LKN_xj

My Jeep is a 1991 XJ limited with the 4.0 and NP242. I should start by saying I had the transfer case out this weekend (needed to check input shaft length because I'm swapping to a 231 and needed to make sure my spare would swap in - so it was a literal take 242 off, put 242 back on while 231 is being rebuilt).

Everything went back together pretty smoothly. I really didn't have any issues other than the wires on top of the speedo gear ripped out of the connector, so i'm running without a speedo right now.

Fast forward to yesterday. It was the first time since the T-case work that the engine really got warmed up and I drove it around for 15 minutes or so. When cold, everything is great. Even when the engine comes to temperature things are still good. But after about 10 minutes when the drivetrain starts to accumulate heat, I start to lose power and I get this strange rough sounding growling noise at about half throttle. Power loss isn't devastating, but it is noticeable (I've driven this jeep for over 10 years so I'm pretty tuned in to what it typically sounds like). The noise actually sounds like the exhaust note changes at mid throttle and gets really throaty. I can't tell if it's something coming into interference and grinding as things heat up, or if it is totally unrelated to my work with the T-case.

I'm looking for things to check. At first I thought I may have bumped into an O2 sensor and it's causing things to run rich, so I replaced that and at first I thought that helped, but then the symptoms came back this morning on the way to work.

Here's what I know:

- Transfer case shifts into all 4x4 settings without hitch, and transmission shifts just fine
- Replaced U-joints on rear drive shaft about 2 weeks ago
- Replaced transmission mount while the T-case was off
- Replaced O2 sensor last night (I only have one from what I can see - at the down pipe)
- Checked transmission fluid and it was nice and red and the level was spot on
- Not leaking transmission fluid from anywhere (not at T-case, not between t-case and transmission)

I'm stumped because it seems very straightforward. If the transfer case mated to the transmission then the splines are engaged and it seems like nothing can bind there. Then I'm thinking about maybe driveshafts but I can coast all day long smooth as butter. One theory I have is maybe front engine mounts got stressed too much while I had the transmission lowered to remove the T-case, and now the drivetrain is kinda in a bind? I don't know.

Hope you guys have some ideas I can check and I appreciate the help in advance!

cmarr6

Might be cat starting to plug. Mine did something similar. Take out 02 sensor and drive a little to see if it runs better.

LKN_xj

Yes! Good call! I thought about this right after I posted and went ahead and got a Cat on order from the store. Went with the high flow while I was at it.

I'll keep you posted on it. Thanks!

madmanmarty

Did you perform the test first?

LKN_xj

Haven't quite gotten the chance hoping the rain holds off tomorrow long enough to do so (my apartment parking lot is my workspace - regardless it has been on my list to replace - sounds like a brick rattling around in there and has been for about 6 months...

LKN_xj

Turns out she needed a cat - installed a new one and she runs like a champ! Thanks fellas!

madmanmarty

glad you got it fixed.