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2004 Jeep Grand Cherokee P0300 Code: Rough Idle, Stalling, Gas Fumes - Stock Vehicle with Aftermarket Alarm Installation

Model: 2004 Jeep Grand Cherokee Fault Code: P0300 Posted: 2013-05-30 22:52

My 2004 Jeep Grand Cherokee Limited 2WD 4.7L V8 (J VIN) with automatic transmission is currently exhibiting severe engine performance issues. The vehicle ran fine until I attempted to restart it after a brief shutdown—upon restarting, the engine began running roughly and frequently stalls. A strong gasoline odor is now noticeable during operation. Diagnostic trouble codes retrieved include: P0300 (Random/Multiple Cylinder Misfire), P0301–P0304 (Individual Cylinder Misfires), P0352, P0353, P0356 (Ignition System Faults), and P0141/P0161 (Oxygen Sensor Issues). During testing, I disabled injectors and coil packs for cylinders 1 through 6 one by one. The engine ran on only two cylinders at a time—cylinders 2 and 4—and showed no improvement when re-activated. Compression test results: - Cylinder 1: 150 psi - Cylinder 3: 135 psi - Cylinder 5: 150 psi - Cylinder 7: 150 psi - Cylinder 2: 0 psi (no compression) - Cylinder 4: 105 psi - Cylinder 6: 150 psi - Cylinder 8: Not yet accessible due to access issues The spark plug in cylinder 2 appears wet and black, indicating possible fuel contamination or poor combustion. I have no modifications except for an aftermarket alarm system that was previously installed. The alarm was patched directly into the power line at the diagnostic connector. I removed it and re-soldered the wires with heat shrink. Could this have damaged the ECU or ignition control module? All dashboard lights, horn, cruise control, and ABS appear functional—except for a strange creaking noise under the dash, which may be related to the AC climate control plenum. I am seeking advice on how to diagnose and resolve these misfire issues. Any guidance on fuel pressure testing, ECU reset procedures, or potential damage from the alarm wiring would be greatly appreciated.

Related fault codes
P0300
Comments (3)
Anonymous 2013-05-31 07:20

The vehicle is stock with no recent repairs. However, an aftermarket alarm was previously installed and wired directly into the power line at the diagnostic connector. I removed it and re-soldered the wires with heat shrink. Could this have caused damage to the ECU or ignition system? Also, please confirm if any other modifications or electrical changes were made—this could impact fault code behavior.

Anonymous 2013-05-31 11:20

No recent repairs beyond the alarm installation. Compression test results show zero psi in cylinder 2 and only 105 psi in cylinder 4, which is significantly below expected levels. The spark plug in cylinder 2 appears wet and black—this suggests fuel contamination or a failed ignition system. I also noticed a creaking noise under the dash that may be coming from the AC climate control plenum. All other dashboard functions (horn, cruise, ABS) appear normal.

Anonymous 2013-05-31 13:50

First step: check engine oil for signs of fuel contamination—this could indicate a leaking injector or internal combustion issue. A fuel pressure test is recommended to identify any faulty injectors or multi-injector leaks. Confirming that the vehicle had no prior issues before the sudden misfire (CEL not on) and then developed multiple codes after restart is critical. The 0 psi in cylinder 2 with a wet, black spark plug strongly suggests a failed ignition coil or internal engine damage—possibly related to the alarm wiring if it caused voltage spikes or ground faults. Consider performing an ECU reset and testing for misfire patterns under load.