Before when I had a bad t-stat, yes I had heat just not intense heat and temp was just below 80, as it is now. But now the only way I get heat, when it drops out, its to recirculate air and have the fan on low. Then I get some heat.

Oddly now when its below 80, I get better heat than last year when heat does work. So its not the same "formula" as last time with a bad t-stat. Also before it would take a while to get heat. Now I still get heat quickly. It just eventually goes away, particularly on the highway.

As for milage, all summer I was getting 18 or more easily. My goal was 22mpg highway only. Got that up to 20 at least. But average was always near 19. I know winter will kill economy, but the first half of winter I was doing just above 18 and seldom in the 17s.

As for climate system, the mono valve is sure flooded with coolant, but when heat dies both the lines and windshield heater lines are just warm, not hot like the washer tank lines.

I think my biggest concern is overall engine heat. It has to be a clue. If engine heat was normal, just above 80, then I would suspect something clogged or air trapped. But the cooler running engine has to be a hint at something.


EDIT: note, I do not have a viscus fan install and have not for months now. Its sorta stuck on all the time, so I took it off this winter. I wonder if the new expansion tank cap is the issue. I dont think these issues started with a new one exactly, but when I remove the new one it will launch off the tank! The only one did not vent much air when the engine was hot. So the new one certainly is holding a lot more pressure. The new one is rated 20lbs and it some aftermarket brand. I did order an OEM unit that should be here next week.

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1991 560 SEL / 185k miles
1992 750il / 17k miles - project car