As far as I know it has had this problem since i bought it, but I haven't driven it that much. If you drive with the heater off it behaves normally, and you can pressurize the cooling system with a pressure tester tool with the engine off and it holds pressure for at least half an hour, but with the heater on you can drive until the engine has has more than enough time to get up to operating temperature and open the hood and squeeze the upper hose and there is no pressure at all, then turn the heater off and drive a bit more and it has the normal amount of pressure it it. There are no visible leaks, it doesn't loose coolant, and no signs of coolant or oil ending up where they don't belong. The thermostat, coolant reservoir cap and radiator hoses are new, the rest of the cooling system hasn't been touched. Any ideas?

With the engine off, key out, the heater valve is wide open.

With the engine running, heater off [edit, corrected from “on” as noted in next post], the heater valve is closed.

There’s only a heater valve on the return side of the cabin heater. Even with the valve closed the heater core is subject to cooling system pressure from the supply side. Coolant just isn’t flowing.

If you know which monovalve pin is which, you can remove the connector and supply 12V directly. Then you can pressure test with engine off, key out, with 12V and without 12V to see if there’s a difference in static system pressure test. It’s another datapoint.

Sixto
98 E320s sedan and wagon
02 C320 wagon

Slight correction to the above:

With the heater calling for heat, the monovalve is open, not closed. Same thing when the engine is turned off, the monovalve is open. It closes (energizes) to stop or control flow through the heater core, such as when running the A/C or the HVAC is turned off while driving. It will cycle open and closed to maintain a given cabin temperature, depending on the mode that the pushbutton unit is in and what it decides to cycle.

If the system holds pressure with the key off, the system is tight. It's possible that with the heater running, it's extracting enough heat to prevent pressure building in the cooling system.

__________________
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
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