Feel your hoses, it can tell you a lot. Quick touch, let go, then slightly longer touch, let go, before it gets uncomfortable. You will not be able to measure temperature accurately by touch, but you can tell if one hose is hotter than another by how long you can your keep hand on them.

Feel the in/ out hose on the monovalve as well as the hoses into the firewall to/from the heater core. They should be similar in temperature. If not, monovalve may be blocked or heater core may be blocked. At full heat, monovalve/ heater core hoses should be less hot than the upper radiator hose (because it is further away from it), but not by a lot. If it is very cool in comparison, you may have some coolant flow issues.

I find it very useful to start the engine cold and feel all coolant hoses as the engine warms up, which shows you the flow path.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked