GOLFMK8
GOLFMK7
GOLFMK6
GOLFMKV

Coolant + Oil mixed and bubbling

mystix

Passed Driver's Ed
Location
Toronto
I need help diagnosing my vehicle's (2015 GTI CXCB) coolant bubbling issue.

Parts Replaced:
-thermostart housing [original vw]
-water pump [original vw]
-engine oil cooler [original vw]
-coolant hose adapters that connect to water pump (all three) [original vw]

Current Scenario:

1. I put the car back together after replacing the above parts. At this point the coolant system is mostly empty and I fill it up with distilled water.
2. I start the engine and at this point the reservoir contains clean distilled water.
3. Engine starts to reach temperature and settles at 90 degrees (celsius).
4. The HVAC is set to high, fan low. Vents blowing out normal air and not hot air.
5. Bubbles start to form in the coolant reservoir and after a few minutes some form of oil+water mixture starts to overflow out of the reservoir.
6. I turn engine off and immediately all the coolant (water+oil mixture) is sucked out of the reservoir. My assumption is that the coolant is going to the water pump.
7. Reservoir remains empty after engine has cooled (checked a few hours after).
8. The dipstick looks like clean oil and not a mixture of water/oil like the reservoir.

Diagnosis?

I have performed a coolant pressure test and it does hold pressure. At this point I can only think of two potential sources below.
i. Transmission Oil Cooler
ii. Engine (Head Gasket and/or Cracked Block/Head)

I did notice oil settled around the transmission oil filter but the transmission oil cooler itselt was clean and dry. This is problably a separate unrelated issue.
I am also thinking about bridging the coolant hoses connected to the transmission oil cooler to rule it out.

Question:
1. Does anyone know why the coolant bubbling and eventually mixing with oil only starts after the engine has reached operating temperature? I ask because if it is a head gasket issue wouldn't the bubbling begin immediately after the car is turned on and not after it has reached temperature, which is when the thermostat would open?
2. Any suggestion on what I can test for next?


Here is a picure of what the reservoir looks like when bubbling.

1688958716328.png


Thanks,
 

mystix

Passed Driver's Ed
Location
Toronto
I bypassed the transmission oil cooler by bridging the hoses using a 7/16 long socket and the system still bubbles a few minutes after reaching temperature.

IMG_3062.jpeg


Observations:

1. I pressure tested the system after bridging the hoses and there was a slight air leak and a hissing sound coming from the hoses I bridged. Overall not too worried about creating a perfectly tight seal.

2. I didn’t drain out all the old coolant because the objective was to determine if the bubbling was due to the transmission cooler.

3a. Upper radiator hose hot.
3b. Middle hose (water pump to reservoir) hose hot.
3c. Lower radiator hose cold.

Question:

Could the thermostat housing / water pump be defective if the lower radiator hose is cold?
 

Knifebright

Go Kart Newbie
Location
Bay Area
My 2015 had a small bit of oil in the coolant bottle. Was in the shop for its yearly/5k maintenance and it turned into a horrific project but seems resolved. I had zero symptoms and the only thing that anyone was certain of was it was not the head gasket. Great, what the hell is the issue.

There was a coolant valve that needed to be replaced under warranty, that’s when they found more oil, remember the bottle is at the top of the system, it gets worse as you go deeper.

We replaced the heat exchanger, a number of hoses and eventually the radiator as it was too gunked up and getting all the crud flowing eventually caused over heating on test drives.

The root cause ended up being the bracket that holds the oil filter housing. There’s a place where oil and coolant exchange there and oil is at a higher pressure.

A few days of flushes and it’s in a pretty good spot. I gotta get a few thousand miles on it and then one more flush and replace the coolant bottle as I am expecting a few more globs to float up but for now, there’s very little oil residue left and we’re running great!

Just needed to replace nearly the entire coolant system. All I need is a heater core and I’ve literally replaced the entire system over the past few years.


