Owner's Problem Description
The owner reported that the check engine light illuminated and the vehicle runs rough at idle speed. However, it performs well during regular driving and on the expressway without hesitation or sputtering. When parked, revving the engine to 4000 RPM results in sputtering. Previous actions included scanning for codes, which indicated a misfire in cylinder #2. The current condition suggests a need for further diagnostics on the ignition or fuel system.
Professional Technical Analysis
The 2007 Chrysler PT Cruiser, with approximately 120,000 miles on the odometer, is experiencing a rough idle condition with the check engine light illuminated. The symptoms are isolated to low RPM situations, particularly at idle, while higher RPMs during driving yield smooth operation. The misfire code for cylinder #2 suggests potential issues within the ignition system (spark plug, coil, or wiring) or fuel delivery (injector malfunction). Given the vehicle's age and mileage, wear in ignition components is common, particularly if the spark plugs have not been replaced in accordance with the manufacturer's recommended intervals (typically 30,000-100,000 miles depending on the type). The charging system should be assessed as fluctuations in voltage can also affect engine performance. During idle, the engine should maintain a voltage between 13.8-14.4V. A thorough examination of the ignition coil resistance (should be around 0.5-1.5 ohms), spark plug condition, and cylinder compression tests are recommended to determine the root cause. If these components are functioning correctly, the fuel system should be evaluated for proper pressure (ideally 35-45 PSI), and any air intake leaks should be checked, as they can lead to lean conditions that might cause misfires.
Possible Causes
Most common causes (ordered by frequency):
- Ignition Coil Failure: The ignition coil responsible for cylinder #2 may be failing, leading to weak or no spark. This can be tested with an ohmmeter, expecting a resistance of 0.5-1.5 ohms. A failing coil often shows signs of cracking or carbon tracking. Replacement with an OEM part (P/N 56028947AA) is recommended if found defective. - Check engine light for misfire code P0302, visible damage to ignition coil, poor spark plug condition.
- Spark Plug Fouling: Spark plugs in cylinder #2 may be fouled due to oil or fuel contamination, leading to inadequate ignition. Typically, a fouled plug will show signs of carbon buildup. Replacement is recommended every 30,000 miles unless otherwise specified by the manufacturer (OEM P/N 68252284AA). - Visual inspection reveals heavy carbon deposits on the spark plug, engine misfire symptoms.
- Fuel Injector Malfunction: A clogged or malfunctioning fuel injector for cylinder #2 can lead to inadequate fuel delivery, resulting in a misfire condition. Fuel injector should be tested for proper spray pattern and resistance (typically around 12-16 ohms). Cleaning or replacement may be necessary (OEM P/N 04891866AA). - Fuel pressure tests show inadequate pressure, injector spray pattern test indicates poor atomization.
- Vacuum Leak: A vacuum leak in the intake manifold can lead to a lean condition, causing the engine to misfire at idle. Vacuum levels should be checked and should ideally be around 19-21 inHg at idle. Use a smoke machine to detect leaks effectively. - Visual inspection reveals signs of cracked hoses, idle air control valve operation irregularities.
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 - OBD-II Scanning: Begin with a thorough OBD-II scan using GeekOBD APP or a professional scanner to retrieve all stored codes. Pay particular attention to misfire codes, such as P0302 for cylinder #2. Review freeze frame data for any additional context regarding the conditions under which the misfire occurs.
- STEP 2 - Visual Inspection: Inspect the ignition components, including the spark plug and ignition coil for signs of wear or damage. Check for loose connections or corrosion at the connectors. If the spark plug shows signs of fouling, replace it and inspect the ignition coil for proper resistance values.
- STEP 3 - Fuel Pressure Testing: Connect a fuel pressure gauge to the fuel rail to ensure the fuel pressure is within specification (35-45 PSI). If the pressure is low, further investigate the fuel injectors for clogging or malfunction. Ensure proper voltage is supplied to the injectors (typically around 12V).
- STEP 4 - Vacuum Leak Testing: Perform a smoke test to check for vacuum leaks in the intake system. Any leaks should be visually inspected and repaired. Use the GeekOBD APP to monitor fuel trims during idle to check for lean conditions that may indicate a leak.
