So. the hypothesis is that an open water valve is causing your problem. Could be, but unlikely, so lets not waste too much time or money. Do the visegrip test I described. Five minutes, most of it driving, proves or disproves the hypothesis.

This is really an exact science.

The valve is in the same pocket under the hood as the battery.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician