Your diesel should run between 80-85C. It if runs over 95C, it is not healthy for the engine. This might cool it down 5C unless you have major problems like cylinder head crack or leaking head gasket.
This OEM thermostat from another engine fits perfectly in the OM617 diesel engine. It does require a special rubber seal, which we include here. These engines spec an 85C thermostat, but if your engine tends to run hot ( you live in a warm climate or run the A/C a lot), this thermostat could lower your operation temp. about 5C. Remember that this may not work if you have a rusty block, bad fan clutch, partially plugged radiator etc.
1978 to 1985 5 Cylinder Diesel 75 C Thermostat for Better Cooling
$28.75
SKU:
DSL-75CTHERM 
Product Details
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$32.50
115 116 123 126 Diesel Thermostat Installation Kit
Fits 115 116 123 and 126 chassis diesel engines, non turbo and turbo, to 1985. This includes the 220D, 240D, 300D 300TD wagon and 300D/300SD turbo engines. Most thermostats only last a few years and should be changed for optimal diesel performance.