Question: Over the past year and a half I’ve had an apparent overheating problem on my 2004 Land Rover Freelander. The temperature gauge does not go to boiling point, but the water in the expansion tank keeps boiling. I have taken the cylinder head, engine block and radiator to be overhauled, but the problem persists. What can it be?
Answer: Well, Lebo, you have certainly covered all the main bases. Assuming all the work was done to professional standards, it has to be a relatively minor thing that’s been overlooked all along. I was stumped. But my wizened old mechanic friend, veteran of many scalded fingers, immediately came up with a theory when we discussed your letter. “What about a faulty pressure cap on the cooling system?” The pressure cap usually incorporates a calibrated pressure-relief valve whose job it is to keep the coolant under pressure, thus raising its boiling point.
At Gauteng altitude, water will boil at about 94°C at barometric pressure. If a further 1035 millibar (15 psi) of pressure is imposed on the water, as you may find in a typical cooling system with a properly functioning pressure cap, the boiling point increases to 119°C. The added pressure allows the coolant to get hotter than its “normal” boiling point without actually boiling. Perhaps the temperature gauge takes this into account and shows the boiling point as being the temperature where the pressurised coolant will boil. If the coolant isn’t pressurised, it will boil before the gauge gets to that point. I suggest you examine the pressure cap as a first step.
The seal may have perished or the spring regulating the relief valve may have become tired. If nothing untoward is found there, you have to look wider, but then you will be revisiting components like the thermostat, head gasket, hoses, and so on that have presumably already been attended to.
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