Radiator and expansion tank caps ratings 1986 V12 B-spec engine

Hi guys,

last year, the cap on my radiator failed, letting hot coolant spray out.
I ordered a new one from SNG and installed it.
Now, with the weather getting warmer, the temperature gauge climbs over 90C (never higher than 110) during prolonged idling and the engine occasionally spews some coolant from the overflow on the expansion tank.
Can this be related to a false pressure value on the radiator cap?

What should the correct values for both caps be?
I know they are not interchangeable…

Not specifically V12 but likely similar, the cap with the spring goes onto the overflow and it has the pressure value on the cap (and a wrong pressure value doesn’t change much - ie the E Type got a higher pressure during production and it behaves the same. It has to open before the coolant hoses break and keep the boiling point up). The non-sprung cap goes on anything that has no overflow drain, so header tanks, thermostat housings etc.

In your case I would suspect trapped air or low coolant first, a clogged or blocked radiator second and a head gasket just maybe. Does the fan work and is the other coming on?
Does the temperature rise and fall or does it stay at 100+ in a stable way? Is it related to power or also curves and inclines? My XJ6 went beyond 100 on hot days, especially during idle, but if it is new behavior be careful.

Only during idling, the temperature goes up to about 110c.

Last summer I had the cam cover gaskets replaced and the problem arose after that. I assumed the shop had overfilled the system, causing the excess coolant to be disposed of…

Verzonden vanaf mijn Galaxy

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Hello Patrick - looked at my service manual for the 1986 V12 in the XJS (i think this would be the same for your car since you have the V12) and the engine filler pipe cap (located approximately to the right of center at the engine front) is set to 1.41 kg/square cm or 20 lb/square inch to ensure a reliable seal for the radiator bleed pipe - the remote header tank cap (located towards the left of the engine front) controls the system pressure to 1.05 k/square cm or 15 lb/square inch, and is retained by a chain to ensure that it is not incorrectly fitted to the engine filler pipe -

if picture comes through it is the page from my manual showing the cold and hot procedure to check and refill - hope this helps you - Tex.

Thapnks for the info!

Verzonden vanaf mijn Galaxy

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last year, the cap on my radiator failed, letting hot coolant spray out.

The radiator on your year model V12 does not have a radiator cap. The coolant filler cap is located on the coolant filler pipe at the top front of the V. It doesn’t matter what the pressure rating is for that specific cap. All that cap serves is to seal the filler neck. It has NO…repeat NO pressure control on the cooling system.

Now, with the weather getting warmer, the temperature gauge climbs over 90C (never higher than 110) during prolonged idling and the engine occasionally spews some coolant from the overflow on the expansion tank.

Excess coolant from the cooling system should be be expelled from the atmospheric catchment tank located in a hidden compartment at the rear of the front driver side wheel well. With the cooling system filled properly, that tank should be 1/2 full when cold. If your expansion tank is expelling coolant, it is likely the tubing going from the expansion tank to the atmospheric catchment tank is blocked. Follow the tubing to find your problem.

Can this be related to a false pressure value on the radiator cap?
NO !!
SD Faircloth www.jaguarfuelinjectorservice.com

SD here now replying to what I just wrote. You said the expansion tank was spewing coolant from the overflow. Where is it spewing coolant ? The overflow should be going to the atmospheric catchment tank located as previously indicated. Any excess coolant in the system should be expelled from the atmospheric catchment tank overflow tubing onto the ground…from below the rear of the front left wheel well. Clarify. the spewing location. SD Faircloth

Thank you for the detailed description. I will check the hose for blockages.

Verzonden vanaf mijn Galaxy

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My system only has the filler tube (with the 20psi cap, now 16) and the metal tank in the left front of the engine bay, with the cap and chain in place. It spews from the overflow of this metal tank.

