I was about to take your advice. I came home today and the solution became really clear. The timing was outstanding. My house has a long steep driveway, I got out at the top and I noticed a huge swath of coolant, but it only started just as I got to the top, about 30 feet back.

There was coolant leaking out below in the same spot only quite a bit of it. I let it cool off for an hour, put some tap-water into it, and it immediately started leaking out below. So I positioned the hose to stay in the top of the radiator, turned it on, and this is what I saw underneath:

https://youtu.be/TozwUYzNpLE?si=8Jy21FtwV5nVAJDy

I took the starter out, wired it up so I wouldn’t have to undo all the cables, and who woulda thought, I was correct on the freeze plug hypothesis:



I went back to where the coolant trail started, and look what I found:



Now all I’ve got do is make sure I know what I’m doing when I put ia new one back in. I’ll buff the inside of that opening with scotch bright. I read mentioned that some people like to use brass replacement items. Not sure.

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Te futueo et caballum tuum

1986 300SDL, 362K
1984 300D, 138K

I thought you said that location was hard to get to? This looks as if it is not even in a vehicle?

Taking the starter out allowed you to see that the freeze plug was the problem, congratulations! In the video it looks like a gusher! Best wishes for a successful repair!

almost forgot, due to the large amounts of coolant/water gushing out over the starter, you may need to replace the starter. I know this one because I once drove through a flooded roadway which took out my starter so I had to replace it (fun).

Thoroughly freeze the new plug to shrink it before installation. Some sealer around the edges helps.

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Are those photos up side down?

.era yeht kniht I

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"Rudeness is a weak man's imitation of strength" - Eric Hoffer

I said upside down, not mirrored

Look at the video - is the first link in that post. The freeze plug was just not visible, the starter was in the way. I took the starter out, and my suspicion was confirmed.

__________________
Te futueo et caballum tuum

1986 300SDL, 362K
1984 300D, 138K

I’m about to get some dry ice at the grocery store. Former member Brian Carlton, a mechanical engineer who builds devices for balancing jet engines once told me that many of the fittings he used were small steel pins, placed in liquid nitrogen prior to placement.

I suddenly have an odd bit of anxiety. My plan is to have the new plug sitting in rice for a few minutes while I’m putting a thin coating of ATV around the opening. I am wondering if the super cool part will somehow burn the silicone? I think I’ll experiment with the old plug, some RTV silicone on it, and dry ice applied.

__________________
Te futueo et caballum tuum

1986 300SDL, 362K
1984 300D, 138K

The awful deed is done. Worked out pretty well I think, looks good, no leakage.



That is an upside down image come to think of it. I am on my back with my top of my head somewhere close to where the crankshaft is, looking up at the side of the block.

I probably wasted my money on the dry ice, it was only $6. I was wondering if the plug being really cold would make it condense moisture, I was correct. I got it in position, quickly wiped off the condensate with a paper towel and shoved it in place, started hammering, I used a half inch drive socket that was the right size to just fit inside. I don’t think it will be a problem, but I was concerned about having moisture in there along with the silicone gasket sealer stuff.

The install tools push on the outer rim. When I was about halfway in, one side was a little higher than the other, so I hammered on a large steel rod on the outer rim wherever it was high, then I reverted to my large socket with an 8 inch long extender.

__________________
Te futueo et caballum tuum

1986 300SDL, 362K
1984 300D, 138K