Owner's Problem Description
My stepdaughter's Saturn is experiencing a rough idle when in gear, just above the stall point, and the SES light is illuminated. Codes retrieved are PO133 (O2 sensor), PO442 (EVAP leak), and PO303 (cylinder 3 misfire). The car runs fine above idle. When shifting from Drive or Reverse to Park, the RPMs spike unusually high. At startup, RPMs also rise significantly, possibly a choke issue. A split hose from the cam cover to the air intake was repaired, improving park idle slightly but not in gear. With budget constraints, I need guidance on the next steps for repair.
Professional Technical Analysis
The vehicle in question is a Saturn experiencing rough idle conditions specifically when in gear. With the current mileage and history unknown, the symptoms suggest an underlying issue with the fuel and air management systems, possibly exacerbated by vacuum leaks. The presence of the SES light and the codes PO133, PO442, and PO303 indicates multiple concerns: the first code points to a failing oxygen sensor which can affect fuel trim, the second code suggests an EVAP system leak that may influence engine performance, and the third code reflects a misfire in cylinder 3. The misfire can contribute to rough idle, particularly under load when the vehicle is in gear, as the engine struggles to maintain combustion efficiency. Additionally, the high RPMs encountered when shifting to Park and during startup may indicate an issue with the throttle body or choke system, further complicating the idle stability. The split hose from the cam cover to the air intake likely contributed to the rough idle, as it could lead to unmetered air entering the intake, affecting the air-fuel mixture. Current vehicle condition warrants immediate attention to the oxygen sensor and the EVAP system, as resolving these could alleviate the rough idle significantly.
Possible Causes
Most common causes (ordered by frequency):
- Oxygen Sensor Failure (Code PO133): The oxygen sensor may be malfunctioning, leading to incorrect readings of the air-fuel mixture. This could cause poor fuel trim adjustments and contribute to rough idle. Typical voltage output for a functioning O2 sensor should oscillate between 0.1V to 0.9V under normal operating conditions. If the voltage is steady or outside this range, replacement is necessary. The sensor, commonly OEM P/N 12570163, typically fails after 100,000 miles or due to contamination. - SES light illuminated, abnormal fuel trims, poor engine performance during idle.
- EVAP System Leak (Code PO442): A leak in the evaporative emission control system can allow unmetered air into the intake manifold, disrupting the air-fuel mixture. Common sources include cracked hoses, faulty purge valves, or leaks at the fuel tank. A smoke test should be performed to locate the source of the leak. Repairs can be simple and often cost-effective, with hoses typically costing between $20-$50. - Visual inspection shows cracked hoses, SES light present, potential air leaks.
- Cylinder Misfire (Code PO303): Misfire in cylinder 3 could be due to several reasons such as faulty ignition components (spark plug, ignition coil), fuel delivery issues, or compression problems. A resistance check of the ignition coil should be performed (should be within 0.5-1.5 ohms), and the spark plug should be inspected for wear. If either component is faulty, replacement with quality parts like OEM spark plug P/N 12621258 is recommended. - Rough idle in gear, specific cylinder misfire symptoms, spark plug condition check.
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 Code Retrieval: Begin with a thorough scan using an OBD-II scanner or GeekOBD APP to retrieve and analyze all stored codes. Focus on the codes present (PO133, PO442, PO303), and examine freeze frame data to understand the conditions when the codes were set. This provides insight into the vehicle's behavior during the symptoms.
- STEP 2 - Visual Inspection: Conduct a thorough visual inspection of vacuum lines, particularly around the intake area, to check for any leaks or cracks. Inspect the O2 sensor and its connections for any signs of damage or corrosion. Check the condition of the spark plug in cylinder 3 and the ignition coil resistance to rule out ignition issues. Use the GeekOBD APP to monitor O2 sensor readings during this inspection.
- STEP 3 - Functional Testing: Perform functional tests on the EVAP system by conducting a smoke test to locate any leaks. Simultaneously, monitor fuel trims and O2 sensor performance with the GeekOBD APP during these tests to understand how the vehicle reacts under load. This can reveal discrepancies in fuel delivery or air-fuel mixture adjustments.
- STEP 4 - Road Test: Conduct a controlled road test while monitoring live data using GeekOBD APP. Pay attention to the RPM behavior during gear shifts and the overall engine performance. Document any abnormalities in fuel trims, ignition timing, and any misfire symptoms experienced during the test drive.
