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My 240 AT has this pattern in providing interior heat: down to ~25 deg F (outside temp) the heat is excellent eg. fan is off, air coming out of the heat vents is warm to the fingers, on a longer drive I move the heat-temp control to the middle position (no fan) and the car stays warm inside. Warm-up is fast and the defroster clears the windshield in ~ 5 miles.
At ~20 deg F there's essentially no heat eg. with the heat-temp control full on and even with the blower on high the air feels cool at best, doesn't warm up the interior of the car, won't defrost the windshield.
The engine feels the same temp in either case ie. I open the hood and touch the intake manifold, radiator, valve cover.
The temp gauge on most of my other 240s always did this (Winter & Summer): slow climb until the needle touches the mid-point mark; stability there for a bit; slight drop until there is a hair's distance between the top of the needle and the bottom of the mid-point mark.
This car has more fluctuation: the needle sometimes goes over the middle mark; sometimes 1/8" under; sometimes at the middle. In traffic or on hills it goes up; coasting it goes down.
If the thermostat were just sticking open I'd expect a gradual - more linear - heat loss over a wider range eg. decent heat at 40 F, fair heat at 30 F, no heat at 20 F.
It's not really a problem - we don't get many 20 deg days in OH - but it's so different from my other 240s that I'm curious.
All I can think of is a failing thermostat: at low temps the cooler coolant from the radiator causes it to stick. And or it could be worn or gunked up and sticking throughout its range - the gauge fluctuations are because it's slow to respond. And perhaps the colder coolant exacerbates this.
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240s: 2 drivers and some parts cars
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