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So I have loosing coolant for a few weeks now (more than 6 or 7 weeks now), I think I found where: the coolant seem to escape the coolant tank reservoir through the overflow outlet (it splash over the oil filter). Now the question is why? I did not overfill it. The coolant system is strange: it is always pressurized, even I park over night, if I loosen the cap in the morning, the pssst sound indicate the reservoir was pressurized and then the coolant flow back from engine into the reservoir. Can anyone tell me why? Is it normal?

lyleswk

I did think to check my coolant system to compare to yours and mine does not stay pressurized overnight. Now, which one is correct is another discussion. Since you are having coolant come out after shutdown, I would assume something is wrong with yours. The question is what. I am afraid I don't have any thoughts on this one.

heateris

Check around the oil cooler also.

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Funny, I did found white hardwater spot on oil filter, but I could not find any wet spots

lyleswk

If you have coolant on the filter, it could be your "J" plug, just aft of the thermostat but in from of the oil filter housing. Wipe it down well and them inspect after you drive.

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Nope, brand new J-plug & thermostat. After a long trip this morning (30 miles), I let the engine running and I came out: it is little wet right at the under the reservoir overflow hole (on the bottom of the reservoir) so definitely it is overflow, but why? Something must be wrong with my cooling system. Water pump dead? New thermostat, new J-plug.

lyleswk

Water pump may be your problem. When was the last time it was replaced? If it is not providing the spec'd flow, the coolant may be in your block too long thus casuing the pressure to stay in the system and "boil" over. Strange that your coolant temp is okay.

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That very likely I review the service record and saw the PO replace the TB at 110K (car is now 130K) but not water pump. Not a good sign. Time to go shopping

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Think back, water pump may not be the issue: my temp gauge is solidly at middle. I can't think of anything else, there is not much in the cooling system: water pump, thermostat, coolant sensor, what else? Radiator?
Why I have so many mystery issue? Misfired only for 10-15 sec and coolant overflow without overheat. I only had 3 issues with mine B5 over 4 years: fuel pump, coolant flange and clutch slave cylinder. Two hard issues just a few months of ownership. I am not lucky.

lyleswk

Love to hear from other folks, but everything you mention including the radiator should have an overheat with it. Do you know what your mix of coolant (I assume you have the proper "pink" coolant) and water you are running?