Radiator caps , anyone know why so many?

Anyone know why there is so many diffrent pressure rated Radiator caps

4lb , 7lb , 13lb , 15lb , as the XK engine’s in diffrent car’s are more or less the same , I would have thought they would have had the same pressure cap !

After all it’s just a basic valve right , to let off too much pressure .

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The higher the pressure the higher the boiling point. As systems and components improved manufactures were able to raise the pressure caps. If my memory serves me correctly modern cars run sealed systems and only have an emergency type PSV to relieve extreme pressure. I also seem to remember that the drive for higher pressure operating systems stems from environmental and engine efficiency goals.
Far more knowledgeable chaps (the various Paul’s) will correct me if I am wrong (again)
Best regards

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Your explanation looks good from my house!


To add to Phil’s explanation, Jaguar used different pressure ratings depending on whether or not the car was equipped with air conditioning. Seven PSI without and thirteen PSI with, IIRC.

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You need to consider the whole system, and not just the radiator cap in isolation.
Once upon a time, all cars ran unpressurised cooling systems, but as the engineering improved, pressurised systems were introduced to increase the boiling point of the water/coolant. Initial efforts were usually limited to 4 psi, as with XK cooling systems, but that involved upgrading the entire cooling system to be reliable and not subject to failure at 4psi. Thus radiator cores, all hoses, hose clip design, pipe design etc had to be improved to withstand 4 psi. The pressure cap ensures the cooling system doesn’t exceed 4 psi, and thus exceed the design of the cooling system. Later on, as there were greater and greater demands on the cooling system, all aspects were improved, allowing higher operating pressures without cooling system failures, thus the 7, 13 and 15psi caps that limited the operating pressure to 7, 13 and 15psi. Its not an exact science, and levels of confidence come into it. I personally will always stay with a 4 psi cap on my XK140, albeit at a touch would probably be able to get away with 7psi, but I wont. I definitely would NOT ru with a 13 or 15psi cap, as that would be extremely high risk of cooling system failure.

Many years ago now I had an XJ40 saloon, one weekend fitted new top hose from a local Jaguar supplier, who had sourced 1/2 price top hoses of same shape/size ex Volvo. Car started up and ran OK, so drove around to a friends, parked car on his drive, went inside and came back to see a gushing pool of green coolant all over the drive way. The new Volvo hose had split along the entire top, hose-clamp to hose clamp.

Seems the Volvo hoses were speced/made for their 7 psi cooling system, and not strong enough for the XJ40s 13/15 psi designed system.

So unless you are prepared to risk all aspects of your XK cooling system, or prepared to upgrade the pressure rating of everything pressurised in system, stick with the original 4psi radiator cap, and hope that any aftermarket hoses etc you buy are well made/good for 4psi. With XKs one of the first places an over pressurised system will fail, apart from cheap hoses, is in the heater radiator core. These heaters were not designed/built for much more than 4/7 psi.

If your XK is overheating, don’t blame the radiator cap - find the real problem and fix that.

Thanks for that Roger
I don’t think Jaguar knew what cap was best , not sure what it was at first on the MK2 , but they changed it to 9lb , then after a few months down to 7lb , then down to 4lb , so I have read !

I will stick with the 4lb .

I can understand diffrent pressures with diffrent manufactures , but not why so many on the XK engine ,

I know a Engine will do more work with Air con on , so may get hotter , but can’t see every hose and heater components being diffrent

I would say Jaguar knew best , but the more I read the less I think they knew :grin:

Data point: my 3.8 E Type, with an after market radiator, ran a 13 psi cap, most of its life.

Not that it was really needed (except when it was being raced) because, with said radiator, the trmp rarely exceeded 90C.

No other issues were ever noted.

My earlier post referred to what was commonly found on E-types. I don’t think Jaguar changed anything but the pressure cap when they went to a 13 lb. cap for AC equipped Series II cars.

I’m hoping my XK120 will be safe with the 7lb cap. I have a very well-made looking NAR alloy radiator, all new hoses and clips, plus the block has just been cleaned out and new core plugs fitted. A slight worry is the heater - a new one was fitted in 1998, though Jaguar Cars fitted the previousl one when the car was just under 2 years old in 1952.

the other issue re radiator cap over 4lb on the XK120 thru early Series 1 EType is the thermostat : XK120 thru early Series 1 E, have a Smiths bellows sleeve type thermostat, which can be damaged if over 4 lb cap, and will be damaged .,.if over 7lb. …when,the coolant does reach a high enough temp to pressurize the system. (WHEN it does…,not if)
see recent post on XK Forum: Radiator Cap for XK120 early 140,

The actual original cap used on a 3.4 litre XK150 is a C.10289 which is
rated at 4 psi, and originals were made by AC (UK) See recent posts on the XK forum,"Thermostats for the XK120 a study,