Coolant overflow

[Fu Manchu]

With a poor seal on the cap. The surface that the cap seats into may be pitted or have minor corrosion and pitting.

A leaky water pump weeping ever so slightly. An old hose. A loose radiator hose. A previously faulty cap that pushed out coolant and drew in air.

A new cap on the system without bleeding out the air.

Also when filling a system air is introduced firstly by having it exposed to air (kind of a big one) and then the filling process both introduces air with the liquid and traps air in the system as it is filled. Like a submarine filling with water. There’s going to be air trapped in all sorts of places tucked away.

The only way to get it out is to attach a bleed bucket and run the car through two or three thermostat openings. Working the hoses with gloves on. Patience.

Or

Using a vacuum fill kit. Requires a compressor. All air is sucked out. Then it has to hold vacuum for 5mins or so. If it doesn’t, there is a leak. Somewhere. Then filling the system under vacuum takes just a minute or so and no bleeding is required.

 

[Revidup]

With a poor seal on the cap. The surface that the cap seats into may be pitted or have minor corrosion and pitting.

A leaky water pump weeping ever so slightly. An old hose. A loose radiator hose. A previously faulty cap that pushed out coolant and drew in air.

A new cap on the system without bleeding out the air.

Also when filling a system air is introduced firstly by having it exposed to air (kind of a big one) and then the filling process both introduces air with the liquid and traps air in the system as it is filled. Like a submarine filling with water. There’s going to be air trapped in all sorts of places tucked away.

The only way to get it out is to attach a bleed bucket and run the car through two or three thermostat openings. Working the hoses with gloves on. Patience.

Or

Using a vacuum fill kit. Requires a compressor. All air is sucked out. Then it has to hold vacuum for 5mins or so. If it doesn’t, there is a leak. Somewhere. Then filling the system under vacuum takes just a minute or so and no bleeding is required.

Whoa time for this fella to stick to cabinet making and let the pros handle it

With a poor seal on the cap. The surface that the cap seats into may be pitted or have minor corrosion and pitting.

A leaky water pump weeping ever so slightly. An old hose. A loose radiator hose. A previously faulty cap that pushed out coolant and drew in air.

A new cap on the system without bleeding out the air.

Also when filling a system air is introduced firstly by having it exposed to air (kind of a big one) and then the filling process both introduces air with the liquid and traps air in the system as it is filled. Like a submarine filling with water. There’s going to be air trapped in all sorts of places tucked away.

The only way to get it out is to attach a bleed bucket and run the car through two or three thermostat openings. Working the hoses with gloves on. Patience.

Or

Using a vacuum fill kit. Requires a compressor. All air is sucked out. Then it has to hold vacuum for 5mins or so. If it doesn’t, there is a leak. Somewhere. Then filling the system under vacuum takes just a minute or so and no bleeding is required.

Whoa, time for this fella to stick to cabinet making and let the pros handle it

 

[Fu Manchu]

Nothing wrong with having a go.
I’ve spent quite a while having a go at cabinet making. I’d like to hand it over to a pro but sometimes learning new skills and taking time the pros won’t afford is worth the result. Getting all the little things done.

 

[Skylarking]

How does air get into the system in the first place ?

As the engine heats up, the coolant expands and the pressure forces some coolant into the overflow bottle. When the engine is shut off, the coolant contracts so it’s drawn back into the radiator by the vacuum that’s created. That’s the breathing FU talked about. It is normal with no coolant lost and no air sucked in.

For this breathing to work well, the radiator cap must function as designed and open at the correct pressure (to let coolant out) but also open when vacuum exists within the system (to draw coolant back).

One indication of a faulty radiator cap is when you see the top and bottom radiator hoses squashed as is a great identifier of a stuffed radiator cap that doesn’t open due to vacuum (caused by fluid contraction as the fluid cools).

But if there is a bigger issue and the engine is overheating, too much coolant can be pushed out into the overflow bottle. This can result in some coolant being dumped on the road as you’re driving. After all, the overflow bottle can only store a certain volume of coolant. In such cases, when you shut the engine off, and it cools down, not all the coolant can be drawn back as some has been dumped on the road. Instead, the vacuum will draw all the fluid from the overflow bottle until none is left and any remaining vacuum within the system will equalise by drawing air from the now empty overflow bottle.

This sucking of air into the cooling system via the overflow reservoir will just cause more engine overheating and is a bad cycle.

So to avoid this cycle, one must ensure that; (1) all air within the cooling system if fully bled out, and (2) the overflow bottle coolant level is at the cold mark when the system has cold (it’s a pita to read the VF overflow bottle coolant level accurately), and (3) the radiator cap works at the correct pressure and vacuum, and (4) the radiator cap sealing surface isn’t scratched or damaged.…

So if all the above checks out ok, then get the cooling system pressure tested both when cold and when hot.

 

[Revidup]

Thanks heaps for the explanation, very detailed