I recently purchased a stock 2006 EVO 9 with 15k miles. I'm sure the car has been run hard but its an EVO so I'm not all that concerned. However I do have an issue I need help troubleshooting.

When the car is cold and I start it the exhaust note sounds like an exhaust leak. Also when I blip the throttle it doesn't seem snappy. It feels like its not firing cleanly. I have done a boost leak check and the car is buttoned up tight.

The strange sound goes away after the car is warm but when the car is warm and its parked idling the entire cabin shakes and the exhaust sounds like a WRX. The idle almost sounds like its cammed but the car is not cammed.

Off idle performance is wonderful. It simply rips and is just as quick as a similar EVO 9.

I have have a wideband and my at idle AFR is 14.2 which seems rich. I've pulled the plugs and they all look normal. They are the stock 15k miles plugs.

I drained the oil and inspected for coolant as well as metal flakes. None. I've drained the coolant and looked for oil. None.

I've reflashed the car with a different rom and I've reset the battery. No change.

Additionally if I don't turn the code off I will get a random misfire at idle. This happens only when its cold as I have a creader to clear it and after the car is warm or on the highway I do not get that code. Also today I got a 0171 when at the drive through. I believe this suggests the front O2 is bad but I'm not sure why that would be. I did log it and it was shifting back and forth indicating life but I'm not sure if there is a specific voltage to watch or not.

So. I'm stuck. Do I need to replace ignition? Plugs? Wires? O2? Do you suspect some kind of head damage? Help!

 

Someone must have a suggestion.

Do you think the front o2 would cause this?

 

If your getting a code for the front O2 then you should change it. Its pretty cheap.

What are the mods done to the car?