Car: 2007 FN2 GT
Join Date: 24th March 2010
Thanked 1,943 Times in 1,635 Posts
Doesn't matter what speed your doing, if your in Neutral and you are pedal free, you should be on either low or high idle based on engine coolant temp - period.
That is not normal fella, my first train of thought would be to look for an airleak between the MAF and the primary o2 sensor in the exhaust. So a split intake hose, a loose inlet manifold, split intake mani gasket, split breather hose, split exhaust somewhere etc,
Basically anything that causes an airleak and confuses the correlation between the MAF reading and the Air fuel ratio read by the Lambda probe.
It could also be a sensor fault, get the car plugged in and scan for stored codes - possibly a screwed TB too.