2007 328i Coolant shooting out of expansion tank
Today I was in a parking lot with the car idling (for about 45min on a phone call) and the low-coolant warning came on all of a sudden. I opened the hood and there was a huge puddle of coolant under the expansion tank and on the pavement. Note that I put in a new thermostat (BEHR) and water pump (Pierburg) myself only 5 months ago ( 70,000 miles; both from bavauto; no problems since).
I filled the expansion tank with distilled water (it took a bit more than half a gallon) and drove it home ~ 4 miles. opened the hood in the driveway and saw coolant shooting and hissing out of the expansion tank bleeder screw - (not from the cap which is where i thought the coolant comes out of when pressure builds up from overheating..). The car did not overheat on the drive home (going ~ 40 mph).
I waited a couple of hours for cool down, then lifted up the car in the garage and inspected the pump and thermostat. everything looked fine, no loose electrical connections, etc.
My first thought was that the new pump let go, so I started the coolant system bleed procedure by cranking up the heat to MAX and holding the pedal down for 10 seconds. The pump turned on and I could hear it working as normal.
The weird thing was that within a few seconds into the bleed procedure, coolant starting shooting out of the expansion tank bleed screw (so I stopped the bleed procedure) - apparently due to the action of the pump..
This is obviously not supposed to happen and I'm wondering now if the thermostat is stuck closed, or if something else is wrong - maybe some kind of blockage between the expansion tank and radiator? Would really appreciate any help diagnosing this. Note the temperature around here (CT) has been extremely cold lately, if that could be related, although today it was ~30F so not too bad.