Overflow tank or radiator? (archive)

I worked at a dealer a few years ago, and we found that there was a flaw in the mold used to make E30 M3 tanks. Inside the neck is a ridge that seals the tank pressure against the rubber ring on the cap. There was an indentation on that ridge that allowed coolant to bow off while building up nearly zero pressure in the system. If you replaced your tank a few years ago, check for this. We had all of the faulty tanks pulled from dealers and the factory warehouse. You will not get the faulty tank today.

If the tank has never been replaced, then this is probably your culprit. The top of the neck is plastic, and it eventually bends upward from the constant spring force under the cap. The neck no longer compresses the spring in the cap, so even a new cap won't hold the correct pressure.

Are you talking about the coolant overflow tank or the radiator? I know this problem was common with the radiators, but I'd never heard of it with overflow tanks. Since the entire overflow tank is plastic, one can only assume the neck is also going to be plastic by design.

Greg.

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