If your engine is taking too long to heat up or is constantly running "hot" it is probably time to replace the thermostat. Kent explains in the video below some other parts you should consider replacing at the same time.
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1986 to 1991 420 560 V8 Engine Water Pump
New water pump and gasket as shown. Plan to change the short hose and thermostat at the same time. Inspect v-belts and change if cracked.
1986 to 1991 420 560 V8 A/C Idler Pulley
The bearing in this pulley wears out at 80,000 to 120,000 miles and can cause rumbling sounds while the engine is running. A good time to plan to replace or at least inspect this is when you change the V-belts.
1986 to 1991 420SEL 560SL, 560SEL, 560SEC Fuel Injector Replacement Kit
PLEASE NOTE: Possible shipping delay. This is a special order item and may take up to 7 days before we can ship it!
380 420 500 560 V8 Fan Clutch Replacement Kit
This fits all Mercedes V8 engines listed in the title from 1981 to 1991 including the sedans, the SL and SEC models. I compare fan clutches to shock absorbers. They wear out slowly over time and you don't feel or see the difference until you actually replace it with new.
1986 to 1991 420 560 V8 Engine Coolant SHORT HOSE Replacement Kit
This critical coolant hose hardly ever gets changed, and when it ruptures, you could experience a catastrophic engine failure. The chief reason is that it is almost impossible to see and inspect.