411 Radiator Caps

[peter dowdle]

Can anybody tell me for sure what radiator caps I should use on my 411.There seems to be varying opinions as to what to use on the radiator and what to use on the catch tank. Some say a blanking cap on the radiator and a pressure cap on the catch tank. Other have the reverse idea. Does anybody know what the factory set up was ?

Regards Peter Dowdle

[Geoff Kingston]

Peter,
Blank cap on the rad pressure cap on the catch tank, that was the set up on my on my 410 and 411 now sold and my two current 603's.
Geoff.

[peter dowdle]

Geoff that is wat I have been using. I dont have any leaks in the system but when the radiator is cold I check thelevel of water and it is always down a bit .I thought the idea was that it should recover water from the catch tank as it cools leaving the radiator full. Any thoughts as to why?
NB I leave the catch tank about half full.

[peter dowdle]

The word is WHAT

[Kevin H]

411 owners manual says 13 psi cap on the expansion tank. It doesn't say what the cap is on the radiator.

[Barrie]

13lb pressure cap on expansion tank and plain cap on radiator.

The plain cap is to refill the radiator only.
The pressure cap is to allow excess pressure in the system to vent.

Expansion of the water from the radiator sends it into the pressure tank and returns when it contracts.
If pressure gets above 13lbs sq inch above bar, first air, then water vent through pressure cap on the expansion tank.
If you put the pressure cap on the radiator then the water in the radiator will only flow to the expansion tank when the pressure is above 13lbs sq inch and, crucially, it cannot return as the pressure cap is a one-way valve and so any water which makes its way to the expansion tank cannot return thus negating its reason for being.

Pressure caps on radiators are only for systems with no expansion tanks.

PS there are at least two different depths of pressure caps. If yours is too shallow it will not allow pressure to build up and so overflow more easily. Identification of the correct one is by observation and feel: when you insert the cap it should protrude above the rim of the aperture and you then press down to compress it and then turn it to close. Also note that the springs in the cap weaken over time so should be replaced regularly.

[peter dowdle]

My local garage fitted a pressure cap on the radiator , Its called a recovery cap allowing the cooling water to be sucked back into the radiator. They also fitted a pressure cap on the overflow tank . The second cap in my opinion should not hold pressure. Apparently this system is used on a lot of modern cars.

My conclusion is as expressed by all replies is the blanking cap fits the radiator and the pressure cap goes on the tank.

I will need to consider further why the water does no return to the radiator.

Could be a very slight air leak .

[Geoff Kingston]

I must admit that I am a bit lazy about actually putting some water in the expansion tanks but on my 410 and 411 when I owned them the water always settled down a bit after I topped upto the rim. This was the same on my first 603 but the second one just bought last Tuesday is different, because there had been slight evidence of a possible minor leak in the pump area before the journey home my mechanic pal at least half filled the expansion tank, after the journey back about 350 miles the level in the rad was the same the next day, just below the rim, later in the day while testing lights etc with the engine running at idle I realised it was getting hot as the otter switch was not working, the override brought the temperature down but water had been expelled from the overflow on the expansion tank. When I took the cap off the rad the following day I expected the level to have dropped but it was absolutely full so a bit of extra water had come back in from the expansion tank.
I have a trip with the car planned next Monday so I won't put any extra water in the expansion tank to see what happens. Still debating whether to change the otter switch of put a more modern controller in the system.
Geoff.

[Barrie]

Hello Geoff. I have always worked on the principal that the radiator should be filled to the brim and the expansion tank should be empty so that water is expelled from the rad to the tank. The tank should be designed to hold only the expanded amount of water that an engine at working temperature provides. If you interrupt this by putting extra water in the expansion tank (as you say) then it will be expelled through the pressure relief cap in the expansion tank.
In short, if, with a cold engine, you fill the radiator to the brim and leave the expansion tank empty that situation should be the same when the engine is cold again. If not it is then it is a sign of an air leak or something more significant like a cylinder head gasket.

[Barrie]

Deleted double post.

[Thomas591]

Hello Barry,
I am sorry, but I don’t think that leaving the expansion tank empty is the right way to do it. I suggest to fill the tank at least one third with coolant.

I checked the owners handbook for the 411 and it confirms my opinion:

Quote:
Fill the system to the top ofthe filler neck and replace the filler
cap.
Run the engine for a short period and then allow the engine to cool off. (Turn the heater ON before filling the radiator).
Half fill expansion chamber when filling radiator.
When the system is cool remove the filler cap and top up to the same level again. Recheck after a short run.

Regards

Thomas

[Geoff Kingston]

Thomas,
The 603 manual says the same and when Mike topped up the expansion tank as I said a fair bit of water ended up on the floor. As someone who has run Bristol V8's for over 40 years now and never until these posts thought of adding water to the expansion tanks or topping them up I am with Barry on this and it is exactly as the manual on a Cadillac I used to run said, Rad full expansion tank empty.
I shall keep an eye on the rad but not the expansion tank, BTW my manual for the 603 says the pressure cap should be rated at 15psi.
Geoff