On my 300CE I had a washer heater that was possibly leaking coolant as the washer fluid was getting dark in colour. A few weeks ago I removed the heater coil and tightened the thermostat bolt that was loose to see if that would cure the leak. I emptied the washer tank with a hand pump and filled it with a bit of water and washer fluid mix so I could monitor it and easily tell if the heater coil was leaking coolant back into the tank.

I haven't noticed any signs of coolant entering the tank so I filled the tank bank up with washer fluid but as I was filling it up I noticed it was leaking out from below the tank. I stopped filling and quickly check to make sure all the hoses were snug. It isn't leaking from either of the pumps or the line that goes to the headlight washer. Where could else could it easily be leaking from?

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1989 300CE

If the washer pump plugs in to the tank, the rubber grommet likely has failed. They are stocked at the dealer, the pump setup is similar to GM and VW so an auto parts might have one.