My wife's XJ has been acting up lately; I'm no stranger to the 4.0L but so far I haven't been able to figure this one out.
Specs on the engine: 150,000 miles. Well cared for, good compression, no noises. Mostly stock except for stainless header and airaid cold air intake. Has run well with this setup for years.
Symptoms: Idles perfectly. Drives perfect, as long as you stay under 1/2 throttle. Above 1/2 throttle, engine begins to miss, and if you go to WOT will eventually die. This happens whether it is parked in the driveway or on the highway. The misfire doesn't seem to be related to RPM level; just throttle position (it will run smoothly at 4500RPM, as long as I keep it under 1/2 throttle). Although it is a little difficult to tell for sure, it seems like I can hear a little bit of detonation through the misfires. After a few days of driving, the misfire codes begin to accumulate; PO301-306. No other codes.
What I have checked and tried:
I have checked fuel pressure at idle and under load while driving, and at WOT. Also did a volume test (passed) while monitoring the pressure. Always stays between 45-50 PSI with very little variance.
Using my scanner, I have tested the TPS, CTS, O2 and MAP sensors under driving conditions. All return expected values thoughout normal ranges, and operate smoothly.
The O2 and TPS sensors are about a year old; O2 was replaced because of mileage interval, TPS because the old one failed.
Cap, rotor, plugs, wires, ignition pickup, and coil were all replaced today. None had any effect on the symptoms.
Computer was reflashed by chrysler about a year ago due to a recall; don't remember the exact details.
I can't figure out what to try next. Any suggestions??
Specs on the engine: 150,000 miles. Well cared for, good compression, no noises. Mostly stock except for stainless header and airaid cold air intake. Has run well with this setup for years.
Symptoms: Idles perfectly. Drives perfect, as long as you stay under 1/2 throttle. Above 1/2 throttle, engine begins to miss, and if you go to WOT will eventually die. This happens whether it is parked in the driveway or on the highway. The misfire doesn't seem to be related to RPM level; just throttle position (it will run smoothly at 4500RPM, as long as I keep it under 1/2 throttle). Although it is a little difficult to tell for sure, it seems like I can hear a little bit of detonation through the misfires. After a few days of driving, the misfire codes begin to accumulate; PO301-306. No other codes.
What I have checked and tried:
I have checked fuel pressure at idle and under load while driving, and at WOT. Also did a volume test (passed) while monitoring the pressure. Always stays between 45-50 PSI with very little variance.
Using my scanner, I have tested the TPS, CTS, O2 and MAP sensors under driving conditions. All return expected values thoughout normal ranges, and operate smoothly.
The O2 and TPS sensors are about a year old; O2 was replaced because of mileage interval, TPS because the old one failed.
Cap, rotor, plugs, wires, ignition pickup, and coil were all replaced today. None had any effect on the symptoms.
Computer was reflashed by chrysler about a year ago due to a recall; don't remember the exact details.
I can't figure out what to try next. Any suggestions??