I find it takes on the order of ten minutes at idle to get to where the cooling fan kicks in. If you put the cardboard behind the radiator you don't have to worry about disabling the fan, it will kick on to cool the engine if needed.

Pros use propane or butane to check for leaks use torque to monitor O₂ sensors which will go rich when they sense unburned fuel.

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