Leaking coolant - causing overheat

[Silicon Voodoo]

New ordeal again, now it's coolant. Once in temperature, it starts to leak pretty fast which allows only 10 minutes drives before the engine-temp's needle starts to shoot up past 90°C.
At which point I immediatly pull over and let it do a "reverse electric pumping" for 5 minutes and retop with water.
I can't put actual ethylen glycol every 10 minutes because of the cost, so I have a very watered down mix right now just to get home. I had to do 2 hours about 100km using this strategy. I even had to stop on the side of the highway to avoid blowing the head gasket when it got past 110° that was stressful.

Anyway I'm not sure of the source of the problem. Hose, expansion reservoir, head gasket, radiator? No idea.

Maybe relevant: Before the leak got bad, the car started a few months back to exhibit a new behavior: it blows the radiator fans after the engine is cut for up to 5 minutes after parking.

I made a video:


There are drops under the reservoir, and it seems to flow against the metal lines until the back of the car.
Any help appreciated.

[Mister J]

Looks to me that your expansion tank/reservoir is cracked. It clearly shows in your video that the drips are running off the tank. I'd start with replacing that tank. Not sure if the dried up coolant running down the side is from pouring, or from a leak. I'd replace the cap too.

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[Silicon Voodoo]

I agree it's a high probability. The rundown on the sides is from me opening that cap multiple time while hot, to refill and reach destination.

[Silicon Voodoo]

I took it to the dealer, and he says it's the thermostat. He thinks the leak is just a normal overflow (when the thermostat fails), and there is no crack and nothing to replace in terms of hoses and expansion tank.

EDIT: just got it back after the change. bill 57000¥ (422$ us). And indeed it now works, and stopped to leak. So the problem was that the thermostat stuck closed forces the flow into the bypass circuit (fast heat) which goes through the expansion tank, and eventually boils -> pressure -> leaks & bubbles -> overheat.

[Xc3]

So there was no code or light on dash?

[Silicon Voodoo]

So there was no code or light on dash?

Correct, there was no code, no MIL. The most surprising was that first time I observed the overheat and stopped on next highway parking area, the expansion tank was completely empty. It should have said "coolant low" since there is an electrolyte sensor in that tank but it fails to do its job.
The thermostat position is perhaps not sensed, so no way to detect a problem?

[Xc3]

Expension tank sensor should alerted the ecu. Yes there is no sensor for thermostat malfunctioning, didn't blown head gasket tho.

[Silicon Voodoo]

Expension tank sensor should alerted the ecu. Yes there is no sensor for thermostat malfunctioning, didn't blown head gasket tho.

I got lucky with the head gasket because, (1) this is a cast iron engine block so it bends less under heat stress. and (2) there was enough remaining water in the radiator and the engine block passages to keep under 105/110° (normally 90°) so not a catastrophic overheat

[Xc3]

I notice small coolant leak by the temp sensor housing flange. I'm monitoring the coolant level. The leakage is like white oil kind but not the coolant color itself. Audi dealer needs to order that housing but thinking of changing it myself. Used little sealent on the housing but still leaks thought.

[Silicon Voodoo]

The coolant becomes white when drying off. It leaves sort of crystals.