On my 3.0 I had a head gasket leak that put exhaust gases into the cooling system causing it to over pressurize resulting in a cracked coolant overflow tank. Never really experienced smoke at the tailpipe.

The Napa blocktest should reveal any gas but in my case the test was negative at idle but definitely getting exhaust in. If you do the test have a helper rev the engine even put it in gear and rev with the brakes on. Truer test. Be careful though.

Also, if exhaust is in the cooling system the radiator hoses will be tight and would maybe not “relax” once cooled. (Mine would stay tight for more than 24 hrs even after just a short drive.)

An unreliable test is to look under the oil filler cap for yellow goo (oil and coolant mixing). Pretty common if you only make short trips or live in cold climate but if all you see is oil residue and no “mayo” you’re probably good.
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