Hi Huw,

I map all my greys cars to idle around 4 - 5% CO. If your car is just running an off the shelf chip or one from another car, then it will only achieve that CO level if the engine is pulling at least -0.65bar at idle. If it is pulling less vacuum then it will be richer at idle than that.

I do not run greys car leaner than that at idle, because the spray pattern of a grey is very poor and due to the variation in idle fueling caused by heat soak and hot restart, a greys YB mapped leaner than 4 - 5% CO will at times idle poorly.