Here’s my total parts list if you find it helpful for


Part Description
HVAC CONTROL VALVE (COOLANT) OIL COOLER GASKET
OIL COOLER CONNECTOR
OIL COOLER SEAL (OFH TO BLOCK) UPPER RADIATOR HOSE
RADIATOR FLUSH
COOLANT HOSE
OIL COOLER
OIL COOLER BOLT M8X20
Radiator
Radiator hose o-ring
 
Last edited:

mystix

Passed Driver's Ed
Location
Toronto
@Knifebright thanks for responding and I’m sorry your solution has been so involved.

I did some more work on the car today.

Started by disconnecting the top rad hose from the thermostat coupling and also disconnected the bottom radiator hose from the radiator.

I checked if the radiator was clogged and it’s not. I took a garden hose and ran water through the top hose and it immediately started flowing out the bottom. I let the water run through until it started appearing clear.

Moving on, I turned the car (with the above mentioned hoses disconnected) and let the engine temp gauge reach 90 degrees. So far no water has exited the upper coupling of the water pump. I then start to pour water into the expansion tank and immediately the water starts exiting via the upper coupling. So I continue to pour (warm tap water; don’t want to crank the engine by using cold garden hose water) water into the expansion tank until the coupler output is also clear water (no longer oil mixture).

I connect the upper rad hose back to the coupling and continue to top up the expansion tank just to confirm that we’re still flows out lower rad coupling, which it does, so I go ahead and re-connect the lower hose back to the rad as well.

At this point I continue to top up the expansion tank until the water level is sufficient and I immediately see bubbling in the tank.

The good news is that the water is clear and I will run the car again tomorrow to confirm it doesn’t get contaminated with oil again.

The bad news is, bubbling continues. Now it could be a head gasket for which I think I’ll need a combustion leak detector to confirm.

The other theory I have involves temp sensors and thermostat housing but this I will pursue if the combustion leak detector comes back negative.

I also want to invest in vcds at this point. It would be helpful to manipulate the thermostat housing.

Some photos …

Clean water coming out the upper thermostat coupling.

IMG_3086.jpeg



Clean water boiling out the expansion tank. The oil in the lip is from before.

IMG_3092.jpeg
 

Knifebright

Go Kart Newbie
Location
Bay Area
@Knifebright thanks for responding and I’m sorry your solution has been so involved.

I did some more work on the car today.

Started by disconnecting the top rad hose from the thermostat coupling and also disconnected the bottom radiator hose from the radiator.

I checked if the radiator was clogged and it’s not. I took a garden hose and ran water through the top hose and it immediately started flowing out the bottom. I let the water run through until it started appearing clear.

Moving on, I turned the car (with the above mentioned hoses disconnected) and let the engine temp gauge reach 90 degrees. So far no water has exited the upper coupling of the water pump. I then start to pour water into the expansion tank and immediately the water starts exiting via the upper coupling. So I continue to pour (warm tap water; don’t want to crank the engine by using cold garden hose water) water into the expansion tank until the coupler output is also clear water (no longer oil mixture).

I connect the upper rad hose back to the coupling and continue to top up the expansion tank just to confirm that we’re still flows out lower rad coupling, which it does, so I go ahead and re-connect the lower hose back to the rad as well.

At this point I continue to top up the expansion tank until the water level is sufficient and I immediately see bubbling in the tank.

The good news is that the water is clear and I will run the car again tomorrow to confirm it doesn’t get contaminated with oil again.

The bad news is, bubbling continues. Now it could be a head gasket for which I think I’ll need a combustion leak detector to confirm.

The other theory I have involves temp sensors and thermostat housing but this I will pursue if the combustion leak detector comes back negative.

I also want to invest in vcds at this point. It would be helpful to manipulate the thermostat housing.

Some photos …

Clean water coming out the upper thermostat coupling.

View attachment 286188


Clean water boiling out the expansion tank. The oil in the lip is from before.