Verzonden vanaf mijn Galaxy

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If the tank ahead of the LF wheel (in the wheel well) loses some coolant after topping up… that’s normal. It’s just finding the level and the catch- and expansion tanks do the rest. Earlier cars vented the excess to the ground during the first expansion, no catch tank.

The cap on your expansion tank should look roughly like that, at a bare minimum it needs the smaller seal with the spring loaded relief. I think it will only really work as intended with the large rubber seal, but maybe the brass works well enough. What is the coolant level now?

Coolant level in the filler tube is at the top of the inner neck.
Expansiontank (plastic) is not installed, metal tank is half full.

Verzonden vanaf mijn Galaxy

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So the coolant level is…

At the top where the blanking cap is

Halfway up inside the expansion tank (the long neck, metal one on the side)

And this one then vents to the ground without any catch tank. This tank is supposed to catch what comes out when all is topped out, so that’s normal.

I‘m going to say that your coolant isn’t a problem then unless it’s as with the xjs where the air can be trapped as the expansion tank sits too low. I think not.
Your radiator is likely clean and clear, the fan(s) spin as they should.

Your radiator might not be as good as it used to be or something else is going on. In any case the temperature during long idle wouldn’t surprise me so much as all my cars get a little warm when they don’t move. Either the fans kick on or it finds some equilibrium then. No bank hot or running lean, what changes when you turn the ac on/off, that is did they unplug a fan sensor or such, if none of that I would ignore it as normal. The cooling system sounds ok to me.

It never gets higher than the middle of the green zone (90-130c).

Fans work, AC works, for some strange reason the fog lights stopped working after the repair, but that should not be related.

New development lately: sporadical quick flashing of the indicators, in combination with sloppy idle and itchy rev counter. This corrects itself after putting some load on the engine.
Will have to check the white wires on the ignition coil near the accelerator assy…

Verzonden vanaf mijn Galaxy

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Somewhat perverse for us xk drivers, Patrick; on the V12 ‘our’ expansion tank is called the ‘remote header tank’ - but it serves exactly the same function on the V12 as the ‘expansion tank’ does on the xk…

Which means that all coolant level checking and level adjustments are done at the remote header tank. Which is fitted with the pressure cap, 15 psi, to maintain system pressure - and relieve excess coolant into the atmospheric catch/recovery tank. Which may or may not be fitted; it’s sole purpose is to catch overflow, to be reused, instead of spilling it on the ground. However, if fitted; it may also overflow if too much coolant is ejected…

Any other filling points, used during the initial fill, are usually sealed by blank caps - not opened after the initial fill. A pressure cap may be fitted here, but to my knowledge; there is no drain at these caps - which means that when forced open, the coolant is just spewed out. Ie, the pressure relief is the cap on the header/expansion tank…

Common features of both the xk and V12 cooling system is that coolant levels should be checked and adjusted with the engine cold. If the system loses pressure the coolant will boil at some 100C - pressurized it will boil at some 120C. If the coolant boils; coolant is forced out by the expanding steam - which will also leave trapped air. If pressure is not relieved; coolant will force its way out at weak points - usually detaching hose joints or bursting hoses. Besides; local temps within the engine is higher than coolant temps indicate - so local boiling may occur…

Some special features of the V12; the two banks are separately cooled, each with its own thermostat, but to a shared radiator. However, the temp gauge is only reading the temp of one bank, the other is not watched - a failed thermostat will cause difference in temps. Also, while the xk engine is basically ‘self venting’; extra care is necessary to evacuate trapped air in the V12 - which may otherwise interfere with coolant flow, causing elevated temperatures…

With all cooling problems; it is advisable to use an infrared thermometer to possibly identify anomalies. Especially with the V12 with only one bank monitored - and in any case; the dash gauge is in itself not very precise…

xj6 85 Sov Europe (UK/NZ)

Hi Patrick,

This is normal.
The excess from the expanded coolant needs to go somewhere…

This is not cool… 110°C is too high.
Aux fan is working?
Radiator in good shape?
You are sure that the thermostats close enough to block the bypass 100%?

