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2002 Chevrolet Malibu High Idle with P0122 Code - No Check Engine Light but RPM Spikes to 2000

Model: 2002 Malibu Fault Code: P0122 Posted: 2018-07-24 17:07

My 2002 Chevrolet Malibu starts normally, but after driving about 15 miles, the engine idle will suddenly spike up to around 2000 RPM. Pressing the gas pedal several times while the car is off usually brings it back down to a normal idle. I've already cleaned both the throttle body and the intake air control (IAC) valve, and replaced the IAC unit—no improvement. There's no Check Engine Light (CEL) on, but my scanner shows an old P0122 code stored in the system. This has been consistent over 8 years of ownership with no prior issues. The car runs fine otherwise. I'm wondering if this is a sensor issue or a wiring problem related to the throttle position sensor (TPS), especially since the TPS data seems inconsistent during high idle events. Any insights on diagnosing or fixing this behavior?

Related fault codes
P0122
Comments (9)
Anonymous 2018-07-24 18:44

What scanner are you using? Can you monitor live TPS data in real time to see how it behaves at different idle states?

Anonymous 2018-07-25 05:14

I'm using an Autel ML629. I'm still getting familiar with its features—I'll check the live TPS readings during testing.

Anonymous 2018-07-25 05:32

Yes, my scanner supports real-time data monitoring and logging of key performance parameters.

Anonymous 2018-07-25 06:48

During a recent road test, the check engine light came on during the bulb test but turned off when I started the engine. After driving about 12 miles, RPMs only reached around 1700. At idle, TPS showed 0%. After letting it run at 1700 RPM with foot off gas, the TPS signal increased from 0% to 0.4% before dropping back to 0%. The throttle body appears closed during high idle, and both throttle cable and cruise cables are free with no tension issues.

Anonymous 2018-07-25 12:10

I don’t have the ability to upload or share a full data log with all PIDs. However, I can provide specific readings if needed.

Anonymous 2018-07-25 19:52

Can you send or post a complete data log? Ideally one that includes all available PID values for accurate analysis?

Anonymous 2018-07-25 21:51

No, I currently don’t have the capability to generate or share full data logs.

Anonymous 2018-07-26 06:31

The TPS percentage is directly tied to the voltage signal sent from the TPS to the PCM. If the throttle plate is fully closed, you should verify that both the 5-volt reference and ground connections at the TPS connector are secure. While a 0.4% reading shouldn’t cause a large RPM jump, it may point toward an underlying issue in the sensor or wiring.

Anonymous 2018-07-26 14:33

With ignition on and engine off, I measured 4.98 volts at the TPS connector—close to 5V—but couldn’t confirm a solid ground connection (may have been done incorrectly). After a 40-minute drive test, idle spiked to 2000 RPM. When I disconnected the TPS connector during operation, idle jumped to 3000 RPM. With engine running, the voltage remained at 4.98V. Reconnecting the TPS didn’t restore normal idle—idle stayed at 3000 RPM even after shutdown and restart. However, when I disconnected the TPS and started the car again without driving, it idled normally. This suggests a possible intermittent signal or grounding issue in the TPS circuit—though I’m not fully confident in my interpretation of this behavior.