2008 H6 LL Bean Overheating / Coolant System Problems

My ’08 H6 is going into service next week and hoping for a few more opinions before they diagnose some weird coolant system issues. The transmission has shrunk from a random incident 12 months ago (the trans coolant line nipple was cut by the rad and dumped my fluid). Otherwise, the car has been the dream I hoped it would be. About 135k miles.

Road trip from BC to California and back this summer. The car would overheat after ~2 hours of continuous highway driving (in red, dash warning light). Now I’d be driving about 80 mph, AC on high, 100 degrees outside, but we’re talking flat roads. I wouldn’t be surprised to see that over a mountain pass… this happened 2-3 times during the trip, and I noticed the rad overflow tank was VERY full when overheated, almost blown, but no sign or smell of anything leak under the hood. Just a lot of heat. Rad is new last November.

The problem gets worse – the car now often goes over or near red in short bursts. Sometimes it swings from medium to red within minutes, back and forth. The main thing is, if I start the car when it’s already hot and try to turn on the cabin heat, it blows chilly. It may pass in hot air in 10-15 minutes or it may not. Rad fans will constantly move to high levels in fall weather from short trips.

Could this all just be a T-stat issue? I suspect it’s the HG because there is virtually no oil loss, 1–2 cups between changes (4000 miles give or take), less than any other Sub I’ve known. T-stat, water pump, timing chain etc are all new with no service plans as it’s an H6… but I’m thinking now that the t-stat + water pump probably have a service interval of around 150k? The main thing is that the local shops only have Gates stats, and I’m pretty sure I don’t want to trade with one of them.

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