AO Platinum Member
Join Date: Feb 2002
Location: The land that Time forgot
You can get localised boiling in the block if the coolant is not being circulated (and cooled) quickly enough. That will cause vapour to be formed which will collect in the radiator, which is why I asked if the rad' felt cool to the touch.
This will also pressurise the system. So the header tank will be cold. The vapour in the rad' will take up volume .. pushing the coolant into the header tank and keeping it trapped (cold) there. Some will be forced out of the cap (at 1 bar pressure) so look for signs of that.
Some vapour will be reabsorbed when the car cools down but generally once you get an airlock while driving it all starts to go downhill... the air pocket in the rad impedes coolant entering it, so effectively you lose the majority of you cooling capacity and the car starts to overheat.
If you ease off the gas, the reduced heat produced allows the rad' (which is mostly full of vapour) to cool the trickle of coolant that is still passing through it.. that brings things under control.
The radiator fan doesn't come on, because the rad is full of vapour (cool) even though the block is boiling. The temp sensor is on the block but the rad fan is activated by a thermo' switch on the rad'.
At 115C the ECU intervenes. It takes it's reading from the temp sensor on the block, regardless of what the fan thermo' switch is detecting. It cuts in the second speed of the fan which sounds as if it is cooling the little coolant in the rad effectively enough.
However, it comes on again soon after, as the volume of coolant it has cooled down isn't that great.. it quickly heats up again.
So.. something is making the car overheat/incur localised boiling...
My theory is STILL the water pump. Everything else is new frankly... and I bet a cooling system pressure and CO tests (for failed head gasket) come up negative.
Is there air in the rad' once it's cold?
Last edited by Ralf S.; 22-08-07 at 08:14.