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P0300, P0302, P0304 Diagnosis - 2001 Jeep Cherokee 4.0L Engine Misfire & Fuel Trim Issues

Model: Jeep 4.0L Cherokee Fault Code: P0300 Posted: 2007-03-06 23:36

Hi, I recently purchased a code reader to diagnose the MIL light and discovered DTC codes P0300, P0302, and P0304 on my 2001 Jeep Cherokee with a 4.0L engine. I also have an evaporative emissions leak code from an August inspection six months ago; the auto service tech replaced the gas cap, which temporarily reset the issue. I don’t believe this evap leak is directly related to the misfire codes. The vehicle has accumulated approximately 270,000 miles but has been well-maintained and does not burn oil. Recently, I’ve noticed a very rough idle when the engine is warmed up—though it starts reliably. Cold starts are better with higher initial idle. I inspected the spark plugs last Saturday (Bosch platinum plugs installed 30,000 miles ago) and found no signs of carbon buildup or wear across all cylinders. The vehicle has primarily been driven on highways. The misfire flashing occurs after driving more than 45 minutes on the highway, especially when coasting downhill or coming to a stop. Acceleration or restarting the engine clears the issue. I have not yet tested swapping injectors to isolate which cylinder is causing the problem. While driving at 2000 RPM and 65 mph, fuel trim Bank 1 reads 8% and Bank 2 reads -4%. Is this imbalance normal? Does a 4.0L straight-six engine have two fuel banks? Additionally, the MAP sensor is reading 27 psi—this seems incorrect as a Jeep with no turbocharger should not exceed 12 psi of manifold pressure. Both O2 sensors appear to be functioning normally, with only the P0300/P0302/P0304 codes present. Any insights or troubleshooting steps would be greatly appreciated. Thanks, Pete V.

Related fault codes
P0300P0302P0304
Comments (2)
Anonymous 2007-03-07 02:49

Hi, Let me address the key technical points first, and leave more complex issues to experts. Yes, the 4.0L inline-six engine in your Jeep Cherokee has two fuel banks. Starting from the front of the vehicle: cylinders 1, 2, and 3 belong to Bank #1; cylinders 4, 5, and 6 belong to Bank #2. You can visually confirm this by examining the two separate exhaust manifolds. Code P0302 indicates a misfire in cylinder #2, while P0304 points to a misfire in cylinder #4. These should be inspected closely. A compression test on cylinders 2 and 4 is highly recommended. Also, please check the condition of your spark plug wires—worn or degraded wiring can cause intermittent misfires. P.S. Useful engine details: - Firing order: 1-5-3-6-2-4 - Coil pairing: (1-6), (2-5), (3-4)

Anonymous 2007-03-07 10:51

I recommend replacing the Bosch platinum spark plugs with cheaper Champion plugs, as they are more commonly used in this engine and may perform better under long-term highway use. Then, conduct a compression test to check for cylinder-specific issues. Also verify fuel pressure—low fuel pressure can cause misfires. Consider having both camshaft and crankshaft position signals monitored via an oscilloscope to rule out sensor or timing problems. A distributor gear issue is possible at 270K miles, but other factors like heavy carbon buildup on exhaust valves, head gasket leaks, or low fuel pressure could also be contributing. Could you please specify which diagnostic scanner you used to read the MAP sensor? This helps assess data accuracy and potential calibration issues.