Hi,
I'm changing all the fuel O-rings on my 96 E300, as well as the delivery valve seals. The usual air in the lines, no problem been there done that. The pre-heater, or fuel thermostat before the pre-strainer looked moist around it's base so I bought the small squareish gasket for it.
My question is; What is this gasket holding back? Do I need to drain some coolant out? I simply do not want to create a big mess when I un-bolt it from the motor.
Lowflyingbird

I think that you've guessed at the correct answer. It does indeed go into the water jacket.