Hello gents, my 1998 XJ 4.0 had developed a slight miss and rough idle. This only occurs in park or neutral and it drives and performs perfectly. I get about 11 MPG, I chalk that up to me liking the skinny pedal too much. The idle also seems a little high. Normally it's about 750rpm but it's like 800-900 now. What could this be, clean air filter and the PCC components where replaced a couple months ago, the CPS was replaced a couple months ago to fix a stalling issue. I haven't changed plugs or rotor. 204,703 miles.

 

Also when idling it will be sorta smooth then a big jolt that is very noticeable. No CEL. Kinda feels like a miss.

 

Have you checked for leaks in the manifold? Mine doesn't have one, but i've noticed a number of people have a crack in their manifold where it's welded/connects to the downpipe before the cat. There's also:

- Check the manifold gasket while you're at it. Turn the car on and carefully move your hand all around the manifold and feel for puffs of air, or just listen for something that doesn't sound right. It gets super hot obviously so be careful.
- Check (might as well replace) the TPS
- Take off/clean the throttle body
- Take out and clean the IAC on the throttle body.

11 mpg seems really low, even for a heavy foot.

 

My manifold is broken and cracked but they where cracked and broken before this started happening. I already cleaned the TB and IAC.

 

Update, so the idle RPMs have gone back to normal, however there is still the slight miss and the shaking.

 

had the exactly same problem with mine!!!!! thought it was something with the spark changed everything and went aftermarket nothing changed!!! somewhere i hear to clean the throttle body. after i cleaned it it ran fine. ended up being the positioning sensor wasn't working properly because of the gunk. or it could be your timing

 

And difference between in Neutral or Drive?

Edit: sorry, wrong thread

 

Can be number of thingsI'd start with:
CLean the IAC and throttle body ( esp plate on both sides), use new gasket on tjrottlebody
If not been done recently do a tune up with fresh plugs and gapped correctly, plug wires,cap and rotorClean up your grounds
Adjust the throttle cable
these are all relatively simple and good maintenance
Let us know if this made a change.