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Coolant System Mystery

754 Views 1 Reply 2 Participants Last post by  Schod
I have an Audi A3 2.0tdi 2014 that has a very frustrating problem.
The car is leaking coolant when on a run, the coolant is coming from the expansion cap and seems to be caused by back pressure building up in the coolant system. The temperature gauge does not move above the normal running temperature though and I have had no problems with overheating
I have also noticed that the cabin heater blowers are not as hot as they used to be and that the passenger side is cooler than the drivers side.
The car has had the expansion bottle and cap changed, the heater matrix pipes are both hot and the garage checked for combustion gasses in the coolant but could not find anything, so hopefully the head gasket is ok?
Has anyone had this issue and managed to find out the cause please, as I am rapidly getting very frustrated that no one can locate the problem?
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Pressure relief valve.
On some vehicles there is a pressure relief valve on the coolant bottle/ header tank / expansion tank cap - there is a pressure test available for the coolant system.

Expansion cap.
The O ring seal and its seat can distort, the relief valve fail.

EGR cooler - unlikely to show pressure in the coolant system.
A leak at the EGR heat exchanger could partially pressurize the coolant system (high rpm and exhaust blockage) but is more likely to show as steam/white smoke from the exhaust and coolant loss.

Head gasket leak.
A head gasket leak can be determined by testing the coolant - colour change in test chemical - . A head gasket leak may only show at a certain temp or and vibration. If you can see into the expansion tank you might see small bubbles rising. Usually the gas is present within the coolant and can be detected using the test chemical.
A head gasket leak gets worse over time. Check oil level. Check coolant loss. Check exhaust colour.
Oil cooler.
The oil cooler can leak oil ( higher pressure than the coolant) into the coolant which may not show till later when it may be seen in the expansion tank.
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