If this were my car, this is how I would diagnose:

1. With a cold engine, remove hose coming out of cyl head (by the oil filter housing) and route it into a 2 gallon bucket. Start engine and measure output/ time.

2. Step one was done at the heater inlet. Next, do it at the heater outlet. Replenish coolant, reconnect hose at filter housing and disconnect hose to monovalve and take the same measurement of output over/time. Make sure the time is the same for both tests.

Output volume should be the same for both tests. If output 2 is less than output 1, heater is clogged.

By comparing outputs to an engine with no heat issues and if the output is less than a healthy engine, then do more diagnosis. It could be worn water pump, clogged engine coolant passages, defective thermostat, clogged radiator.

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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