Tom: Why do you think you have a head gasket problem? I,ll assume you have a 4.0 L6. By the way, to anybody posting a question: Wouldn't it be nice to give at least make, model, year and engine size? Anyway, I,ve seen quite a few of these get an exhaust manifold leak- a crack where the 6 runners converge into one. Fumes could be getting through heat/ac system; coming out vents. If you get
it checked out, make sure mechanic can check it COLD- (takes 8 hr. for a warmed up engine to cool down). Also, make sure mechanic checks for leak as soon as he gets it up in the air 'cause leak may close up as engine warms. Unfortunately these manifolds are really expensive new- maybe go junkyard- if this is your problem. Thomas- you have an UNSAFE condition here, no matter how much you try to take precautions.
Get this problem solved ASAP. Let us know what upshot is 'cause I'm curious as to whether my guess is right or not . Good luck. P.S: Keep heater/ac on recirc- even if you don't use it. And please forget about this miracle liquid crap- no offense. This stuff could clog up important orifices in engine.