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.
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.
