Hi, I would really appreciate your opinion about a coolant problem I’m having with my car.
I have a 2014 Volkswagen Golf 7 GTD, 2.0 TDI 135 kW, engine code CUNA, manual transmission. The car has around 220,000+ km.
The main problem is that I have been losing a lot of coolant. At one point I was losing around 6 litres in 5 days. I initially thought it was an external leak, but the problem was difficult to locate.
Here is everything that has already been checked/replaced:
The head gasket was replaced.
The cylinder head was also replaced/repaired because there was a problem with the head.
The head was properly installed with a new gasket.
The EGR cooler was replaced with another used unit.
The water pump is new.
The small radiator/coolant circuit was checked and there were issues with it previously.
Some coolant hoses were found to be damaged/perforated and were replaced.
The coolant expansion tank was replaced with a new one.
The cap was also replaced.
A cooling system pressure test was performed and no external leak was found.
I have done two CO₂ tests for combustion gases in the coolant, and both were negative.
There is no visible oil contamination in the coolant.
After the head gasket and cylinder head repair, everything was completely normal for around 2,300 km. The engine temperature was stable and I had no significant coolant loss.
After those 2,300 km, the problems started again — the engine started overheating under load and I started losing coolant again.
The overheating now behaves strangely. If I drive normally, the temperature is stable, but if I push the engine a little harder, the temperature starts rising again. If the temperature reaches around 100–105°C, it drops back down very quickly as soon as I release the throttle and drive another 100–200 metres with less load.
However, if it gets as high as around 130°C, it does not cool down nearly as quickly. Before the head gasket/cylinder head repair, I had a similar problem where the temperature would rise under heavy load and then drop again when going downhill or reducing the load.
So the fact that it can heat up under load and then cool down very quickly when I release the throttle is still present, although the problem was completely gone for approximately 2,300 km after the repair.
However, I still have a coolant problem.
There is noticeable pressure in the expansion tank. Even after the car has been sitting overnight following a drive, there can still be some pressure when I open the cap. When the engine is hot, there is obviously more pressure, and if I open the cap, the coolant level rises significantly.
The strange thing is that the coolant sometimes appears to come out around the expansion tank/cap area because the level rises and the system builds pressure. I have also seen coolant leaking underneath the expansion tank area after parking the car. The expansion tank is new, but it feels somewhat loose/poor quality to me.
When the engine is running and warming up, I can also see very small bubbles/foam-like bubbles in the coolant, especially around the return flow into the expansion tank. It looks like coolant and air are circulating together rather than being completely clear coolant flow.
The car can sometimes lose a noticeable amount of coolant, but the pressure test does not show an obvious external leak.
Because the head gasket and cylinder head have already been done, and the CO₂ tests were negative twice, I am trying to understand what else could cause this combination of symptoms.
In your opinion, what would be the most likely cause at this point? Could this be a cooling system circulation/bleeding issue, faulty tank/cap, trapped air, EGR cooler, oil cooler, thermostat/housing, or could combustion gases still be entering the system despite two negative CO₂ tests?
What would you check next before replacing any more parts?
Thanks, I’d really appreciate your honest opinion.