Radiator catch tanks

[Dudley Smith]

My 105 has been fitted with a pressurised catch tank fed from the radiator overflow pipe.
When the engine is hot, water flows into the catch tank, but as the engine cools down the water does not flow back. As hot water flows in, air is expelled past the pressurised cap, but as it cools down air cannot come back into the catch tank to let the water flow back to the radiator. Instead the system sits with negative pressure until it is warmed up again.
I wonder if it would work better if the catch tank was open to the atmosphere. The system would not be pressurised, but the catch tank water should be able to flow back as it cools down.
Has anyone else had this problem?

[jameswheildon]

Hi Dudley,

Thinking about this, in the original system, as the water warmed up it would be expelled through the ball valve at the top of the radiator, onto the road. For a catch tank to work it needs to be connected to the radiator overflow but with the ball valve removed or fixed open. I have seen this done using a small zip clip fed down by the ball to stop it from seating. The beauty of this is that the system can easily be reverted to original. The catch tank needs to be large enough for there to always be some air in it to be compressed as the water expands. I don't think air should be expelled from the system when hot, just compressed. The tank needs to be high enough for the water to "fall" back into the radiator on cooling. Does that make sense?

[jameswheildon]

Just rereading that, the catch tank must be mounted above the header tank to allow the water to return under gravity.

[Dudley Smith]

Hi James,
The ball valve has been removed on my car and although the tank is fitted as high up as possible on the bulkhead, it isn't actually any higher than the radiator.
I agree that air compressed in the tank should push the water back on cooling but it doesn't happen on my car. I think some of the air must escape. I am reluctant to run the system with very high pressure, so I may experiment with a cap that lets the tank breath.

[jameswheildon]

Hi Dudley, The system should work perfectly well with the cap just loosely fitted, not sealed. If the water does not then return to the radiator it can only be because the tank is not high enough. If the system was designed to work pressurised like a modern car, then it is losing pressure somewhere. However Talbots were not designed to run with more than perhaps 2 psi and it would be a major task to upgrade, especially the water pump spindle, although some cars do appear to have a full pressure system. I think the car that used to belong to Mike Pierce is one such. Ian Potts might be able to throw some light onto it.

[dick campbell]

Each to his own but I cannot see the advantage of these expansion tanks. If the system is filled to the top of the radiator when cold and then the engine run up to temperature, thermal expansion of the water will push out about a half pint through the ball valve. After the engine is cool and then on inspection the water appears to need topping up…..if you do then another half pint will come out….and on and on. If left and then checked hot, carefully, or ran up to temperature without the radiator cap on the water level will be full. I have not put any water in mine for 5 years, and only then when I re-sealed the top hose.

[Dudley Smith]

I have now fitted a cap to the catch tank which is not pressurised and allows the system to breath through the catch tank overflow pipe.
Now when the engine is thoroughly hot the tank is three quarters full (about 3/4 litre). When the engine cools all the water is sucked back into the radiator, so the system is now working in a better way.
Nevertheless, whether I have driven 10 miles or 100 miles it always takes a pint to fill the system up again. I suspect that it just chucks out what it doesn't need and then stays at the same level. The good news is that I have had no cooling problems at all having just returned from a 600 mile trip to North Wales on the STD National Rally.
I wish there was a way of knowing how much water is in the radiator.