Very first thing to check is the color of the coolant, often it's muddy and do won't transfer heat well .

The next thing to do is a proper cooling system cleaning, use Citric Acid and water, no need to buy the $pendy Mercedes Citric Acid, it's cheap at baking supply stores or on E-bay etc.

Once the system is clean your heating issues should end but very often lack of proper maintenance over the decades meas corrosion between various alloy parts and rubber or the engine proper .

Any visible cracks in a hose, no matter how tiny, means the hose is kaput and time for a new one .

The thermostat uses an O-Ring to seal not a gasket .

Quite often when there are coolant leaks the two facing machined surfaces are corroded, you can repair them by polishing the pitted surface against a piice of window glass using 1,000 or finer wet/dry abrasive sheet .

Last time I checked Mercedes still sold all the OM616 hoses and alloy housings your 240D might need, better than repairing the old pitted ones but you might now have the $ to buy new .

Remember : any time the temperature gauge begins to rise, you can and should turn the heater on ~ you needn't turn on the fan if the hot air from the dash vents bothers you but the heater core is a good little radiator but itself .

NEVER run the AC if the engine's to hot

__________________
-Nate
1982 240D 408,XXX miles
Ignorance is the mother of suspicion and fear is the father

I did then what I knew how to do ~ now that I know better I do better