XK 120 pressureless cooling system

I have used EVANS waterless coolant( boiling temperature 180° C ) for 3 years in my XK 120 OTS .Recently I discovered a small pool of coolant on the floor and located a hole in the quite old radiator as a reason for that . It has been soldered meanwhile .
The system is upgraded with an expansion tank and 4 psi cap .
I am considering now operating the system with atmospheric pressure .
Could I create other troubles with this measure ?
Any advice is appreciated .


As liquids expand when the temperature rises (even Evans waterless coolant), you need some kind of protection against coolant loss. So a cap has to be placed in any case to allow the system to “breathe”.

As far as I know, early Jaguar Saloons had a screw cap on the radiator with a small hole and these engines had non-pressurized cooling systems (see photos of an early 1½ Litre radiator with screw cap. Note that the extra holes in the radiator cap are for fitting an ornament or even a “Leaping Cat”).


I assume you’ll have to modify your expansion tank to receive a new cap. Don’t know whether there are RC5 (or do you use RC4?).caps for non-pressurized systems.

Bob K.

Yes, all the side valve and pushrod models up through the Mark V had non-pressurized cooling systems with an overflow tube at the top of the filler neck.

I don’t quite see the value in an XK120. I have a recovery tank that keeps the level in the radiator up to the top.

If you operate a barometric cooling system evaporative losses become a possibility. Just because you operate below the coolant boiling point does not mean it will not evaporate over time. You have to be diligent about checking the fluid level. That is much less of an issue with a sealed system.

Thank you for all the hints .
Bob , my radiator has a small tube welded on top close to the filler neck . This tube was connected with a 0,25" hose to a 1" diameter tube about 12" in length , which is located on top of the firewall behind the heater valve . I have never seen such and have no idea about the function . I took this hose and connected it with an expansion tank from an E-type . In this way i avoid any overflow loss of coolant .
As you mentioned I will now simply drill a hole in the cap .
Rob , this are good news . If all this Jaguars had a pressureless system it must work .
Mike , I didn’t think about evaporation . I will check the level frequently .

This sounds correct. It would normally run over to the left, then down along the left rear corner to spill the overflow on the ground.
I connected my overflow tank to this tube with a short hose.
The original cap was 4 psi, and larger than a normal car cap.
Stant makes a replacement for trucks which fits and is sold by Moss and others.
I believe the idea in pressurizing the system was to get it to run a little hotter for better air/fuel mixture and better combustion.
I don’t understand why you would want to drill a hole in your cap.

Sorry for mine imprecise expression . My radiator has 2 tubes . This one which you describe has no function since I installed the expansion tank . The other one is about 1" right hand side from the filler neck seen in driving direction with same pressure as inside the radiator . This tube is connected with the E-Type expansion tank which is closed with a 4 psi cap . I have an old cap . I would drill a hole in that and replace the 4 psi E-type cap . The original XK cap remains untouched . Of course the exp. tank has an overflow tube .The only idea is to achive as much as possible protection for the core with “zero” pressure . But your advise about fuel economy makes me on the one hand uncertain . On the other hand the effort is just a hole in an old cap . I will try it .
A pic would be self explaining . Unfortunately I am too putty to add one . Sorry

I have used Evans in my Cooper S, sixties Mustang and Cobra, Land Rover and others for over ten years now, all running zero pressure systems. The coolant still expands, so each vehicle has a simple overflow bottle to accommodate this which returns the coolant to the rad as it cools down after the engine is turned off. The radiator cap is either non-valved, or I have removed the pressure spring.

I’ve never had any problems with this and the cars have been fine in all sorts of road and temperature conditions. The XK will be getting the same when I get to the cooling system.

As I understand it, cooling systems were (are) pressurized to raise the boiling point of the water/ethylene glycol mix to try to reduce the risk of boiling. With Evans (propylene glycol neat) this is not necessary as the coolant won’t boil until 180C, or thereabouts.
No, I don’t work for Evans!

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Thank you Roger ,that is very useful and reassuring . I too have only best experience with Evans in both of my cars - the E-Type and the XK .

Hand made overflow tank high in drivers side wheel well works well for me.