Owner's Problem Description
The owner reports that while idling, the engine RPM drops to a level where it feels like it might stall. The vehicle accelerates well initially, but when the driver takes their foot off the gas, the tachometer surges up to 4000 RPM before settling down. The owner has replaced the spark plugs, wires, cap, and rotor, but the issue persists. They suspect an electronic problem and are seeking advice on where to start diagnosing the issue.
Professional Technical Analysis
This analysis pertains to a 2001 Nissan Altima with a 2.4L four-cylinder engine, experiencing intermittent stalling and RPM surges at idle, indicative of potential misfire issues. The vehicle has approximately 120,000 miles on the odometer. The symptoms manifest primarily when the vehicle is stationary, suggesting a fuel delivery or ignition issue rather than a mechanical fault. Given the reported P0304 code, indicating a misfire in cylinder 4, further investigation into the ignition system shows recent replacement of components like spark plugs and wires, which typically address common failure points. However, the persistence of the misfire suggests deeper issues such as a faulty fuel injector, compression problems within the cylinder, or potential air leaks from the intake manifold gasket. Notably, Nissan’s 2.4L engine has documented instances of injector failures, especially when subjected to high mileage. The current condition poses safety concerns, especially in traffic situations where stalling could lead to accidents. A detailed examination of the ignition coil for cylinder 4, fuel rail pressure (should be 35-45 PSI), and cylinder compression (should be 150-200 PSI) is recommended to isolate the issue. Additionally, checking for vacuum leaks using a smoke machine can reveal any intake gasket failures contributing to the misfire.
Possible Causes
Most common causes (ordered by frequency):
- Faulty Fuel Injector: A malfunctioning fuel injector in cylinder 4 can cause inconsistent fuel delivery, leading to a misfire. Injector resistance should be between 12-16 ohms. If the injector fails to open or close properly, it can lead to a rich or lean condition, causing the RPM surge observed. Testing can be done using a noid light to confirm injector operation. - Intermittent misfire codes, inconsistent fuel trim readings, injector resistance tests outside specified range.
- Compression Issues: Low compression in cylinder 4 can lead to misfire symptoms. Compression should be within 150-200 PSI for optimal performance. A compression test revealing below-normal values could indicate a bad valve or piston ring issue. - Compression test results show readings below 150 PSI, potential oil consumption, or excessive exhaust smoke.
- Vacuum Leaks: An intake manifold gasket leak can cause incorrect air-fuel mixture, leading to misfire conditions. Listening for hissing sounds or using a smoke test can help identify potential air leaks affecting cylinder 4. - Hissing sounds during idle, fluctuating RPM, and smoke test revealing leaks at manifold joints.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Scan: Begin with a comprehensive OBD-II scan using GeekOBD APP to retrieve any stored codes. Pay particular attention to the P0304 code and freeze frame data for RPM at the time of the misfire. This will provide critical insight into engine behavior during the symptoms.
- STEP 2 - Live Data Monitoring: Use GeekOBD APP to monitor live data parameters, including fuel trims, RPM fluctuations, and ignition timing. Pay attention to any irregularities in fuel trims, which may indicate a rich or lean condition affecting cylinder 4.
- STEP 3 - Compression Testing: Perform a compression test on cylinder 4. Expected readings should be between 150-200 PSI. If the reading is below this threshold, further investigation is needed to assess the condition of the piston rings and valves.
- STEP 4 - Injector Function Test: Use a noid light to test the operation of the fuel injector in cylinder 4. If the injector is not firing correctly, it may need replacement. Verify the injector resistance as well, ensuring it falls within the 12-16 ohm range.
