Evo 8 idle problems

[Rob197]

Hi I'm needing a bit of help. I have a Evo 8 MR fq340 and it idles fine until I drive it hard or on full boost. If I do that the car wants to idle at 2000 which makes slowing down for roundabouts quite fun..
If I wait 5 mins and turn it off and back on again the idle is fine. ICV tested and its working but I've fitted a new one just incase problem was intermittent. Thanks

[GreenV8S]

Does the high idle remain until you switch off?

[PositronicRay]

Any codes recorded? Purge valve sticking open?

[Rob197]

Yea idle stays at 2000 until I turn it off

[GreenV8S]

If that was me, I'd try to work out whether the idle was being raised by the normal idle speed control mechanism. Do you know what method that engine uses for idle speed control? I guess use either throttle-by-wire, or an idle air control valve. If it's an IACV, I'd try to find out how it's controlled. If it's PWM, you could use a scope or smart meter to monitor the duty cycle and see whether the ECU is raising the idle deliberately. If it's stepper motor controlled, disconnect when the car has established a hot idle and see whether that solves the problem.

If it isn't the idle speed control, you might be looking for things that can physically prevent the throttle from closing, or possible air leaks from PCV check valves, EGR systems, carbon canister purge valves and so on.