![]() The 99-01 XJs have a coil-on-plug ignition pack for all six cylinders instead of a distributor and With the obvious problems ruled out, it was time to start more in depth diagnosis. The pressure stayed steady at 48 psi, which indicated that the fuel pump or filter were not the culprit. I connected my fuel pressure gauge to the test port on the fuel railĪnd checked fuel pressure, both at idle and under load. Fuel PressureĪ clogged fuel filter could cause fuel rail pressure drop under load, which could explain the misfire on the highway. Unfortunately, although this smoothed out the idle, it did not solve the misfire entirely. Replacing all six with fresh NGK ZFR5N V-Power copper plugs was in order. I pulled all six spark plugs, and all had a large gap of more than 0.040”. Bad spark plugs can easily cause misfires,Īnd in this case the Jeep had 275,000 miles and spark plugs of unknown age. In my case, the obvious things were spark plugs and fuel. ![]() When diagnosing a misfire, you should check and fix the obvious problems first. ![]() They are not necessarily perfect, but I hope they can help anyone else with similar symptoms on their Jeep. These are the steps that I followed to track down this issue. It shuddered slightly at idleĪnd got bad fuel economy on the highway. When scanned with OBDII, the ECU reported P0300 (Random misfire) and individual misfire codes for each cylinder (P0301 - P0306). ![]() Here is the procedure that I used to diagnose and solve the problem. It had a light misfire at idle and would blink the check engine light under heavy acceleration. I recently bought a 2000 Jeep Cherokee (XJ) with the 4.0l inline-6 engine. Diagnosing a light, random misfire on a Jeep XJ 4.0l Engine ![]()
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