I like the caps to fit really tight but find it hard for my old arthritic hands to do & undo - so a made a tool for the task. Wordless workshop:
The 7lb cap goes in the header tank, NOT the radiator. The radiator gets a blanking cap.
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David and Michael are spot onâŚ
Also open the heater valve and make sure the core is not plugged so you get flow through the whole system.
Pull your dip stick.
If its foamy or you smell antifreeze its a head gasket issue.
If just oil.
those two puddles are antifreeze running into oil leaks along the wayâŚ
Ive heard that happens once in a blue moon.
HA HA HA âŚ
Once the dip stick is only oil, a lot of questions are answered.
pressure cap at the expansion tank and blank at the radiator.
13 if a/c car 7 if non a/c.
Use a quality cap, stay away from repro till its sorted, the repro are crap.
gtjoey1314
I had also tried bending the tabs but did not work for long. I also had leaks at rivet, had a large collection of failed caps until I got the Waxstat. My original cap has lasted 40 plus years until the rubber seal tore.
Glenn
In case it helps here are photos of one that is decades old. Rubber seal tore a few years ago but I had another one lying around that I robbed a seal from. It fits nice and tight with no leaks.
David
68 E-type FHC
I have used a reproduction cap from Moss (Classic Gold brand) for 6 years and 10s of thousands of miles w/o a problem.
What PSI on those caps? Do keep at same PSI as factory fitted. A more PSI rated cap to raise your boil point is not the best practice. The cooling system is designed for factory spec pressure, If it is overheating to the point of boil with anti freeze 50 50 that is around 223FâŚ, 1 PSI provides about 3o F to boil.point, ( 1,000 ft elevation takes away 3o F), something else needs to be remedied. Very early Series 1 are 4lb, and thermostats in those cars are a different type and will be destroyed if much over. Other issues with too much PSI, core plug leaks, head gasket leaks, and thinking it is now safe to run at 220F or more. It is not ok.
Nick
If youâre asking about the Classic Gold I pictured - it is 7 psi which is correct for a non-AC S2.
Last month I drove through Phoenix and Vegas on successive days in 100°+ temps with no issue other than I got a bit hot.
Not the case. Bear in mind that Jaguarâs cooling system designs were hardly the world standard even at the time, and thereâs decades of experience now.The stock S2 cooling system carries a 7lb cap for no reason other than Jaguar had a supply of 7lb caps left over from S1 production. When the car was equipped with A/C, the cap was changed to 13lbs with no other changes to the cooling system. So we know a 13lb cap can be used without any problem at all. The difference between a 7lb and a 13lb cap is 12 degrees F using 50/50. That brings the boilover temperature to 260F, which may save your bacon in a traffic jam or at the track. Beyond 13lbs, you are in terra incognita.
Itâs important to remember that installing a higher pressure cap doesnât raise the actual system pressure, it only raises the upper limit. Pressure is a function of temperature, so your normal operating procedures, which should include keeping an eye on the gauge, will keep your pressures low.
Makes sense Michael,âŚthanks for the addtl info on theS2 system. Just as best practice,m .I will still maintain that to install a greater PSI rated cap than factory, in order to raise your boil point is not the best practice. .Iâd hate to see 260F on a early JaguarâŚNick
Itâs what I used on Tweety, though with his giant NASCAR radiator, in the hottest traffic, it rarely ran hotter than 95C.
Is that the blanking cap on the reservoir tank?
No, that is the pressure cap on the Series 2 reservoir or header (like the OP and Carlo have).
David,
Old post but curious how it all ended with your PS system. As a ânewâ owner of a 1970 S2 2+2 as well, I did the EXACT same thing a couple of days back. Realized my mistake before I drove the car and did my best to siphon off all the coolant from the reservoir. We seem to have lots in common 
So did your PS system end up alright? Anything I should be bracing for?
Congrats on the new car. Hope you start a thread introducing you and the car (with pics).
Besides feeling stupid and embarrassed, no issues once cleaned. I took it to the mechanic and he drained/flushed the system with no issues. My leak was all PS fluid and once cleared the coolant mixed in didnât cause additional problems.
Keep you head up high
Thanks for the welcome, David. Will definitely send an intro email this weekend. Turns out that our PS system has a leak somewhere ( not coolant related - I was able to flush most of that out). Letâs see how bad it turns out I lost almost all the ATF I added to the reservoir within a couple of days - I was clearly leaving a trail wherever I went
I also saw your message about the Aircon. Did you manage to finalize the replacement/approach? One more similarity - my car came with AC option but has not been working for years I think! I need to figure out a way to make it work since my family is having a tough time joining me in my âpleasure drivesâ in the 100 degree heat we are experiencing here in California. Ideally, I would like to get some R12 freon and add it to see if it the components work but looks like finding R12 is like finding henâs teeth!
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Iâve replaced my rack and ps hose/pump but still have a very minor leak. The new components are just not great. My initial leak was in the rack that had already been rebuilt to an inch of its life.
AC is still a challenge. It works. But spits and sputters in high setting. Itâs very hot in DC and the new AC is just not up to the task, imo. I hope to have some updates by years end.
R12 is pretty available but expensive (~$90/lb). You may need to rebuild your unit under the dash (there are threads on updated fans and switches). Probably also need to redo the hoses and belts and compressor. So basically, everything
Which replacement AC did you go for? Retroair? Would you recommend it?
My AC system seems to be working based on a recent diagnostic but all the hoses need replacement since there are leaks. Iâm in a dilemma whether to invest the $$ in replacing all hoses or just upgrading to a new system. Iâd estimate the latter to be atleast 2X the cost of overhauling the current system. Any suggestions?
I have the RetroAir (which is now classic air). I canât really recommend it. Some people seem to have had great success but I have not.
If I had to do it all over again, Iâd refurb my original unit, maybe upgrade the fan and compressor and see how that was.
Go to a hydraulic shop with your hoses and have them replace the rubber with barrier hoseâŚtheyâll know what to use. The hose ends and fittings will be reused. This will save you a bundle and will probably last longer.
I wouldnât recommend trying to replace your system with something else. All of these conversions involve modifications to the interior, most commonly to the radio console. If you switch to a different system, youâ'll find that the modified interior bits may not be compatible. Best to solve the problems you have than pay for a lot of new ones.