I've been pondering using one of these kits as well on my 300CE.

Yes, put the elbow on the expansion tank and it should work just fine - it's the same pressure cap-type opening as on a traditional radiator.

Once we figure out which hose to splice the tee into, it should be pretty simple. But, of course, you want to close off those drain plugs before you start flushing with the garden hose.

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Jay
1998 S320