Join Date: Jul 2003
Location: Gloucestershire - Cotswold Cockney
Car: MG Montego Turbo; 620ti x3; MG ZS; MGF and MG ZT 1.8t x2; MG ZT-T x1
So, in answer to the OP's question, it is the expansion of the coolant as it heats up that causes the system to pressurise. It is normal in a healthy engine. That additional pressure enables the coolant to operate at a much higher temperature without boiling than it would do at normal atmospheric pressure.
However, should there be a leak in the sealed cooling system, then it is not sealed. That means normal healthy operating pressure is never reached and that leads to premature boiling of the coolant.
Leaks come in many forms of course. Anything from a badly fitted coolant hose, a worn out radiator to a worn Water Pump allowing coolant to escape. A cracked or warped engine part allowing pressure to escape or compression gases from the cylinder to enter the coolant passageways will also cause coolant to boil over badly by overpressurising the system.
Two so called CHG 'failures' I investigated on "Spares or repairs" Rovers I've bought were as a direct result of incompetent repairs by professionals. One was a T-Series Turbocharged 600ti which had a poorly skimmed cylinder head ( close to £1000 Invoices for the work done ! ) allowing powerful compression gases into the coolant passageways once fully warmed up. The other a simple new exhaust manifold gasket incorrectly fitted on a K-Series which meant the Coolant "Elbow" could never seat properly on its own gasket allowing a steady loss of coolant once the engine warmed up.
Oh yes, worth mentioning two other so called pro repair "failures". One was a low mileage MG Montego Turbo which the previous owner had been charged £500 to replace the cylinder head gasket but it still boiled over once warmed up. Trapped between its Cylinder Head surface and Head Gasket was a small pea sized bit of dirt crushed near flat but allowing compression gases into the coolant!
The other a work colleague's one owner from new MG Montego 2.0 EFi which only needed a new Water Pump, not the £600 cylinder head gasket renewal he was quoted by a couple of repair shops. The water pump cost less than twenty quid and less than an hour to fit on the O-Series. Simple inexpensive repair.
I could mention other examples but the above will serve to alert folks that all too often the cars are wrongly blamed because as everyone knows following widespread and hugely negative media publicity....
THEY ALL DO THAT!
From the best evidence available to me, that of my own eyes and direct experience.... they do not all do that. The real failures lie elsewhere.
Apparently ... in a parallel universe MGJohn drives .........A.......
Last edited by MGJohn; 30-12-2012 at 10:33.