Looking for advice on a strange issue with my 2005 Jeep Wrangler TJ 4.0L, manual transmission, with approximately 77,000 miles.
I’ve replaced or checked quite a few things while trying to diagnose it:
I’m still dealing with an intermittent vibration and strange fuel-trim behavior.
The Jeep generally drives fine and has no check-engine light. A vibration mostly happens when the engine is fully warmed up, especially after accelerating and then coming to a stop.
For example, when I accelerate up the slope into my garage, press the brake and put the transmission in neutral, the fuel trims suddenly go very negative—sometimes around -16% to -17%. As the trims gradually return toward zero or positive, the vibration slowly disappears.
The fuel trims also go significantly negative under acceleration. While driving, my long-term fuel trim is usually around -16%, and the short-term trim can also drop to approximately -16%. At idle, the short-term trim is normally closer to 0, but can go randomly in the negative.
Once the Jeep is fully warmed up, I also sometimes feel a hesitation during acceleration, especially when shifting from first to second and then from second to third. It feels like the engine hesitates or briefly struggles when I get back on the throttle after changing gears. This is much less noticeable when the engine is cold.
Other things I’ve noticed:
I'm honestly clueless of what it can be.
I’ve replaced or checked quite a few things while trying to diagnose it:
- New fuel pump
- o2 sensors
- Fuel injectors
- Spark plugs and coil rail
- MAP sensor
- IAC valve
- EVAP purge valve
- CPS/crankshaft position sensor
- Coolant temperature sensor
- PCM (from WranglerFix)
- Intake manifold gasket, although I’m not completely confident that I installed it perfectly
- OPDA
I’m still dealing with an intermittent vibration and strange fuel-trim behavior.
The Jeep generally drives fine and has no check-engine light. A vibration mostly happens when the engine is fully warmed up, especially after accelerating and then coming to a stop.
For example, when I accelerate up the slope into my garage, press the brake and put the transmission in neutral, the fuel trims suddenly go very negative—sometimes around -16% to -17%. As the trims gradually return toward zero or positive, the vibration slowly disappears.
The fuel trims also go significantly negative under acceleration. While driving, my long-term fuel trim is usually around -16%, and the short-term trim can also drop to approximately -16%. At idle, the short-term trim is normally closer to 0, but can go randomly in the negative.
Once the Jeep is fully warmed up, I also sometimes feel a hesitation during acceleration, especially when shifting from first to second and then from second to third. It feels like the engine hesitates or briefly struggles when I get back on the throttle after changing gears. This is much less noticeable when the engine is cold.
Other things I’ve noticed:
- The vibration rarely happens when the engine is cold.
- Turning on the A/C makes the vibration noticeably worse.
- The hesitation becomes more noticeable after driving in traffic and getting everything fully hot.
- Disconnecting the EVAP purge line and capping the intake side seems to reduce how often the vibration happens.
- It drives normally at highway speeds and generally feels fine once cruising.
- I have no misfire codes or check-engine light.
I'm honestly clueless of what it can be.
