I was wondering if somebody here could help me diagnose this issue. I've been at it for a couple of months and have not come to a definite solution yet. I apologize in advance for the lengthy post. It is a 2001 XJ 4.0 Automatic.
The symptoms are as follows. Rough idle around 500 rpms, miss cylinder 1 2 3, codes P0171, P0131, P0137, Fouled out spark plugs cylinder 1 2 and 3, Black sut on tailpipe and when I step on it and last but not least HORRIBLE gas mileage.
Things I have done. Checked the crank position sensor, Tune up, Changed the all the spark plugs and 1000 miles later the first three spark plugs were completely fouled out. Changed both 1/1 and 1/2 oxygen sensors with OEM sensors. Switched the relay and fuse. Took the XJ to the Dealership and had them check the wiring to the oxygen sensors and they said they would need at least 2-3 more hours of diagnostic time and can't promise results. I checked all the vacuum lines, cylinder leak down test and compression test. Every turned out fine. Also did an intake manifold vacuum pressure test and that turned out fine. I also cleaned the IAC and the throttle body. The only things I could imagine being wrong is either the wiring to the 02 sensor or ground, catalytic converter on that side being bad because the 02 sensor was ran bad for so long, unsuffiecient spark to those three cylinders or the worst an ECM.
I am very stumped and strapped for cash so I don't have money to throw at it. Anything else you recommend me doing? Forgot to mention I have tried reseting the computer and I have been checking the codes and voltage using a Snap-on Solus diagnostic tool. I hope you XJ gurus could help me out. This is my fourth and final XJ. Plan on taking it to the grave with me.

 

Referring to my trusty Haynes manual:
1. Properly gapped plugs?
2. Shorts or bad grounds in ignition system?
3. Faulty coils?
4. Fuel leaks?
5. Clogged injectors?

The black fouled plugs and awful gas mileage obviously point to a rich running condition. The MAP, MAT and coolant temp sensors all play into air/fuel mixture ratios. Maybe giving them a look with your multimeter will help eliminate them...

 

Spark plug gaps good. The only issue about the map and other sensors I need to find something that only effect the front three cylinder ( 1 2 3) I have run injector cleaner but will do the injector cleaner through the fuel rail this time. Or just replace all with 784. Yes I agree its running really rich.

 

Maybe grab a noid light and check your injector function, but if you're swapping them out anyway, wouldn't be much point in that. Seafoam is the best stuff out there for cleaning anything in your motor...I have removed the fuel rail, and filled it with seafoam a couple times. Works great. I was thinking the same thing with it only being 3 cylinders...so it would have to be the wires/coils on those 3, injectors or plugs....what kind of plugs did you put in? Even so, that doesn't explain the rich running condition. Multimeter time!

 

Concentrate on the "rich" mixture. You could verify that with an exhaust analyzer at a shop to be sure, but it sounds like it truly is running rich.

Two things out of the gate that are involved with a rich mixture are the map sensor and the fuel pressure. Both should be tested.

The map sensor can be tested with a meter and be sure the vacuum line leading to it is pulling good vacuum.

Fuel pressure absolutely must be checked as well and it must be done with a gauge. Should be 49 psi, plus or minus 5 psi. If you have regulation issues with fuel delivery, that can be problematic.

Lastly, you're not using any antifreeze are you? If you had a cracked 0331 cylinder head, the leakdown test should have showed it, but keep in mind that the #1 symptom of a cracked 0331 head is unexplained coolant loss.

 

since you only have a problem on 1-2-3. i would concentrate on what is swapable.
swap injector from 1-2-3 to 4-5-6 and check if problrm follows. if so you know you got bad injectors.
your compression test was good, so you know the valves are eaqual good, so they are out of the list to check.

do the swap of let us know.

 

you have the two pc exhaust manifold correct, are 1,2,3 the same manifold?

 

What type of plugs are you running, and also have you checked the plug boots and springs on the coil pack.Also have you checked the coil packs out?

 

Plan on replacing injectors 784, yes it is a two piece manifold and I already checken for leaks and did a vacuum pressure test. How do I check the coil packs? I will switch around the injectors before I get the new ones.