It all points to something wildly altering the the fuel/air mixture causing the engine to smoke and even backfire. The only thing that makes sense is the vacuum thats created due to engine braking, that vacuum is sucking either extra air, fuel, or oil into the combustion, until there is enough positive pressure to bring everything back to equilibrium the engine does weird stuff. On that theory, it somehow has to make it into the top of the cylinder since thats where the combustion takes place, at least on DI engines like the 1HDT, therefore it makes the most sense that it has to come from the crank case, which is full of oil. My theory is that oil is somehow being introduced, which explains the white smoke and the backfire if its really bad.
As I've said early, I've had the same symptoms, without the backfire, and after a head job, where I replaced the valve seals and head gasket, and changed nothing else with the engine, it no longer does it. Doubt the head gasket had any affect, so what I concluded were the valve seals allowing something to enter the combustion chamber when under vacuum. Are you positive the valve seal were replaced with the head job, or replaced correctly? I know it sucks to have to question that sort of thing, but stranger things have happened.
As I've said early, I've had the same symptoms, without the backfire, and after a head job, where I replaced the valve seals and head gasket, and changed nothing else with the engine, it no longer does it. Doubt the head gasket had any affect, so what I concluded were the valve seals allowing something to enter the combustion chamber when under vacuum. Are you positive the valve seal were replaced with the head job, or replaced correctly? I know it sucks to have to question that sort of thing, but stranger things have happened.
