The strength of the BMW M4 Coupé is evident in every detail.
Coolant loss is a cause for concern, but many live with it as “just one of those things” and are quite happy to check and top the system up regularly.
When I remarked to a client that his car’s coolant level was low, he shrugged and said he kept his eye on it and refilled when necessary. He said the engine never overheated, even around town, and there were no obvious leaks.
He had had the system pressure-tested and no leakage was shown. There was no steam from the exhaust either, so he assumed the coolant loss was normal.
The expansion tank filler cap is a prime suspect when coolant loss occurs. Some people, as a temporary measure, tie a plastic bag over the end of the overflow pipe to see if any coolant is escaping.
Rather than fiddle around like that, I would fit a new cap.
Let’s face it, if the engine is losing coolant, it must be leaking out or leaking in. In serious cases of the latter, it should show up on the dipstick.
Instead of the normal oil colour, there will be white emulsion on the dipstick and the oil level will rise.
Another possibility is that it is leaking into the induction system and this would make itself known by the previously mentioned steam from the exhaust.
Combustion pressures leaking into the cooling system would cause it to become over-pressurised and if you suspect this, ask at your friendly local garage mechanic to hold an exhaust gas analyser over the coolant in the reservoir.
This will clearly show if combustion pressures are leaking, probably past the cylinder head gasket, into the coolant.
IT’S NOT NECESSARILY THE RADIATOR
I suppose it’s natural when coolant loss occurs to immediately imagine a radiator fault. But rather go for a pressure test before laying out money for a new or reconditioned radiator. We have come across leaking hoses; some of them looked fine but were porous.
A corroded thermostat housing has been responsible for many a leak and is far less expensive to replace than a radiator. You might have a tough job if the bolts have corroded into the housing, but with penetrating oil and patience this can often be overcome.
Overheating for any reason, if allowed to continue for long, may damage the radiator, water pump and head gasket, so it’s a catch-22 situation. Many water pumps have plastic impellers and heat does them no good at all. When overheating occurs, check to see if the radiator gets hot at both top and bottom as it should.
SQUEEZE THE HOSE
Poor circulation may be caused by an air lock, blocked radiator or failing water pump. Note that water pump failure could cause the cam belt to break and possible engine damage. If in doubt about coolant circulation, with the cap off, squeeze the bottom radiator hose several times. The level should rise and fall as you squeeze and release. Drain and flush the radiator with a flushing compound.
Fans can go wrong too. Make certain that electrical type is working. Mechanical types with a central clutch may appear to rotate even if this clutch fails, but not powerfully enough to displace air. If in doubt seek professional advice. - Star Motoring