have a 04 grand am 3.4 which keeps overheating and losing cabin heat. flushed and backflushed heater core, rad, motor, and all lines and hoses. pressure tested for a leak cause it keeps losing fluid, held 30 psi for 45 mins. I fill it, purge air, topoff, and let it idle. temp never goes over 210 while idling. I get heat again in the cabin, but after driving maybe 30 mins or so, loses cabin heat again and starts to overheat. i go and check coolant and it's low again, but not always. I keep getting air from the bleeder but have no clue how it's getting in there. no coolant on passenger floor. no coolant on ground. oh, and both cooling fans work. yes both high and low speeds work. smelled the exhaust, has NO hint of coolant to it. oil not foamy. trans not contaminated. what am I missing? almost forgot, t-stat is new and so is cap for degas bottle. *update* just used a exhaust gas tester and fluid in tester stayed blue. added a pic of the stuff I got out one of the small coolant lines on top of the degas bottle.
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try a chemical test to rule it out. They are simple tests and will detect exhaust gases in the cooling system.