Best way to bleed coolant system?

[JamesQB]

Hi,

This is a general car question... is there a 'proper' way to bleed coolant systems after something like a rad change? Spent hours tonight helping a friend change his radiator and don't know of any approved methods for bleeding so spent ages mucking around.

Put new part in, filled expansion tank, looked for bleed valves and found none (although in the past, these haven't helped me much). Got engine to temp with cap partly screwed on expansiion tank. When thermostat opens and fan kicks in, water erupted from expansion tank with air bubbles too. Assumed air expanding in system was forcing fluid out. As engine cooled from fan going, remaining liquid suddenly sucked back into system leaving expansion tank empty.

Repeated and same thing happened. Got friend ready with coolant to fill it as soon as it finished erupting and started sucking back into the system. Thought that must have done it, but system erupted next time thermostat opened. And so on.

In the end, stuck expansion tank cap back on and let it get to temp over and over. Liquid level didn't budge. Is my mistake leaving cap partly undone for air to escape? Is it air trapped causing these eruptions? What's the proper method!?

Got him home ok, so don't know what to think.

[Homer]

You don't say what car it was, there are techniques for certain cars, especially if the header tank is below the highest level of the water system. In which case you need to somehow extend the header tank.

If you have got the car to running temp without the cap on then the water could be boiling (the cap keeps the system pressurised which raises the boiling point).

Shouldn't need to get the thermostat to open in order to fill the system properly.

In the absence of any bleed points I would disconnect a pipe at the highest point of the system while filling.

[citronut]

as Homer says depends on what car,

if PSA i never have trouble by first filling with the top end bung removed from the rad, fill till coolant comes out and stops sinking,

then open thermostat and heater bleed valves and keep filling till coolant come out at these points, if it doesnt then satart the engine and maybe bring the rev's up a bit, once coolant comes out at these points close those valves,

carry on running engine with cap off and once the thermostat opens top up as required till it stops sinking, replace cap and run till fan/s cut in and back out,

now gently open cap it might erupt the first time if so top up and the lst stage again,


regards malcolm