Owner's Problem Description
The owner reports that their 1999 Saturn SC2 is idling rough and burning a little oil. After performing a compression test, they found dry readings between 195 to 210 PSI, but significant increases to 270 to 300 PSI during the wet test. The owner is unsure about the reliability of these results and questions if too much oil was added during the wet test.
Professional Technical Analysis
The 1999 Saturn SC2, equipped with a 1.9L I4 engine, typically exhibits compression specifications of 185-205 PSI. The reported dry compression results suggest acceptable cylinder sealing, while the wet test results indicate a significant increase in pressure, pointing towards potential wear of piston rings. However, such a high increase (75-90 PSI) can also suggest overfilling oil during the wet test, which can lead to flooding and misinterpretation of results. Additionally, the owner's observation of rough idling and misfire codes specifically for cylinder 1 raises concerns about possible ignition system issues or fuel delivery problems. This could be exacerbated by the high compression readings, indicating possible carbon build-up on the valves or excessive oil consumption affecting combustion. Given that the vehicle only runs rough at idle and runs smoothly at higher speeds, the issue may be linked to airflow or fuel mixture problems, warranting a thorough inspection of the ignition components, including spark plugs and ignition wires, as well as a review of the fuel injector performance. The rough idle indicates a need for further diagnostics, potentially including a leak-down test to confirm the integrity of the cylinder sealing under operational conditions, despite the owner's concern about the necessity of such tests based on the compression results.
Possible Causes
Most common causes (ordered by frequency):
- Worn Piston Rings: The significant increase in wet compression readings suggests potential wear in the piston rings. If too much oil was added during the wet test, it could lead to misleading results by creating a seal that masks underlying wear. Typical wear patterns are evidenced by oil consumption and reduced performance. In this case, a compression reading increase of 75-90 PSI is unusually high, indicating that the rings might need inspection or replacement. A standard test should involve adding only a small amount of oil to avoid flooding, with expectations of a 10-20 PSI increase at most. - Noticeable oil consumption and rough idle at low RPMs, significant wet compression increase indicating potential ring wear.
- Ignition System Malfunction: The misfire code for cylinder 1 could indicate issues with the ignition coil, spark plug, or associated wiring. Since the owner has replaced the spark plugs and performed a continuity check on the wires, it’s essential to verify the ignition coil's resistance (typically 0.5-1.5 ohms for the primary winding) and the secondary winding (around 10,000-15,000 ohms). A failing ignition coil can lead to poor spark energy, especially noticeable at idle, causing rough running and misfires. - Misfire code for cylinder 1, continuity checks showing normal resistance on wires, and replaced spark plugs.
- Fuel Injector Issue: The fuel injector for cylinder 1 might not be delivering adequate fuel due to clogging or electrical faults. The measured resistance of 12 ohms is within specification, but it’s crucial to check the injector's flow rate and pulse width. A clogged or malfunctioning injector can disrupt the fuel-air mixture, particularly at idle, leading to rough operation. - Normal resistance on the injector, potential rough idle indicating inadequate fuel delivery, especially at lower RPMs.
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: Use a professional OBD-II scanner or GeekOBD APP to retrieve any stored trouble codes and monitor live data. Check for misfire codes and related fuel trim values. This will provide insight into whether the rough idle is related to a specific cylinder or a broader engine issue.
- STEP 2 - Compression Testing Verification: Review the compression test results and ensure that the proper procedure was followed. Confirm that only a small amount of oil (1-2 teaspoons) was used in the wet test. If a significant increase in compression is observed, consider inspecting the piston rings and cylinder walls for wear patterns. A follow-up leak-down test can be performed if compression issues are confirmed.
- STEP 3 - Ignition System Inspection: Inspect the ignition coil and associated wiring for any signs of wear or damage. Measure the resistance of the ignition coil to ensure it meets specifications (0.5-1.5 ohms primary, 10,000-15,000 ohms secondary). If the ignition coil is found to be faulty, replace it with a quality OEM part (e.g., AC Delco 19119334).
- STEP 4 - Fuel Injector Function Test: Test the fuel injector operation using a noid light or by measuring the fuel delivery rate. If clogging is suspected, consider cleaning or replacing the injector (OEM part number 217-3009). Make sure to check for any fuel leaks after reinstallation.
