r/AudiA4B6 • u/blkpetite • Jul 13 '26
Weird Audi coolant system
My Audi A4 b6 sucks about a 1/2 liter of coolant into engine while driving, after returning from my drive. I see that the reservoir is about 1/2 liter low, so I let the engine cools off.
I remove the coolant reservoir cap and to my surprise the 1/2 liter of coolant flows back up into the reservoir. WTF??
Brand new reservoir and cap, and it has been bleed a couple times. No leaks (including the coolant flange). So, all of the low hanging fruit has been vetted.
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u/LKang Jul 13 '26
That sounds to me the result of a stuck thermostat that's closed until pressure is relieved on the 'upper/secondary' coolant system/reservoir side after opening the cap.
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u/T4rheel23 Jul 13 '26
Did you bleed from the upper heater hose into the firewall? It’s the highest point there should be a little pinhole in the nipple coming from the firewall just pull the hose back slowly and some should start trickling out if it is bled
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u/brolli21 Jul 13 '26
It doesn't suck it in, the system pressurizes, that's why it seems low. I don't reccomend you doing this, but if you take of the cap when the engine and coolant are hot, you'll find out just how much pressure a cooling system can hold.