I recently bought a 99 XJ with 180mi on her. It has a rough idle, and is throwing codes P0100, P0303 and P0304. So she is misfiring on cylinders 3 and 4, and I am also getting a MAF sensor error. Reading what threads I could, I have so far tried replacing MAP sensor, IAC sensor, TPS, as well as dist cap & rotor, cables and plugs with no help. What else should I be looking at? Thanks!

 

Check the ground cable from battery. One of my 98's was running perfect. Disconnected the battery for a day hooked it back up and it ran like crap. Now it's to the point where I turn it on and it has power. The second I turn the key it's cuts power. Have to jiggle ground cable to get it to start. Haven't pulled codes yet but it's definitely misfiring.

 

Check all the major grounds in your engine. Esp the data link ground on the side of the engine block below you sparks and the engine to firewall on the back of the block. I had an electrical gremlin after cleaning my battery cables and swapping starter. After cleaning up 2 grounds I had no electrical issues as well as improved idle, restored power, etc.. just a thought. I read that those 2 grounds can throw a lot of codes and cause a lot of problems.

 

Ps. I would try cleaning those grounds first before replacing more stuff.
The expert guys on here may be more help if the issues persist... or correct me if I'm wrong. Hope you figure it out.

 

How many grounds are there in the engine bay that one should check?

 

This is a list of ground locations I made a while back for my '00 XJ from the wiring diagrams in the factory service manual. Hope it is of some use...

G100 by battery
G101 x2 on engine near ops
G102 firewall near ops
G104 t/o at engine starter motor, t/o on battery harness
G106 by pcm
G107 glovebox
G108 drivers right knee
G200 under passenger seat
G300 t/o for airbag control module
G301 near amp t/o
G302 behind spare wheel
G303 under driver's seat
G304 behind spare wheel
G306 under driver's seat

I should add, my XJ is a UK right hand drive. Can't remember if that changes any of the locations or not.

 

Time for a compression test. It's an easy test and can tell you a lot.

The spec for the 4.0 is 120-150 psi, with no more than a 30 psi variation between cylinders. Can't ignore the possibility of an internal issue like a head gasket, etc.

If compression checks out, then I would swap the injectors from 3 & 4 with two other cylinders and see if the misfire follows.

 

Not many but they are important. It may not be the issue but worth looking into.

https://www.cherokeeforum.com/f2/engine-ground-firewall-130071/index2