2011 Audi A8L 4.2L | M1 0W40 | 4,900mi | Coolant in oil!

Jul 16, 2022
Good afternoon all. Looks like I got a pretty bad report for my first UOA. During this interval, the low coolant warning came up on the car twice. I topped it off both times and eventually had my mechanic replace a coolant union pipe behind the engine (a common point of failure on the 4.2 V8). This was done right before the oil change when the sample was taken. I thought this was where the coolant was leaking but now I'm worried it was instead leaking into the engine. Some research shows that the oil cooler seal can fail on these cars, as well as the PCV valve (which has a coolant pass-through, apparently). I don't think a failing union pipe can allow coolant into the engine. The engine has just under 130k on it. Since I have owned it, starting at 99k mi, I have done oil changes with a vacuum extractor via the dipstick. There is never any buildup under the filler cap and the oil does has never appeared milky. The car has never had an overheating issues and has no warning lights or drivability issues.

I also replaced the high pressure fuel pumps right before doing this oil change and sample. The car had an occasional rough warm start issue that can come from old, leaky fuel pumps. So hopefully that took care of the fuel contamination issue. The warm start issue has been resolved. However, the larger coolant issue is outstanding.

Finally, I used a bottle of Techron before the coolant pipe, fuel pump and oil change was done. How much trouble am I in? Should I stop driving the car or just carry on and send another sample in 3k miles as advised by BSL? I am going to change the PCV valve as it is relatively inexpensive and easy to do (knock on wood). I will also get in touch with my mechanic who did the union pipe on Monday. Thanks in advance.

This is the PCV/separator on the rear of the engine not under the intake correct? These can leak but not as common as the under manifold one.
This type of plate oil coolers are an issue on many cars not just Audi.
Drain the oil but do not refill, leave the drain plug out so you can see if coolant begins to come out, pinch off or remove and bypass the coolant lines at the PCV and pressurize the system, do the same with the oil cooler lines, which ever one stops the bottle from going down is the culprit.
There are other possibilities but these are the most common.
Thank you very much for the reply. Yes, the PCV I plan on replacing at the moment is the one at the rear of the engine, on top. Same as the one shown here:

I will try your advice. Should those steps be taken ASAP? Is 0.7% coolant in the oil a dire situation?
Absolutely, do it yesterday. Small leaks can turn into large leaks very quickly, coolant does play well with engine bearings other lubricated internals.
You already know this but to be clear pressurize the system with a hand pump not by running the engine with no oil in it.
Last edited:
Does that car have the oil cooler that has the coolant lines running through? I had my oil cooler crack in my 2001 S4 and my pink coolant looked like chocolate milk.
It sure does, and I've read the seals can go b/w the engine and oil cooler. It's quite a job to get to it:
Is it possible my vacuum syphon oil changes via the dipstick haven't been getting very small amounts of coolant getting in through a bad PCV valve over the course of 30k miles, as the coolant would settle at the very bottom? It seems like entire capacity is coming out when I change it, and adding the 8.1 qts called for results in a perfect oil level in the MMI.
That job is a big PITA. The guy in the video say they use plastic seals, they are in fact petrified rubber. I probably is not loosing too much coolant right now. If it were the cooler it would have to be where the plates that carry the oil ruptured in the coolant, the usually blow the seals out on the block causing a big oil leak not coolant.
The more I think about it at this age I would change the PCV anyway, change the oil and run it 2K, check for coolant loss and do another UOA.
Good point. I think I will do the PCV and send some more samples in, maybe sooner, while keeping a close eye on the coolant level.
I got another two oil reports done, one ~500 miles after PCV replacement, and another ~2500 miles after that. Unfortunately, it looks like the problem remains. The low coolant light has not come back on but the level does seem to be going down ever so slightly. I'm going to drop it off at my mechanic for a pressure test to start the diagnosis next week. Since the PCV is brand new, the most likely culprit is the oil cooler and its seals? I can't find a single instance online of a bad head gasket on these.

I got another two oil reports done, one ~500 miles after PCV replacement, and another ~2500 miles after that. Unfortunately, it looks like the problem remains… I'm going to drop it off at my mechanic for a pressure test to start the diagnosis next week.

Well that’s not good… but it would be interesting if you do find an internal leak and end up fixing it and watch the signs of coolant leave from the UOA reports. 👍🏼