2001 Saturn SC 1.9L 4-Cylinder High Idle Issue: P0118, P0507, P0301 - Diagnosis & Fixes

Are there any current error codes displayed on your OBD2 scanner? If not, it might indicate the issue is intermittent or related to sensor calibration.

I only currently have P0301 (cylinder 1 misfire). I swapped spark plugs between cylinders 1 and 2. The plug wires are relatively new, and the idle behavior remains consistent with previous issues. Thanks for the input.

P0301 appears to be the primary concern here. I recommend following the official P0301 troubleshooting guide and reviewing live data and long-term fuel trims (LTFT). If the engine is running lean—especially with LTFT values above +15%—a vacuum leak could be causing excessive idle speed.

I'm working on resolving P0301, but I can't pass emissions testing due to the persistent high idle at 1200 rpm. I've sprayed water across all hoses and intake components while running—no change in RPM. This suggests a non-vacuum-related root cause.

Regarding vacuum leak testing, instead of using water, try spraying propane or another combustible substance and listen for an RPM increase or monitor LTFT changes. If the engine has a misfire code, emissions may be excessively high due to incomplete combustion. Also, did you replace the IAC valve? If not, cleaning or replacing it is a common fix for high idle issues. You should use a scan tool during troubleshooting to get accurate live data and freeze frame readings.

I cleaned the IAC valve and replaced the EGR—now the engine idles at 2000 rpm instead of dropping to 1200. I don’t have a scan tool, only a code reader. The P0301 has cleared (hopefully), but now I’m seeing P0507. No vacuum leaks detected when spraying carb cleaner on the intake and TB area. I may need to give up soon—this is becoming very frustrating. Thanks.

P0507 (high idle) could be caused by a sticky or malfunctioning IAC valve. After cleaning, the idle remains at 2000 rpm. No other error codes are set post-IAC replacement. You didn’t disconnect the battery after replacing the IAC—this may have prevented ECU relearning. Try disconnecting the negative battery for 1–2 minutes to clear stored values and allow the ECU to recalibrate idle settings. This process might need multiple drives to stabilize. Also, try disconnecting the IAC electrical input while running: if RPM stays high, it suggests a faulty valve or wiring. Was there an EGR-related code in the past? That could be relevant.

I disconnected the battery and restarted—no change in idle behavior. The IAC is new, so the engine idles at 2000 rpm in park and drops to 1200 when in gear. Previously, it would rise to 2000 then drop; now it stays high in park. I’ve tested with a propane spray—no RPM change. P0507 has returned. The EGR was replaced too. When driving at 30 mph on flat ground, the engine holds that speed smoothly—like cruise control—which puts significant strain on brakes. Thanks for the help.

The P0507 code is typically triggered when actual idle RPM exceeds the desired setpoint by more than 200 rpm. This can stem from multiple causes: faulty IAC, incorrect throttle linkage, high fuel pressure, TPS misreading, or even a weak generator output. Without live data, diagnosis becomes difficult. Check long-term fuel trims (LTFT)—if they are significantly lean (>+15%), it may point to vacuum leaks or poor combustion. Verify that the IAC valve is properly connected and that all connectors (MAP, TPS) are secure. A full IAC relearn procedure should be performed after replacement. At normal operating temperature, slowly cycle from 3500 rpm down to idle three times to allow proper calibration. Did you adjust or check the throttle stop screw during installation? Also, ensure the throttle body is fully seated and not obstructed. Fuel pressure, generator output (check with a voltmeter), and TPS signal readings should be evaluated—use a multimeter to test TPS voltage. A high TPS reading could cause excessive idle. Consider using propane for vacuum leak testing instead of water. Are you noticing unusual fuel consumption or increased fuel usage?

P0507 has returned—now with a sustained 2000 rpm idle. The throttle butterfly is fully closed and seated correctly at idle, resting on the throttle stop screw. All connectors (MAP, TPS) are properly reconnected. I’ve completed the IAC relearn procedure—no improvement in idle behavior. Thanks for the guidance.

If you can access a scan tool, it would significantly improve troubleshooting. While Autojoe may be able to guide you through step-by-step diagnostics, live data and freeze frame readings are essential for identifying root causes—especially for P0507 and idle stability.

Have you performed a test drive above 40 mph? If not, try it. Also, disconnect the negative battery terminal for 10 seconds and reconnect to reset ECU memory values. This can help trigger relearning of idle settings.