Coolant loss

I am writing to you from Singapore, where I drive a 2007 Saab 9-3. My cooling system is a completely pressurised one – in common with most European models – and the reservoir is also pressurised. Assuming there are no coolant leaks (ie, no visible leaks or strange odours), is it normal for my Saab to lose some coolant over time? Is evaporation of coolant still a factor with pressurised systems?

Edward Tow

You do not mention the amount the coolant that is required for topping up, but this should not normally be required on a regular basis. During normal operation, evaporation of coolant in a pressurised system would be minimal. The sealed cooling system should not lose fluid, providing the expansion tank has sufficient room for the quantity of coolant when hot. If the cooling system is overfilled, the excess will escape via the valve in the system's pressure cap when warm.

Cooling system loss can sometimes be difficult to pinpoint, especially if the loss is minor. If possible, the system should be checked with a pressure tester and its components should be examined for signs of leakage. Coolant will normally leave small tell-tale marks in the form of white stains around the area of the leak. Also carefully examine the seams of the radiator, which may allow slight loss without leaving an easily visible trace.

Another possibility is that the water pump is weeping slightly. You do not mention which engine combination you have in your vehicle, but it is likely your water pump is hidden behind the timing cover. There is also, of course, the possibility that the head gasket is starting to fail, allowing coolant loss through the engine. This is normally accompanied by overheating, a possible misfire and heavy smoke from the exhaust – symptoms which you do not appear to have.

Finally, the cooling system cap may be failing to retain the pressurised coolant.