As I have learned from a number of you, my car (1992 300D) has a coolant overflow reservoir. Overflow coolant goes into this tank when hot, then is sucked back into the the expansion tank when the system cools.

I strongly suspect that this system (either hose or tank) is leaking and that that accounts for my loss of coolant. The reason I suspect this is that the loss rate has substantially decreased as the coolant level when cold has approached the seam in the expansion tank. Currently the level looks to be about 1/8" inch below the seam.

My question: how low would the cold level have to get before I could conclude that the overflow system is NOT the problem?

Thanks.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I am thinking that pressure testing you coolant system would tell you if there is an external leak.
They replace the Overflow Tank Cap with one that has a fitting to hook to a hand pump. You pump on the hand pump till it is close to the pressure of your system (or follow the instructions in the manual) and see if the pressure drops and look for leaks.

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84 300D, 82 Volvo 244Gl Diesel

Until you find and fix it!

After you pressure test it as 911 suggests...
take out the fender liner on the passenger side wheel well and inspect the
overflow tank and hose.
Most likely failure point for the hose is where it passes through the Body
Sheet metal to the Wheel well,area.

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'84 300SD sold
124.128

Really? Some have suggested that, if that's the problem, eventually I will stop losing coolant. Not sure what to think....

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 158k miles
06 E320 CDI "Rutherford", Black on Tan, 177k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 144k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 72k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

I'm in the,"I don't wanna find out how low or the end result" school of thought.

"Obsessively observing Coolant system,even with new surge tank and all new hoses."

It will eventually Stop Leaking,whether you fix it or not.("They" are correct about that)

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'84 300SD sold
124.128

Yes, when there is not enough water left to cool your engine. If there is a leak, when the engine is hot, vapor will leak out once there is insufficient water to leak out.

Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)

It stops leaking out just above the low coolant light sensor. For me anyhow, my temp stays below 85 and I'm sure if it went above that more would leak out.