Overheating under load! Chasing for 2+ years

Air trapped after an initial fill is supposed to be “ejected” into the header tank, if everything is not plugged and working properly. As coolant heats up and expands, some coolant ,and the air, then is pushed through your plastic clear line into the reservoir behind the LHS front wheel. (If it is not plugged and rusted shut.)
As the engine cools, fluid is drawn back into the header tank, leaving the air behind it, so after a few cycles, your clear line should show only coolant. See if you can blow through the clear line going to the reservoir behind the LHS front wheel. Half-way full is not normal.

wouldnt it be easier to remove both thermostats, cut round piece of metal/plastic and using silicone close the entry to the bypass, remove thermostats and see what happens?
that however will make filling up and checking coolant level quite a challenge,

2nd option is to seal exit from corsspipe and seal entry of the waterpump, remove thermostats.

ok, its going sideways, i forgot that i tried with 2 different pairs of thermostats, inspected closely both housings, cleaned the thermostat seats and tested all 4 of them in the pan. I think its enough work even for XJS

as i questioned previously

“Radiator on another hand, lets think together, if its a bottle neck, would it cause worse cooling (not enough) under load, and sufficient without any load??”

any thoughts?

now we talking, you are 100% right, however even after several runs its only 1/3 with coolant.

But everything else what you said is crossed out
plastic tank cleaned, transparent line blown through, everything like it should

What poundage pressure cap is on the header tank? And one other thing- A Jag v12 sidetanks on the radiator need special attention, and need to be left in place during a recore.

What brand and part number are your thermostats ?

SD Faircloth

Ever tried “WATER WETTER”? Used in my race car

I always wondered about this. How? Unless you’re hitting 16 ftlbs of pressure in the cooling system, the cap keeps that tube to the reservoir closed at all times.

I just checked my reservoir when doing my coolant change, and it was empty. The line to it is fine. My engine has never got over 205F in two years, so it may have never got above 16 ftlbs of pressure?

On a side note, i went ahead and filled it halfway this time.

I think i asked somebody else this question, how do you know its overheating? Are you going by temp gauge alone? Are your sender and gauge accurate?

I installed aftermarket temp gauges first year i got mine so i can know EXACT temps on both banks. It has proven invaluable.

You answered your own question, Greg. As the engine heats the coolant the pressure rises and at whatever pressure cap you use the cap will open against the spring pressure and let coolant(and air) escape to the plastic reservoir. Doug posted a very clear picture of the pressure cap function on another recent post. I use my windshield washer reservoir for an overflow, and the level of coolant in it rises and falls each time I drive the car any distance. As it should.

I’m using a 16lbs pressure cap. Its been tested. So what does it mean if I’m not reaching 16lbs and/or my reservoir was empty? Maybe i should install a clear tube as many have. Glad i filled the reservoir halfway this time.

Memory is back! Forgot that the cap has a vacuum valve that allows coolant reverse flow.

Not 16 ftlbs, just 16 lbs, actually it is 16 psi, as in pounds square inch pressure.

Valve in center of cap operates on pressure differential
Look at image in Doug Dwyers post of a few days ago.

I have only seen one mention of timing, has this been checked because all I have seen from the posts is that the OP has been concentrating on the cooling system.

He has a Marelli car, so nothing to adjust. However, it might be worth verifying what it is at idle, to make sure it is within spec.

If the core is not the correct one, the undersized rad will not have the capacity to cool the engine at full load / max heat output.

No it’s not normal, it would mean that the atmospheric tank does not have enough coolant inside, so when the engine cools down it sucks back air instead of coolant.
For starters, keep topping up until you see that the plastic hose is full.
Also check that the hose it tightly sealed.

Adding a bleeder valve at the heater hose is highly recommended.
You will be surprised with how much air is there and how easier refiling will become.

BTW, did you ever open and clean the Tefba filters?
Had the exact same issue when my filters got plugged from all the debris they collected, they are a double edge sword.

Also very important, check the banjo bolt at the top of the rad that it’s not blocked and that it can actually let air through, it can be misaligned by the factory.

Is your front spoiler installed?
Check Kirby’s book.

And lastly, do check your centrifugal and vacuum advance as already mentioned.

Good luck.

How i would top it up until line is full, if the coolant level is already at max \

I will definitely add the bleeder valve at heater hose, i just cant figure out how to do it, since there is a 60 degree bend in the highest point of the heater hose ;/

i did clean filters once, but will do it one more time as i see it collected some more debris.
banjo bolt is new, but what about misalignment, that small hole should be in a certain direction? can you please explain/

A banjo bolt is normally wasted where the hole is this allows the bolt to tighten in any position and allow liquid to flow, as has been suggested, if you haven’t downloaded Kirbys ‘Book’ then you need to.

ahh that, sure it is. book i went through 2 or 3 times, cooling section maybe 10 times.

I had the same issues before my V12 overheated. Now I am concerned she doesn’t get hot enough. When I swapped out the lumps, I had the radiator re-cored from 2 to three core, I made sure the foam around the radiator actually blocked the air to stop it leaking through to the engine and ensure that all the air goes through the rad. Water pump was changed and everything flushed through. She’s been run at high speeds in hot climates, sat in London traffic and yes, whilst I still watch the temp gauge religiously (i think this is second nature for a Jag owner) its been fine ever since.

If you decide to use a clear line, reduce the size from 3/8 to a smaller diam. It is possible for a slug of air to travel back and forth in a 3/8 line and not escape. The smaller diam line, the more movement there will be.