I have an 84 300D which has mysteriously developed a coolant leak on the front of the engine at what SEEMS like to be the hole that the arrow is pointing at. This is on the front of the engine just under the water pump:


What do ya'll think this is?
So here is the story:
I had just four days prior done a system flush, drained out all the old green crap, and had degreased it, had run a jar of coolant flush (was waiting for the citric acid to arrive). I replaced the thermostat as well. All was well in the world. Then I drove the car out of town, and when I parked it, there was a nice leak coming from the front of the engine. I checked every hose, the thermostat area, etc, but it doesn't seem to be coming from any of those places. It seems to be (but it is so cramped I can't tell for sure) to be coming from that hole. What is that hole? It seems to stop leaking when the reservoir tank is only about a 1/3 full, and will leak if I add more to it. I drove it 30 miles like this, and it was fine (no overheating etc) I stopped to check on everything, and added some more water just to be sure, and added 1/3 a cup of citric acid, just because I could. Well within 400 feet my car went to 110 degrees, so I pulled over just to see that now the leak seemed worse, but not that much. I pulled the thermostat (which is brand new) and now it doesn't overheat, but it still leaks in that hole. I have since drained everything out, but the leak is still there. What gives?

I disconnected the top hose from the radiator with the engine running, and it sprayed water everywhere, so I'm assuming the water pump is still good.


I'm about to pull the fan off and try to plug up the hole with some high temp muffler putty, but I wanted some input first. Thanks!