Installing new coolant
 

The vechicle is a 420 SEL, Engine 116.965

I have just finished installing a new water pump and I'm about to install the coolant. The engine block was drained on both sides and the only liquid remaining is in the heater core. In examining the old water pump, no coolant can enter the engine until the thermostat starts to opens, which according to the "Mercedes Service Manual CD" is 75 C (167 F). There is no by-pass.

As I understand the refilling procedure, the coolant is to be replinished by refilling the expansion tank to the max. mark and then the engine is to be cranked.

In thinking through this, It would seem that since the engine has been drained of all coolant it's going to sit there and cook until the hot metal transfers heat to the thermostat and it eventually responds and suddenly admits water to a hot engine.

I'm inclined to go back and temporarily remove the thermostat and fill the engine with coolant before cranking.

Maybe I'm missing something. Comments would be appreciated.

William:

I own a '91 300-SEL which has the smaller 6 cyl motor vs. your 4.2 litre V-8, but...no matter.

Your idea is fine and is how alot of folks refill the 6 cyl motor.

Fill the overflow tank and also fill thru the thermostat area if possible. I'm assuming the 420's is horizontal, thereby allowing for refill thru the t-start area. Once it's full in the t-stat area, reinstall the t-stat and cover. Leave the cap off the overflow tank and start the car. As it warms up, you may see bubbles in the tank - a sign air is escaping the system...good thing. Reinstall pressure cap after "burping the air.

Mike:

Thanks for your response and helpful suggestion. I plan to proceed with the refill as you outlined.

William