1997 Jeep Wrangler 4.0L - P0301 Cylinder #1 Misfire Only in Warm Weather

Techo wrote: I'm experiencing a persistent idle roughness issue on my 1997 Jeep Wrangler 4.0L, and it only occurs during warm weather conditions—particularly in the Northeast U.S. After performing three key-on cycles to retrieve a code, I initially got code 43, which translated to P0301 (Cylinder #1 misfire). This was confirmed at an auto shop in August 2006. I've already replaced basic components including spark plugs, wires, cap, and rotor—no improvement. I then installed all new Bosch fuel injectors, still no fix. O2 sensors were also replaced with no change. Previously, I performed a valve spring replacement on cylinder #1, which provided slight improvement temporarily. However, as temperatures dropped, the engine ran smoother. Now that warm weather has returned, the rough idle and check engine light are back. The issue appears to be temperature-dependent—why does it only happen in warmer conditions? I'm at a loss and seeking insight into possible root causes. I recently conducted a compression test and recorded 50 PSI on cylinder #1—a reading that suggests significant internal leakage. Given the vehicle's over 130,000 miles, I suspect valve wear or carbon buildup may be contributing to this issue. The 4.0L engine is known for exhaust valve carbon deposits due to stiff valve springs. Could a burnt valve or poor valve seat sealing be causing the misfire specifically in warm weather? Any advice on diagnosing or repairing this would be greatly appreciated. Try tapping the ECU gently—sometimes it can temporarily stop or reveal a misfire. I’ve seen similar cases where replacing the ECU resolved the issue.

Have you checked compression and cylinder leakage? The 4.0L engine is prone to carbon buildup on exhaust valves due to stiff valve springs. A misfire in warm weather could stem from a valve seat or face that isn't sealing properly—these issues may not always show up during standard compression tests.

Thanks for the input. I've already run a decarbonizer through the engine and performed a thorough cleaning. Today's compression test showed only 50 PSI on cylinder #1—well below acceptable levels. With over 130,000 miles, I believe a valve job is necessary. This seems to align with the symptoms of a burnt or worn exhaust valve.

I've seen several engine heads warp on older 4.0L Jeeps, though my 1994 Cherokee has over 340,000 km and still hasn't had a head gasket issue—so it's not universal. Still, valve wear and thermal expansion in warm weather could exacerbate misfires in high-temperature conditions.