1987 300SDL loosing coolant slowly but getting worse, smell of coolant in the passenger compartment, slight hissing sound heard immediately after shutdown, coolant dripping under car after shutdown. Can't have this aluminum head overheating!
Quick check of monovalve area shows no leaks. I assume that my heater core has sprung a leak.
You know where that is. Takes almost a day to remove the underdash parts and ashtray, radio, center console, upper ( this upper was stuck something fierce and took hours of very careful pulling and prying to get loose without breakage) and lower wood switch panels to get access to the heater /AC box. I was eventually able to reach into the box where I find it dry! I find and disconnect the drain from the box and its dry! What the *%@! ?

I stop where I’m at and crawl under the car. Remove the panels and start searching for the origin of the leak. Looks like it’s coming from above the tranny. While I'm laying on my back a drop of coolant lands on my arm! A thorough search leads me to a drain emptying into the passenger wheel well which partially empties onto the plastic belly panel. This drain empties the windshield wiper motor/mechanism bay just under the windshield front.

Remove the wiper arms the foam seal strip and the plastic grating and I find fresh coolant in the bay area draining towards the passenger drain and a large cutout in the center. a close examination shows fresh coolant on a small 1/4 inch hose coming from a fitting just below the windshield. I replace the battery and start her up. I can observe what appears to be a slow leak from the fitting at the upper end of the hose with coolant running down to the lower end where this short (5") hose attaches to a hard pipe. I watch for five minutes where it drips steady with the engine running.

I shut down the engine and I'm looking at the area when al of a sudden I see a small jet of coolant shooting from under the edge of the windshield back towards the firewall! This creates a steady flow of coolant running out of the wheel well drain and down over the tranny. I get a small mirror and look up under the windshield edge where I can see a small hole corroded into the flat bottom side of the defroster heat exchanger. It appears that immediately after shutdown the coolant in the block gets hotter without water pump flow and pressure increases pushing the expanding coolant out of this hole. I had already noticed this post shutdown phenomena when I watched the coolant reservoir fill up right after a shut down.

Does anyone know if this failed heat exchanger can be replaced without removing the dash or windshield? If so how? It looks like a large, thin, flat sandwich where the coolant is inside. It has an inlet and an outlet formed into its forward bottom edge. The inlet is connected to the small fitting at the bottom of the monovalve and the outlet is connected via a small piece of hose to a hard return pipe leading into the engine compartment.

It looks like I will just bypass the heat exchanger for now but I'd like to get it fixed right as I will assume that the defroster efficiency with be reduced without it.

Any experiences or opinions are welcome and appreciated. Thanks.