There is no such thing as overfilling.
Underfilling on the other hand, quite possibly.
Before anything make sure you don’t have air in your system.
Best is to install a flashing T at the heater hose. Details in the Book.

All the best.

Yes there is, and the excess is vented on the ground past the cap, and then the system is filled to the correct level once the coolant had its chance to expand fully for the first time. That’s how it should work.

Expanding fully also means it‘ll vent some more when it gets hotter than usual. As you say, both nothing to worry about.

I filled the wing tank to the brim today, the the filler pipe. I drove for about 50 minutes after that and the temperature gauge now clims steadily to 90 and stays there, no matter what. Took about 6 liters of coolant to fill her completely.

Furthermore, it looks like the aftermarket 16psi cap i ordered from SNG has a torn membrane though. So I ordered an OEM one as replacement.

Verzonden vanaf mijn Galaxy

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On cars designed by rational people, there is a reserve or overflow tank made of translucent plastic so you can see the level, and there are marks for “Low” and “Full” – full being nowhere near the top of the container, it’s usually about halfway up. On some cars these reserve tanks are pressurized, on others they are not, either way works.

Jaguar devised just such a reserve tank, calls it a “header tank”, and made the @#$%^ thing out of metal so you can’t see the level without opening it. And, understandably, owners conclude it’s supposed to be full right up to the cap, and are constantly popping the cap and adding – probably contributing to the huge loads of minerals plugging up the radiators.

Jaguar eventually realized that scheme wasn’t working so they added an “atmospheric catchment tank” – basically a reservoir for the reservoir. They made it out of translucent plastic – then hid it within an enclosed compartment on the XJ-S so you couldn’t see the level anyway! And then connected it with two different size hoses with an adapter that gets plugged up and causes blown radiator hoses. And they hid that adapter where you can’t easily find it, either. Of course, owners still pop that radiator cap on the header tank to check the level, which causes the coolant to drain back into the atmospheric catchment tank. Then they top it up. Repeating this a few times results in the atmospheric catchment tank overfilling and dumping coolant within the bodywork, where it leaks out of everywhere behind the LF wheel. One could reasonably conclude that Jaguar was deliberately out to cause problems with this system.

There are lots of proposed fixes, but none of them involve leaving the system in OEM configuration! In an ideal world, someone would make or find a translucent plastic container to replace the metal header tank, complete with reasonable “Low” and “Full” markings, and the atmospheric catchment tank would be jettisoned, a simple overboard hose installed instead leading out the bottom of the engine compartment. Failing that, you could fairly simply modify the OEM metal header tank by brazing in a fitting near the bottom and another just below the radiator cap and connecting these two with a length of clear tubing. Hence, you could easily see the level within without uncapping it.

Others have suggested replacing the hose between the header tank and the atmospheric catchment tank with a clear hose so you can watch the coolant move back and forth in it. Of course, ya gotta do something about that plug-prone hose adapter in the line, preferably tossing it over the hedge and running the larger size hose the whole way.

Mine is still “old-skool” British engineering, eg only has the metal tank and the filler tubes.
No plastic overflow reservoir.

Because I suspect the shop to have underfilled or not bled the cooling system, yesterday I hooked up our automatic filler/bleeder and flushed the entire system. Turns out they actually didn’t do their job correctly, as I had to add about 3 liters of coolant just to get it to the right coolant level. As compensation for the air possibly still trapped in the system, I intentionally overfilled the system and let atmospherics do their job ( in a controlled environment, catching all excess coolant ofcourse).
Now the coolant level is where it should be and the engine never goes above 90c anymore.

I ordered a new, OEM cap for the filler tube, as a membrane seems to have ruptured on the old one…


The pressure cap goes on the ‘remote header tank’, Patrick - your ‘filler tube’ is where? All caps, except the header tanks, should be blanks - there should be only one venting outlet
in the set-up…

xj6 85 Sov Europe (UK/NZ)