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1998 Chevrolet Grand Prix P0507 Code: Idle Control System RPM Higher Than Expected

Model: 98 Grand Prix Fault Code: P0507 Posted: 2007-07-19 07:52

I'm experiencing a P0507 diagnostic trouble code on my 1998 Chevrolet Grand Prix, which indicates the Idle Air Control (IAC) system is not maintaining an appropriate idle speed. My scan gauge shows that the IAC valve is actively moving when the accelerator is pressed — the higher the throttle input, the greater the reading. However, when the pedal is fully released (foot off), the IAC reading drops to zero at 2200 RPMs, which is significantly above normal idle speed. I've already performed a thorough vacuum leak check and confirmed that TPS and foot-off sensor readings are showing 0% under idle conditions. The car idles around 2200 RPM when cold, and attempts to 'tap' the throttle to lower it result in an increase rather than a decrease. I have cleaned both the IAC valve and the throttle body, and inspected the air intake snorkel for splits or damage — no issues were found. A carburetor cleaner test on the upper engine components also revealed no significant changes in idle behavior, suggesting no major vacuum leaks are present. My question is: what could be causing this persistent high idle despite a clean IAC and throttle body? Is it possible that the IAC motor itself is faulty or not receiving proper signals from the Powertrain Control Module (PCM)? Any insights would be greatly appreciated.

Related fault codes
P0507
Comments (8)
Anonymous 2007-07-19 09:04

Does the vehicle physically idle higher than its expected idle speed under normal conditions?

Anonymous 2007-07-19 10:24

Yes, it runs at approximately 2200 RPM when cold and stays high even when I try to gently tap the throttle — in fact, it increases instead of decreasing.

Anonymous 2007-07-19 13:25

A 2200 RPM idle in park is a strong indicator that either the Idle Air Control (IAC) valve or its electrical circuit is malfunctioning. Have you tried cleaning both the IAC valve and throttle body? Also, check for any cracks or splits in the air intake snorkel or vacuum lines.

Anonymous 2007-07-19 13:30

Yes, it idles at 2200 RPM when cold. I inspected both the IAC valve and its seat on the throttle body — they were clean. I also used carb cleaner to test the upper engine components for vacuum leaks; no noticeable changes in idle speed occurred, so there are likely no significant vacuum leaks.

Anonymous 2007-07-19 13:31

What happens if you disconnect the IAC valve while the engine is running? Does the idle behavior change?

Anonymous 2007-07-19 13:32

No — after disconnecting the IAC, the idle remains at 2200 RPM and does not drop or stabilize.

Anonymous 2007-07-19 15:55

If the IAC is disconnected and idle remains high, this suggests that either the PCM isn't sending proper control signals to the IAC, the IAC motor itself is faulty, or there's an underlying issue preventing the system from regulating idle properly. High-quality no-idle light kits with IAC pulse indicators can help verify whether the PCM is actively pulsing the valve. If not, a digital multimeter (DVOM) and a wiring diagram are needed to test the circuit continuity and signal voltage between the IAC and PCM.

Anonymous 2007-07-19 16:50

I considered tracing the wiring from the IAC to the PCM, but heavy rain prevented me from doing so today.