Filling the Radiator

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Original submission by T Mar 5th 2007:

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Radiator Cap pressure details are stamped onto the Radiator Cap. Click to Enlargen. Photo by Todd Rogers, image enhanced by T.

Filling the Radiator:

Trapped Air:

When refilling the radiator on an Old Holden after a full draining, certain steps need to be taken to purge the air from the water jackets in the block and cylinder head(s). In Old Holdens the top of the Radiator is higher than the top of the Engine which allows air to naturally make its way to the Radiator Cap.

The Heater Must be Turned On:

Turning the Heater to the on position will make sure that no air pocket is left. Any air that is left in the Heater Core or Block will make its way into the Head(s) and cause overheating at an embarrassing time.


Before Starting the Engine, pour Coolant into the Radiator until it's full. The Radiator will bubble Air out as you fill it and need  extra Coolant to bring the Level back up. Once the Radiator is full, start the Engine and allow it to warm up.

Trapped Air:

Air will be trapped in the block and Heater Core. This air will be pushed around the block and make its way to the base of the Thermostat as the Engine warms up.
Coolant Recovery Radiator Cap from a 6 Cylinder UC Torana. Note that the cap has a large rubber seal in the top to force the overflow coolant out of the overflow pipe as well as a smaller seal to provide the pressure seal against the Radiator. It also has a brass valve in the centre to allow the Coolant to flow back into the Radiator when the Engine cools down. Also note the presence of contaminant under the central valve which has been drawn back from the Recovery Bottle. This can cause the cap to lose pressure  when the Engine is hot. Click to Enlargen. Photo by T.

Fitting the Radiator Cap Last:

When the Thermostat opens, trapped Air flows through to the Radiator Top. By keeping the Radiator Cap off until the Engine heats up, this Air will be allowed to escape with ease rather than remaining inside the Radiator causing trouble. The Radiator is full when the Engine is hot and no more Bubbles come out. You will see the Coolant start to overflow. This is a good time to fit the Radiator Cap.

Coolant Recovery Bottle:

If fitted, note this is labelleled Max Hot and Min. Air is purged from the Radiator when the Coolant Pressure comes up. When the Engine cools down again, Coolant will be drawn into the Radiator to replace the expelled Air. That means you will need to top up the Coolant Bottle after the Engine has cooled down from the first run.I favour fitting a Coolant Recovery Bottle, and filling it right up to the top before Starting the Engine and letting the first run cool down take care of the Level. It's better to have too much than rather too little Coolant in the Engine. The Coolant Level will drop noticeably during the first cool down.

Radiator Cap:

As the Engine heats up, make sure the Radiator Cap holds Pressure by occasionally squeezing the Top Hose. If the Hose doesn't become harder to squeeze by the time the Engine is hot, the Radiator Cap is leaking. The Cap may be faulty, or  have contaminant preventing it from sealing. Fit a new Cap or clear the sticking item otherwise the Cooling System will not behave correctly.


The Cooling System is up to Temperature when the Radiator Top Hose is hot to touch. This means the Thermostat has opened. The Top Hose is where the hottest Coolant is found.

Special Cases:

Note that Nissan powered VL Commodores *must* have their cooling system bleed holes purged of any air otherwise the cylinder head will crack. This is because the Engine is higher than the top of the Radiator in these cars and provides no natural way for any air to escape. The air must be permitted to escape through the bleed holes in the inlet manifold.


Coolant Recovery System



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