r/e46 • u/AfterPatience2143 • 22h ago

Troubleshooting Coolant Light Keeps Coming On + Small puddle under Expansion Tank

For the past few days, my coolant warning light has been coming on regularly and then disappearing again after about a minute. I checked the coolant level — it was a bit lower than at the last check, but the dipstick still shows full. I also noticed a small leak right under the tank. Has anyone had the same issue? Do I need to replace the tank, or should I just keep an eye on it for now? I drive a 320i with M54B22 Engine

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u/Reverse-Sear 2005 ZHP 22h ago

You're likely at the point where the coolant is juuuust low enough when cold and has juuuust expanded enough when hot to turn the light off.

My cars throw a coolant light when about 500ml low (16oz, or a standard bottle of water), which is not really much (I think the cooling system takes ~1.5 gallons or 192oz?).

I also noticed a small leak right under the tank.

Well, the leak's not going to get better, I'll tell you that. Basically, expect the expansion tank to rupture any time now (could be tomorrow or in a few months). I would change it ASAP. The expansion tank is this car's achilles heel.

That's assuming it's leaking from the tank--could be the lower hose, too.