Gray smoke from rebuild engine

Oil buring is blue smoke, white smoke for antifreeze, and black for too much gas. Gray could be a mix of too much gas but not so much to be black, any codes on the PCM? I'm thinking you are in open loop after warm up(very rich). Can you scan to see what mode it's in? If your scan tool can look up fuel trims and IM readiness it will reveal if this is correct info. Fuel trims will be a high positive number. IM will state what mode you are in.