PeachParts Mercedes-Benz Forum - View Single Post - 300SDL leaking cooling from expansion tank

Jeremy5848 · #4 06-16-2009, 06:07 PM

First, let's get the terminology straight. The "expansion tank" sits on the passenger inner fender in the engine compartment and has a pressure cap. It is under pressure at operating temps; leaks in it suggest an age-related crack that can be disastrous if you happen to be looking closely when it goes "boom."

However, the tank that is "behind the passenger fender," if that's how you literally mean it, is the "overflow tank," which has no cap and is never under pressure. It acts simply to collect any overflow from the expansion tank and return it when the system cools down. If it is dripping, it may simply be that the piece of hose from the expansion tank to the overflow tank has rotted--that's what happened in my '87 300D Turbo (W124, OM603). I removed the fender liner, took off the tank, cleaned it out, and replaced the hose. Documented it here. Another possibility is that the overflow tank itself has cracked. You may be able to apply some silicone sealer or you may have to replace the tank.

AFAIK, my expansion tank has never gotten full enough to utilize the overflow tank since I have had the car; you may have too much coolant in your system. The coolant is supposed to come up to the joint in the tank, about half full, when cold.

Jeremy