foreveryoung001

Picked up a 96 xj 4.0 a couple weeks back and noticed, after driving it for a bit that the idle would jump between 1100 and 1400 when I let off the gas going down hill.

After reading a bunch of threads, sounded like it might be a bad TPS. Thought I would just change it up, since they aren't that expensive, until I priced the Mopar TPS and thought, a little testing was in order first.

Well, last night, it started doing some other strange things, wanting to shift at odd points, running a little rougher at speed, and just feeling "wrong" if that makes sense. Since I figured it was the TPS... just getting worse, I decided not to wait, and picked one up this morning.

My dealer can't/won't order the correct Mopar part for me, so stopped by NAPA and got an Echlin, which seems to be favored by a lot of folks on the forums, if OEM isn't an option, and then headed to work and switched it out in the parking lot.

I don't have access to many tools at work, but knew, from reading, that I was going to need a T20 torx and brought that from home. Got it switched out and took it for a quick test drive.

Things have improved dramatically... heck, things I didn't even know were wrong with it are now working right... running much smoother, shifting correctly, and is actually a lot more responsive when first accelerating. The jumpy Idle, while coasting down hill, is still there though. Not as bad... yesterday, it just kept doing it until I hit the gas again... with the new TPS it would do it once... act like it wanted to do it again, but then settled on an idle speed and that was that...

Better... but not perfect. My question is, before I start trying to diagnose a problem that might not be there, is do I have to let the ECU learn what to do with this new TPS. I know I was supposed to disconnect the battery cable for a few minutes to let it reset, but without access to my wrenches or even a pair of pliers, I wasn't able to do that. Will disconnecting the battery when I get home be sufficient, or at this point is it better to just let the ECU figure things out with the new TPS and see if that jumpy idle issue goes away in a day or two before I start looking for other issues?

Roler

Do you also have irregular idle at stop, or only when coasting?

weebur

I would take a look at your Idle Air Controller, easy to clean/replace.

foreveryoung001

It only jumps when coasting downhill... Once in a great while it will do it while I'm coasting on a flat surface, but usually not... and never when going uphill.

foreveryoung001

I'm sure the whole throttle body needs a good cleaning too, but I just haven't had the time to pull it all apart, but would the IAC cause the rpm's to jump like that... its not just a rough idle that goes up and down... it is a fast jump.

At first, I thought it was the transmission jumping out of overdrive, but now I'm thinking it wasn't actually doing that, and that its probably the torque converter locking and unlocking. I've read several threads about it, and my symptoms are very similar... Seems like the majority of them were able to cure it with a new TPS

kjc

Next time you're out and it's doin' the funny idle thing, lift up on the brake pedal with your left foot and see if it acts better. The brake stop light switch also controls the torque converter lock/unlock, and OD. It could just need adjusted.

Roler

I'd clean the IAC and throttle body and valve
You could test the TPS (although it appears to be working ok): best is with a analogue meter, to visualize the meter 'sweep' as you open the throttle.

foreveryoung001

I was able to disconnect the battery for a few minutes, but as soon as came back inside, my wife sent me to the store, so hooked it back up and took off. Even those few minutes seemed to have helped a little bit.... or maybe the ECU is just learning how to deal with the new TPS.

The store is a 5 mile trip, mostly downhill from my house, and I only noticed it doing the idle thing a couple of times... the rest of the time it was behaving better. Still not perfect, but better.

Its supposed to be extremely cold tomorrow, but I'm thinking I'll still pull the throttle body and clean that up, along with the IAC. Even if that doesn't have any effect on my problem, it should probably be done. The engine was rebuilt 15,000 miles ago, but from all the gunk built up around the IAC, I'm thinking they didn't do anything to clean up the throttle body when they did the engine.

It was done at a shop in a nearby town, so I'm thinking they just did as little as possible to get the $$$ from the PO.