Coolant tank cap is a big deal(!)

A6 All/ S6 All
#1

I had a small coolant leak a couple months ago—added 800ml on the first time, but I would lose maybe 1cm from the tank every week. Then finally my old coolant expansion tank cracked at the inlet. I had it replaced, but the slow leak remained, as evidenced by a small wet spot that appeared each night.

It turned out the Meyle expansion tank cap I bought may have been the culprit. It screwed on very tight and did not have a click at the end. Regardless, the pressure release was not working properly and somehow I was losing coolant through the overflow in the tank. I got a VAG cap yesterday and the floor of my garage is now dry.

#2

Also, I noticed on 2 occasions that the temp gauge climbed to almost 3/4 on a 30C+ day, only to drop to normal when the AC is turned off.

Checked both engine and AC fans and they appear to run correctly. But I noticed that the air coming past the passenger side by the valve cover block is significantly warmer than the driver side.

Is this normal or maybe a blocked radiator core? Any thoughts?

BTW, This could still be a pressure issue caused by the faulty coolant tank cap. Still waiting to see if it happens again after I replaced it.

Coolant Temp rises during long idles
#3

I may have spoken too soon. After 5 hours in the garage, no leak. But the next morning, the same wet spot.

#4

Oh man, coolant leaks… The single biggest, and most frequently recurring issue with my S6.

As you know, these engines run HOT, and I think that results in the coolant system parts aging and cracking/breaking right around the 100k-130k miles timeframe.

You fix one, then you get a new one somewhere else. Fix that one, and a new one pops up.

It’s entirely possible (likely, even) that your coolant cap DID solve the problem, and now that the coolant system has started keeping pressure where it’s supposed to, the next weakest link broke, and happens to be in the same general area as the last one.

I can’t even tell you how many times that happened to me. At least 4 or 5.

Somewhere buried in this forum, I started a coolant leak thread. Check it out - see if anything in there sounds like what your problem is.

For what it’s worth, after my radiator sprung a leak 2 weeks after replacing a bunch of coolant lines, I haven’t had a single coolant issue in the last 10 months. So, eventually once you get them all resolved, you shouldn’t have to deal with it again for a long time.

#5

Later that day, my car took a piss all over the road. I was driving uphill in sport mode.

It turned out the thermostat that I installed together with a new water pump about 15000 km ago was stuck closed. The hose from the engine (top) blew completely off, and the coolant poured out.

A tow and a new thermostat later, I’m back on the road and now testing for any other problems…

#6

That will do it, defective parts are the worst. Hopefully the shop / part company warrantied the situation.

Glad to see you back on the road again.

#7

The part was from The dealer, but after 15 mos, they did not offer anything.

#8

Did they give you the updated thermostat? There was a revision, and the part number was 079 121 115 BA

I had my radiator blow in September 2016, and had both a crack coolant reserve tank and faulty cap. Later I tested my thermostat and it was sticking, I do believe that they are electronically controlled, but not 100% sure. Maybe someone can chime in.

#9

Yes. That was the part number for the replacement thermostat. The previous one came together with the water pump. (079121013N)

BTW, there was also a coolant tank which cracked at the inlet pipe—it was replaced a month earlier.