If your loosing coolant...then you need to find out where its going. Sometimes the head gasket just cracks slightly internally. In that case what happens is it seeps into one of the cylinders and sits on top of the piston. So when you start the car it will smoke alot of white smoke and you can smell the antifreeze...but it gets burnt out of the cylinder and shows no sign of a leak. I had this problem with a 4.0 litre ford explorer...and I was loosing coolant ever so slowly and could not track it down until it got worse. When I finally removed the heads, I found it was not just one head but in fact both heads had cracks between the intake and exhaust valve ports. The gasket was split between the ports and it was leaking down into the cylinder and was being burnt. What I am saying is...that your coolant is not evaporating...your loosing it somewhere, and a pressure test would be a good start, as well as a inspection of the plugs and wires. Would your O2 sensor cause your problem?> Im not sure. I would also remove the throttle body...get a new gasket and make sure your egr port onthe rear of the throttle body is not plugged..that woulld cause a rough start up, but would not explain where your coolant is going.
