Idle Air Control System RPM Higher Than Expected

The PCM has an idle target — usually 600–800 RPM warm. It controls idle by moving the throttle plate (or on older stuff, an IAC valve) to let just enough air in. When the engine is idling higher than target and the PCM has already commanded the throttle fully closed, that means air is getting in somewhere it shouldn't. Almost every P0507 is a vacuum leak. Could be a torn intake boot, a cracked PCV hose, a stuck-open EVAP purge valve, or a brake booster diaphragm. The PCM is basically saying 'I'm not letting this air in — but it's getting in anyway.'

Actual idle RPM exceeds desired idle by ~200 RPM (varies by manufacturer) with throttle position at or near minimum and no driver input.

Don't just throw parts at it. Walk through these in order — each step tells you whether to keep going or stop and fix what you found.

1. 1Look at long-term fuel trim at idle. A vacuum leak shows positive LTFT (lean) at idle that drops to near zero at 2500 RPM. That pattern alone narrows it down fast.
2. 2Pinch off the EVAP purge line. If idle drops to normal — purge valve is stuck open. Replace it.
3. 3Smoke-test the intake. Watch around the intake manifold gasket, PCV system, throttle body gasket, and brake booster hose.
4. 4On drive-by-wire, check commanded throttle position vs actual. PCM should be commanding minimum. If commanded matches actual at minimum and idle still hangs high — air is leaking past.
5. 5Do a throttle body relearn after cleaning. Many vehicles need a procedure or the idle won't come back into spec.
6. 6Brake booster trick: with engine idling rough/high, pinch off the booster hose. If idle changes — booster diaphragm is leaking.

• Vacuum leak — intake manifold gasket, PCV hose, brake booster, intake boot
• Stuck-open EVAP purge valve (very common)
• Dirty / carboned throttle body needing relearn
• Failed PCV valve stuck open
• Cracked intake plenum (common on certain V6/V8 plastic intakes)
• Failed IAC valve (older non-DBW vehicles)

Purge valve $40–$150. Vacuum hose $20–$80. Intake gasket $200–$600. Throttle body clean/relearn $80–$200.