View attachment 286189
Ya man, essentially my car hates coolant. I feel like we got to a place of resolution but like I mentioned, I’m not going to be shocked if I get a bit more residue and need to swap another bottle in a few thousand miles. My radiator wasn’t clogged until it was, there was just a bunch of gunk hiding somewhere that got dislodged in flushing is my guess. 4 water pumps, a bunch of hoses and the rest of the coolant parts I mentioned over the past few years. I’ll let you know if I end up with that heater core lol but I do think it’s fixed now…

Seems like you’re making progress though! Rule out head gasket when you can. Just about everywhere oil and coolant overlap, oil is at a higher pressure so hoping you can rule in or out head gasket so you can get to the bottom.

Good luck man, I swear they designed these things to be a fucking mess if you try to flush coolant!
 

DSC808

Autocross Champion
Location
HI State
Car(s)
2016 GTI SE MT

YamR1rider

Drag Racing Champion
Location
Tampa, FL
Car(s)
2017 GTI Sport DSG
Forgive me if I'm stating a basic obvious which you have accounted for already, but this bubbling up is still occurring with the reservoir pressure cap on, right? Because if you are running the car up to temp with the reservoir pressure cap off, it will eventually boil up and over at temp, normal behavior. If cap is on.... could it be something as silly as the cap is faulty? I've had one fail before (on a different car) causing overflow issues.

As for the oil/water mixing, I had it as knifebright above describes - the fix for me also was to replace the perished oil cooler to bracket seal, and also the one behind, i.e. bracket to the block (changed the oil cooler as well to be sure while in there, but the one that came off seemed fine). I did a recent post about it with pics on the 1.8 engine forum.
 
Last edited:

YamR1rider

Drag Racing Champion
Location
Tampa, FL
Car(s)
2017 GTI Sport DSG
Ya man, essentially my car hates coolant. I feel like we got to a place of resolution but like I mentioned, I’m not going to be shocked if I get a bit more residue and need to swap another bottle in a few thousand miles. My radiator wasn’t clogged until it was, there was just a bunch of gunk hiding somewhere that got dislodged in flushing is my guess. 4 water pumps, a bunch of hoses and the rest of the coolant parts I mentioned over the past few years. I’ll let you know if I end up with that heater core lol but I do think it’s fixed now…

Seems like you’re making progress though! Rule out head gasket when you can. Just about everywhere oil and coolant overlap, oil is at a higher pressure so hoping you can rule in or out head gasket so you can get to the bottom.

Good luck man, I swear they designed these things to be a fucking mess if you try to flush coolant!
I think I flushed my daughter's through about 4 times over a couple of weeks driving before it was clear of all the oil contamination (and it really wasn't too bad in my case). Also swapped out the heater matrix as thay wasn't working(fiddly job) ... It had some oily gunk in it but turned out it wasn't that which was causing the heating problem it was a stuck N82 valve a bit further up the line... More fiddliness but now all is great. Ha yep never a dull day in VW cooling system land 😁
 

mystix

Passed Driver's Ed
Location
Toronto
Some notes:

I took the thermostat out of the housing and the part number on the thermostat is 06L-121-113A and when you google that part number, you will notice that it triggers at 113°C.

What does it mean for me? Well I have testing with distilled water so far which boils at 100°C.

Anyhow, I am waiting for the combustion leak test kit to arrive. Will do this first and if it’s good then I will fill with coolant and hopefully all my bubbling complains are a result of my own doing.
 
Last edited:

DSC808

Autocross Champion
Location
HI State
Car(s)
2016 GTI SE MT
Only option now is to build the motor.
 

tmw2442

Autocross Newbie
Location
Approved NPC storage contanier
Car(s)
Mk7 R
Bummer. Headgasket failure seems almost unheard of on the ea888 motors it seems like
Very rare...and unusually induced my something, they don't normally, just fail. Could also be a failed coolant/oil heat exchanger, to cause the mix; but not in this case, apparently.
 
Last edited:
Top