Yes my 1991 560 SEL is also doing the same thing as yours. The heater is not putting out any heat. My temp gage was only reading about 40c and I thing it should be at 80c.

For a temporary fix I just blocked off the airflow to the radiator with a large piece of cardboard and that helped raise the engine temp a enough to get a little heat from the heater.

I just started to replace the thermostat and broke a bolt off in the water pump so now it has to be replaced also.

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Gerry

59 Ranchero
83 928 Euro S
84 928 Euro S
87 300D Turbo-Frybrid "WVO, Tastes Great - Less Filling"
90 750 IL
91 560 SEL