Many if not all my TJ issues were solved by this forum so a big thanks to all of you - the information here is priceless.
Unfortunately, I am at my wits' end with the current issue.
What are the symptoms:
Random Misfire - P0300 - CEL blinking. Misfire and loss of power always start at 1800-1850 RPM (with or without load). Occasionally recovers after 2000 RPM - no actual pattern. Sometimes it runs fine after 2000-2200 RPM, sometimes not, sometimes it recovers at 3000RPM, sometimes not.
Misfire occurs with the engine cold and warm.
P0174 was set once or twice but P0300 sets every single time.
The issue occurred once 7 days before it permanently settled. First, the CEL blinked a few times, P0300 appeared and after a restart it went away for a week until it started doing it more and more often to the point where it became permanent.
Car has 125k miles
No major work done in recent time except an oil change
What has been done:
- Replaced spark plugs with Autolite XP985 gapped at 0.035". Old ones were ~25,000miles and the same brand/gap.
- Replaced ignition coil with known good (Mopar) and brand new (MSD)
- All ground points cleaned (engine near the oil sender, body near the cowl, hood)
- Replaced all sensors with known good sensors (all Mopar): Crankshaft (tried 2 different ones), Camshaft, MAP, TPS, Intake Temp., NTK Upstream O2 sensors (both banks)
- Removed catalytic converters (ran without cats using a 2002 TJ exhaust) for testing purposes
- Replaced fuel pump with a new Bosch unit (old pump (likely original) was was tested and found to be pushing 58PSI but I suspected intermittent failure)
- Compression test (see below results). I realize cylinder 4 isn't at its best but I can't imagine it could cause this issue.
- Plugged all vacuum ports (Brake booster, EVAP canister/solenoid, HVAC control/Cruise Control) for testing purposes
Known issues:
- The PCM throws P0135, P0141, P0155, P0161 which all mean O2 heater circuit malfunction. These errors appeared all at the same time about 15,000 miles ago or so (Jun 2021). Car ran perfectly fine. I did investigate the issue (including with Mark from WranglerFix) and concluded it's something wrong with the PCM. When this issue occurred all 4 O2 sensors as well as the catalytic converters were 10,000 miles old and ran perfectly fine before. (NTK sensors and MagnaFlow cats).
Using the compression test method in the service manual (rotate the engine with the engine starter motor for three revolutions):
1: 160PSI
2: 130PSI
3: 150PSI
4: 120PSI
5: 150PSI
6: 160PSI
Testing until pressure gauge needle stabilizes (6-7 revolutions):
1: 195PSI
2: 160PSI
3: 180PSI
4: 140PSI
5: 175PSI
6: 190PSI
Unfortunately, I am at my wits' end with the current issue.
What are the symptoms:
Random Misfire - P0300 - CEL blinking. Misfire and loss of power always start at 1800-1850 RPM (with or without load). Occasionally recovers after 2000 RPM - no actual pattern. Sometimes it runs fine after 2000-2200 RPM, sometimes not, sometimes it recovers at 3000RPM, sometimes not.
Misfire occurs with the engine cold and warm.
P0174 was set once or twice but P0300 sets every single time.
The issue occurred once 7 days before it permanently settled. First, the CEL blinked a few times, P0300 appeared and after a restart it went away for a week until it started doing it more and more often to the point where it became permanent.
Car has 125k miles
No major work done in recent time except an oil change
What has been done:
- Replaced spark plugs with Autolite XP985 gapped at 0.035". Old ones were ~25,000miles and the same brand/gap.
- Replaced ignition coil with known good (Mopar) and brand new (MSD)
- All ground points cleaned (engine near the oil sender, body near the cowl, hood)
- Replaced all sensors with known good sensors (all Mopar): Crankshaft (tried 2 different ones), Camshaft, MAP, TPS, Intake Temp., NTK Upstream O2 sensors (both banks)
- Removed catalytic converters (ran without cats using a 2002 TJ exhaust) for testing purposes
- Replaced fuel pump with a new Bosch unit (old pump (likely original) was was tested and found to be pushing 58PSI but I suspected intermittent failure)
- Compression test (see below results). I realize cylinder 4 isn't at its best but I can't imagine it could cause this issue.
- Plugged all vacuum ports (Brake booster, EVAP canister/solenoid, HVAC control/Cruise Control) for testing purposes
Known issues:
- The PCM throws P0135, P0141, P0155, P0161 which all mean O2 heater circuit malfunction. These errors appeared all at the same time about 15,000 miles ago or so (Jun 2021). Car ran perfectly fine. I did investigate the issue (including with Mark from WranglerFix) and concluded it's something wrong with the PCM. When this issue occurred all 4 O2 sensors as well as the catalytic converters were 10,000 miles old and ran perfectly fine before. (NTK sensors and MagnaFlow cats).
Using the compression test method in the service manual (rotate the engine with the engine starter motor for three revolutions):
1: 160PSI
2: 130PSI
3: 150PSI
4: 120PSI
5: 150PSI
6: 160PSI
Testing until pressure gauge needle stabilizes (6-7 revolutions):
1: 195PSI
2: 160PSI
3: 180PSI
4: 140PSI
5: 175PSI
6: 190PSI
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