2002 Jeep Liberty 3.7L Misfire & Knock Codes P0302, P0325, P0138 - Fuel Injector or Engine Issue?
Hi everyone, I'm new to this forum and recently started working on my 2002 Jeep Liberty with a 3.7L engine. The car has been sitting for about 2-3 months and has accumulated 182,000 miles. I've just begun driving it again after a long period of inactivity. The vehicle is now displaying three diagnostic trouble codes: P0302 (Misfire Detected - Cylinder 2), P0325 (Knock Sensor Bank 1, Sensor 2 - High Voltage), and P0138 (Fuel Trim Bank 1, Sensor 2 - High). The P0138 code has been intermittent for the past two years. I've replaced the spark plugs with new NGK V-Power units, but the issue persists. Given that the car was idle for several months and only driven on short runs, I suspect a clogged fuel injector might be the root cause. I have access to multiple scan tools, a multimeter, and a Spark Tester Kit (S+G Tool Aid). I'm currently planning to order NOID lights to test injector operation. I've also performed a combustion gas test in the coolant using a NAPA kit — no gases were detected. However, I do notice a small amount of coolant loss, which I believe is coming from the exhaust pipe (no visible drip on the driveway). My goal is to get another year out of this vehicle and obtain an inspection sticker for roadworthiness. I'm hoping it's not a head gasket issue. I'd appreciate any advice on how to confirm whether a fuel injector cleaning or replacement is needed, especially given all three codes appearing together after long-term storage. Thanks in advance for your insights! P.S. The engine has been well-maintained with recent suspension, steering, and brake work completed.
If you suspect a clogged fuel injector, it's not a definitive test — but here’s a practical approach using your OBD2 scanner: note the stabilized short-term and long-term fuel trims while the engine is running. Then, disconnect each fuel injector one at a time (in sequence), wait 1–2 minutes for the trims to stabilize again, and record the new values. Calculate the total fuel trim change by adding the short- and long-term trims together. The difference between your original reading and the post-disconnection value should indicate each injector’s contribution to overall fuel delivery. If one cylinder shows minimal or no change in fuel trim, that may point to a misfire or poor injection. A quick test: disable all injectors, hold the throttle wide open, and crank the engine. You should hear a steady, even cadence of 'WA WA WA WA WA WA'. If you instead hear an irregular pattern like 'WA WA WA weee WA WA', this may indicate a cylinder with poor compression or misfire — possibly due to mechanical issues. The 'wee' sound could be caused by the starter working too easily during the compression stroke on one cylinder. This is essentially a relative compression test and suggests further investigation, such as a full compression test, might be needed.