I know my gas engine didn't operate properly when ECT stayed below 80 and when it's below zero F I definitely block my radiator to help keep ECT in the operating range of the m thermostat. I will cover just half the radiator for temps under 10-20F but below 10F or -12c I'll switch to full coverage. I've never seen ECT get over tstat control when radiator is blocked and temps are super cold. I intentionally don't make a perfect seal so some air always gets to the radiator.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)