I've seen bad tanks BRAND NEW... (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.

You can also rent/borrow a coolant pressure tester from some auto parts stores--Autozone and Trak Auto are two that loan tools. Pressurize the system to 20psi. If it holds for more than 5 minutes, you're probably fine, or you may have a blown head gasket. If it doesn't hold, you have an external leak or blown head gasket. Remove the plugs. If one looks like it has white deposits or is really clean compared to the rest, then you have a blown head gasket. The cylinder pressure bleeds into the coolant system and does a good job of blowing coolant out of the overflow.

I had a hard time finding the coolant leak on my fiancee's V6 Mustang. The pressure check found nothing, but the #3 plug was very clean. After removing the heads, we found the #3 head gasket blown pretty badly. Why did the system hold pressure? Maybe it would not leak when stone cold, or maybe the valves on cylinder 3 were closed.

Or like someone else previously suggested, check the overflow hoses and line from the radiator. The hose clamps are those crappy one-time-use things. Replace the soft hoses and all clamps if you find a leak. Inspect the plastic tubing for kinks, cracks, abrasion, and replace if necessary.

Good Luck!!
James
james_535@yahoo.com
88 M5
88 M3
83 535i

Follow Ups:

Overflow tank or radiator? Greg I 12:12 2/13/2001 (0)

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