I have a 2007 AP2 S2000. It's drive by wire. When I start the car in the morning after it's been sitting all night, as soon as it starts the idle fluctuates back and forth between about 1200 rpm and 1700 rpm about every second. If I press the throttle, it fluctuates at a higher rpm at about the same rate, After several seconds of doing this, I can hear a distinct click noise (from the engine compartment?) and the engine idles steadily for a short period and then it reverts back to the abovementioned fluctuating idle. This can happen several times the engine returns to a normal idle. Afterwards the car idles and responds normally. This problem doesn't occur unless the car has not been started for many hours or longer - cold engine. I had the battery removed from the car for an extended time before this problem occurred, but I performed the idle relearn procedure specified in the service manual recently and it hasn't helped with fluctuating idle when the car is started cold. As a related question, I'm curious how the idle (especially the cold idle) is controlled on the DBW S2000.

A final note - The car previously had a severe loss of coolant and blown head gasket. Could this problem be related to the front O2 sensor being coated with coolant - reads wrong when cold but somehow after brief warm up, it reads correctly?

 

You are getting no CEL codes?

 

I have a Hondata Flashpro and I'm seeing no CEL codes.