BLEEDING HELL

▶ The final repair task of bleeding air from the system can be annoyingly arduous but do not underestimate its importance. Be familiar with the location of the bleed screws on your car and follow any filling advice. Some older cars dictate that the screws are open, to help expel air as you add new coolant. Modern thermal management systems may need their filling mode activating diagnostically, via the EOBD socket, to ensure that any valves are kept open, or the appropriate electric water pump(s) are activated, when required. Again, you must research these considerations, before starting work. The engine should then be run (not driven) as the system is bled further, following the manufacturer’s instructions.

Large air pockets impede coolant flow, causing localised overheating at the very least. This is more of an issue with modern cars that possess heat exchangers that transfer heat energy to and from the coolant.

To be effective, these units contain very thin aluminium walls. Should air be present, localised overheating causes the aluminium to crack. In the case of an automatic DSG transmission coolers, for instance, an internal breach would permit coolant to enter the gearbox, risking rapid destruction of the internal friction coatings. Alternatively, transmission fluid could also contaminate the thermal management system.

Should you find that your coolant level is falling and you cannot fathom why, do not snap-diagnose a faulty cylinder-head gasket. Look closer at these heat exchangers and be extra vigilant to bleed out as much air as you can, when conducting a coolant change.