So my normally smooth running high-miles ZJ suddenly developed a driveability problem two weeks ago that had all the hallmarks of a classic cap/rotor/wires ignition problem:
- started well
- idled a little lumpy but okay
- stumbled under accelleration
- stumbled and bucked in top gear at highway speeds
- fuel consumption very high -- about 12 mpg (18 to 22 normally)
- check engine codes for misfires on cyl 6
- check engine codes for multiple random misfires
A check of spark plugs revealed cyl 6 plug wet with fuel, but all others looked okay. Compression checked out okay. I put new Mopar cap/rotor/wires and new Champion plugs in, with no real improvement in driveability. A new coil improved things somewhat, but there was now a definite miss in one cylinder at idle, and highway driving was slightly smoother but still generating codes for multiple random misfires. I changed the fuel filter, cut open the old one, and found it very clean inside the pleats.
I was done throwing parts at it, so I consulted my Jeep guru, AlteredXJ. Under his advice, we ordered one new fuel injector for cyl 6, plus all new o-rings for the injectors. We pulled the fuel rail and found the fuel it contained rust colored and full of sand-like debris, probably rust particles from 15 years/195,000 miles of service. This discolored and dirty fuel was especially concentrated at the number six position in the rail. After cleaning this out, cleaning the injectors, and reinstalling with a new injector in the six position, and new o-rings, the old girl started and ran like silk. Mileage and power are back to normal. Problem solved.
Analysis: It may have been bad o-rings; they were clearly overdue for changing. It may have been a bad injector on six, we didnt test it, just replaced it. Most likely, though, it was a starved injector on six due to a build up of debris. The number six injector is at the rearward end of the fuel rail, which is fed at the forward end by the fuel supply line. It stands to reason that the injector at this "downstream" position in the fuel rail may be more vulnerable to an accumulation of debris, as fuel flows past the other five injector positions, washing them somewhat.
Conclusion: Pulling the fuel rail for inspection of the fuel quality in the rail and manual cleaning of the injectors is a low cost diagnostic step that would have saved me time and money in prematurely replacing ignition components. Given the fuel quality of today, and the age of my vehicle, going foreward I will probably pull the rail for inspection at 25,000 mile intervals.