Changed out the duovalve and the coolant on the 98. Had a very difficult time getting the new coolant to flow into the heater core. Up on ramps, revving engine, etc... When I finally got some heat, and as I continued to add coolant to the expansion tank, I noticed coolant was leaking beneath the car, just under the expansion tank. I'm pretty sure the duovalve connections are not leaking. It got too dark so I gave up for today.

During the period of time when coolant was not flowing into the heater core, is it possible that the coolant I was adding just got "dumped out" somehow? Or is it likely that i have an actual leak somewhere?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 154k miles
06 E320 CDI "Rutherford", Black on Tan, 172k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 142k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Your eyes are the best troubleshooters you have. Wait until morning and look for leaks. It's easy to get sidetracked and forget to tighten up a hose clamp!

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)