Coolant Leak: a mini series

[Gertiethegolf]

2014 VW Golf TDI 100,000km. This is a doozy please read carefully.

Serviced on 03/02 with no prior issues and nothing mentioned by mechanic.
On 19/02, drove Drouin > Traralgon > Pakenham > Warragul with no dramas. When I went to start my car to leave Warragul the low coolant alarm sounded. Popped the hood and the reserve was empty. However I could see there was coolant at the very bottom of the tank feeding into the engine, so it was just the reserve that was empty.

Top it up, take to mechanic, passes all pressure tests and does not leak. Mechanic replaces radiator/reserve cap.

I take it back and check it before and after every drive. It holds solid at the max until 06/03 (so again, 2 weeks) I drive from Drouin to Moorabbin and when I check, it's dropped to min. I think it odd but decide to wait til it's time to leave. 3 hours later I come back and just like last time it's empty with coolant just at the very bottom of the tank and feeding into the engine. This time, I can see coolant is sprayed all around the engine bay and around the bottle as though it burst out from the top of the bottle.

Back to mechanic she goes and this time he had the car for a whole week and drove it in lieu of his own car to try and replicate the fault. Coolant doesn't budge.

Car was due for timing belt/water pump so we did those on 24/03, and that night when I drove the car home, I checked under the hood and what do you know the tank is empty and coolant is sprayed all around the tank but I can (thanks to some dye put in the tank) see the coolant is all at the bottom of the car on the frames and a small amount on the floor.

This time took her to another mechanic who has not been able to fault it at all.

Key things to note:

Always once car is turned off
Sudden leak, not slow
No over heating
Radiator cap replaced
Water pump replaced
Intermittent, not happening with every drive.
Seems to be a pattern of every 2 weeks.
No wet carpets/fluctuating temps/steam/smoke/etc
External leak, not the head gasket (also no milky oil)

Pleeeeeeeeeeeease help I (along with seemingly all mechanics) am at a loss

[Lucas_R]

I would change the reservoir bottle as the next step. Not only is it cheap and easy to do, but they are prone to deteriorate over the years with heat cycles and it may have a pin hole or hairline crack in it somewhere. The new coolant reservoir tank also comes with a new coolant level sensor which is another common thing to fail (although it seems like yours works as its telling you the coolant is low).

You said that you have seen on 2x occasions that the coolant has sprayed out in the engine bay, and one one of those occasions it looks to have come from the reservoir bottle.

5Q0121407M - Coolant Reservoir, Audi 8V S3/TT & Volkswagen MK7 GTI/R
– VAGPARTS Australia

If that doesnt sort it, then perhaps its time to venture towards Melbourne and have a VW/Audi specialist look at it. Someone like Underground Performance who are in Cheltenham perhaps.

Otherwise, it could be this (or one of the other plastic components)
Plastic housings and connector in a Golf 110 diesel

[gregozedobe]

I've read some reports of plastic coolant fittings cracking, especially one at the back of the engine near the turbo. Apparently they get brittle after a while.

As you have had the water pump replaced I'd be suspicious of a leak from the thermostat housing (the seal on those is notorious for leaking), and also all the clips/clamps on all coolant hoses that would have been disturbed during the replacement work.

Like Lucas_R I'd also be suspicious of the coolant reservoir (again, made from plastic which can get brittle after many heat cycles).

If you had access to a flexible borescope/snake cam that might help you locate the source of the leak, although if coolant is splashed around everywhere it might be hard to see where it is coming from.

Good Luck, and please add to the knowledge base by telling us where the fault was and what the fix is when you eventually find it.

[Gertiethegolf]

We both thought it was the bottle too, but it seems to be over pressurizing... and it's like whatever is causing to over pressurize, is forcing it out the bottle but also down the side of one of the metal bits (I know that description does not help) and finishing at the bottom of the car/frame/floor. It's so odd -.-

I would change the reservoir bottle as the next step. Not only is it cheap and easy to do, but they are prone to deteriorate over the years with heat cycles and it may have a pin hole or hairline crack in it somewhere. The new coolant reservoir tank also comes with a new coolant level sensor which is another common thing to fail (although it seems like yours works as its telling you the coolant is low).

You said that you have seen on 2x occasions that the coolant has sprayed out in the engine bay, and one one of those occasions it looks to have come from the reservoir bottle.

5Q0121407M - Coolant Reservoir, Audi 8V S3/TT & Volkswagen MK7 GTI/R
– VAGPARTS Australia

If that doesnt sort it, then perhaps its time to venture towards Melbourne and have a VW/Audi specialist look at it. Someone like Underground Performance who are in Cheltenham perhaps.

Otherwise, it could be this (or one of the other plastic components)
Plastic housings and connector in a Golf 110 diesel

[Gertiethegolf]

Update: Replaced the reserve tank, still happening. It appears to be over pressurizing so I'm starting to suspect the head gasket although it passed a Co2 test earlier on..

[MIG]

I wonder if the heat soak when you shut off the engine is causing the pressure to spike and leak out the weakest link in the cooling system.
I think those TDI use an Aux water pump but do not run it after shutdown like the V6s.

Have you tried taking it for a run (Try and get it hot)and watching it like a hawk with the hood open after you shut it off?