I had a similar coolant leakage issue for the last year or so and could not
figure out where it was happening. In July I had to make an 800 mile
roundtrip on a hot and humid weekend (92F +) .... My first long trip
in my 400E since I got it.

On the return home I popped the hood (after the engine had cooled) to check
coolant level and noticed (for the first time) that there was a distinct white water mark/residue on that plastic elbow area (literally a spray pattern) on top of the Behr radiator (this elbow has a hose clamped to it that goes to the expansion tank) Found out that that elbow is available for a few bucks at most on-line parts providers....Replaced it alongwith a coolant flush and fill and seems like my coolant level has stablized. The old elbow just crumbled away while I was removing it.

Looking back I always used to notice a couple beads of moisture up on the rubber "pad" on that is attached to the hood. When the hood is closed this is lines up approximately on top of the radiator...but since I could not see any issue with the top of radiator I never connected the moisture on the pad with the coolant loss.

So I think the long trip finally deterioted the elbow to the point it was spouting enough coolant to leave a residue...I am glad that elbow did not go while I was on the road. So my point is that old elbow plastic looked just fine looking it...except that it had "rotted" from the inside out....

Just my 2cents worth.

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Matt
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92 400E 128K
99 GMC YUKON 88K