Your 2021 Buick LaCrosse used to idle quietly and smoothly. Now there's a noticeable vibration, and the check engine light reveals P0507—idle speed higher than expected. While high idle might seem like it would run smoother, the accompanying engine management struggles often create vibration and rough running.
What P0507 Indicates
P0507 means the PCM has detected that idle speed is higher than the target RPM by a significant margin. The system expects idle around 650-750 RPM (varies by condition) but is seeing speeds 100+ RPM higher. The PCM is trying to bring idle down but failing.
Why High Idle Causes Vibration
The engine vibration with P0507 typically results from the PCM's compensation attempts rather than the high idle itself:
- Erratic spark timing adjustments
- Rapid fuel trim corrections
- Throttle hunting as the system tries to reduce speed
- Uneven combustion from inconsistent mixture
Common Causes
Vacuum Leaks
Unmetered air entering past the throttle body increases idle speed. The PCM sees the air (via MAP/MAF) but didn't command it through the throttle. Common leak sources include intake gaskets, vacuum hoses, and PCV components.
Throttle Body Problems
A stuck-open throttle blade, carbon buildup preventing complete closure, or throttle position sensor issues can prevent proper idle control.
Idle Air Control Issues
On engines using separate idle air control, a stuck-open IAC or problems with the bypass circuit raise idle speed.
Faulty Sensors
A failed coolant temperature sensor reporting cold engine constantly can cause elevated idle as the PCM enriches for "cold" operation.
Intake Manifold Gasket Leak
A leaking intake manifold gasket is a significant vacuum leak source.
Diagnostic Steps
- Monitor actual idle: Compare actual RPM to target RPM using a scan tool. Verify P0507 condition exists.
- Smoke test: Introduce smoke into the intake system to identify vacuum leak locations.
- Throttle body inspection: Check for carbon buildup preventing full closure. Verify throttle position sensor readings.
- Sensor check: Verify coolant temperature sensor reports correct temperature.
- Listen and spray: Spray carburetor cleaner around intake connections while listening for RPM changes indicating leaks.
Repair Solutions
Vacuum Leak Repair
Identify and seal or replace leaking components. Cost: $100-$400 depending on leak location.
Throttle Body Cleaning
Clean carbon from throttle bore and blade. Perform idle relearn procedure. Cost: $100-$200.
Intake Gasket Replacement
Replace leaking intake manifold gaskets. Cost: $300-$600.
Sensor Replacement
Replace faulty coolant temperature or other sensors affecting idle. Cost: $100-$250.