Ok I had this issue posted on another thread but I got 0 replies and I have more info. to add. I thought my car was just slightly overheating with no leaks. But today there was a good size leak on the driveway. It is coming from the rear drivers side from under the coolant reservoir. I just changed the coolant flange, and replaced the reservoir cap. It will only leak after fully heating up and after I let it sit for at least 10 mins. I feel around the coolant flange and it is completely dry. All hoses from oil cooler and heater core have no leaks, tears, etc. The only thing I can think of is the reservoir is cracked somehow but it has no visible cracks. I have no idea where this leak can be coming from. Am I missing something because I see no place that it could be leaking from where the puddle is except for the reservoir... Any ideas please share.. Car is 01 A4 1.8t AWM

 

Remove the 3 screws out that hold the reservoir on & see if is leaking. remember not to OVER FILL the reservoir! If it is overfilled when the coolant expands it will push it out the overflow in the reservoir. Another place to check is the coolant temp sensor at the back of the cylinder head. There is a rubber o-ring that seals it.

 

i'd wrap a rag light enough to spot leaks around the reservoir, let it run to temp, then check the rag. possible you have a hairline crack in the reservoir and its only forcing coolant out when its hot and pressurized.

 

I just replaced the coolant flange and replaced all o-rings on that so it's not the CTS. The reservoir has no visible leaks/cracks like I said. I don't think it has to do with pressure b/c it doesn't leak when it is running only when I shut the car off and let it sit for a while. I haven't tried wrapping the reservoir in a towel yet, but I did try putting some clean towels under the reservoir and under the oil cooler housing last time I shut the car off after a long trip and I checked them in the morning and they were completely dry but there was a puddle in the driveway. That's why I am baffled.

 

Sounds like something that you'll find when it gets worse. And it will!!
I was going to suggest the paper towel under the resovoir trick, but sounds like you already did that.

 

there is nothing else on the drivers side aside from these lines and the reservoir itself..

 

Thanks Lornnn. I was looking for that diagram. Ya I spent like 20 mins looking for any other coolant lines going on the drivers side of the car but I couldn't find any. I believe the only thing it could be is the reservoir leaking somehow, so I'm going to order a new one and hopefully solve the problem. Thanks guys.

 

Here's something else: a couple weeks ago I too replaced my coolant flange. I had a leak just from refilling coolant. Spent a while doing some research and figuring out what the problem was, and it turned out to be the O-ring in the heater core hose that connects to the bottom/rear of the coolant flange. This O-ring should be replaced when swapping the coolant flange, but it is not called out by itself ANYWHERE because normally this O-ring is ONLY a part of the heater core hose assembly.

So, I'd check to see if that heater core hose is wet. If it is, go to your local auto parts store (a hardware store could even suffice) and find a 7/8" x 1 1/8" x 1/8" O-ring, pull out the old O-ring from the hose (it's probably gonna be a PITA for you being AWM), stuff the new O-ring into the groove in the hose, and slide the hose back on with the clip in place.

 

Good call..this o-ring isn't in any audi/vw repair guides and the dealer doesn't carry it as a part. But the o-ring is a 22.5mm o-ring and yes I replaced it when I replaced the coolant flange. There is no coolant on the tranny or anywhere around there to indicate the coolant flange leaking.. I'm fairly sure it's the coolant reservoir.