final thought,new lifts can cause driveline issues. I would start with the easiest items to check and keep knocking them off the list 1 by 1.

TheKingdomofShenan

Thanks for the replies. I'm planning on spending most of Saturday chasing this issue and will work my way down the list.
Is there any place that could allow driveline movement besides the 2 motor mounts and the transmission mount? The vibration at a stop will be noticeably worse if I brake hard to a stop and can be made to go away by jostling the car between idle and hard brakes to find its "happy place." It also stops completely when shifted into park or neutral.
I plan to check all the mounts as well since they were done by the PO.

madmanmarty

rule out 100% flexplate

TheKingdomofShenan

A while back I went a pulled the inspection plate, checked the flexplate bolts and added locktite. I tried my best to inspect the flexplate but I agree, it's hard to see. Would a broken/warped/cracked flexplate be exacerbated by a TC drop and intensity of braking?

ehall

check fuel pressure