Overheating under load! Chasing for 2+ years

excellent advise! taken! On another note, do you have that T air valve near heater valve? would be nice to see how you have done it!

Thank you

Many owners do have a “T” bleed in the heater hose.I do not, as the heater on my car moves any air out without a problem. Greg will probably respond.

Bleeder ports. Oh boy. Do NOT get the cheap plastic tees you see with the faucet hose thread fittings. After a couple years of opening/closing plastic cap, it cracked on me and ended up leaking coolant into transmission dipstick tube! Luckily I caught it in time.

I found a tee made out of aluminum with an aluminum screw stop for about $30. Go to z1motorsports.com and search for “Aluminum Coolant Bleeder Port”

I find the Tee very useful. When re-filling coolant, after I’ve filled expansion tank and capped, I jack the left/front of car up, open the bleed valve and open the tee. With engine OFF, I fill up crossover until heater tee overflows and I screw that down. Then I continue to fill up until bleeder valve on left side of radiator overflows, and I screw in the cap there (always with new copper washer).

I go for a short drive to warm up, let it cooldown for a few hours, and top off with about a half cup of coolant. Always works for me.

any chance you can share the picture of the one installed in your car?

i have been struggling and unable to drive my car for 2 years, i would now consider anything that might help

guys on this thread came to 2 conclusions - air in my system and coolant flow, so i will try to tackle this every possible way =/

When your cooling system is filled properly, and on a cold engine, the atmospheric catchment tank should be appx 1/2 full. When the engine is started and fully warm, the coolant will expand into the atmo tank the coolant level will rise. It would NORMALLY not overflow, as the atmo tank capacity ( one US gallon) is sufficient to hold the expanded coolant. When the engine stops and begins to cool down, the coolant will be sucks back out of the atmo tank and the coolant level should return to 1/2 full.

If the atmospheric catchment tank is overfilled on a cold system…then on the next extended drive and fully warm engine, the excess coolant will be expelled from the atmo tank onto the ground…at which time, once the coolant and engine are again cold, the atmo catchment tank coolant level should be +/- 1/2 full.

Never did see an answer to my prior question…what brand and part number are your thermostats ?

SD Faircloth

The thermostats are a good question. They should be 180F or 190F. They need to move 41mm to close the bypass when the thermostats are fully open. They also need a jiggle pin, or hole drilled, at 12 o’clock so that air can get out as the engine cools after a shutdown.

[quote=“gregma, post:46, topic:445397”]
Go to z1motorsports.com and search for “Aluminum Coolant Bleeder Port”

Aluminum Coolant Bleeder Port

Lack of jiggle pin or escape hole may be at issue. Good thinking

or jiggle pin present but too low (thermostat not in the right position, with pin/hole at the top
(I found this on bank B when I swapped the stats while I replaced all coolant hoses

I have a potential cause, but really wish I’m wrong :
very early head gasket failure can manifest itself by creating extra heat of the cooling system during heavy load use of the engine
(without any other symptoms as coolant overflowing, or mayonaise from oil mixing with coolant)
This has been suggested to me for a non Jag engine, the mechanic considering this test more accurate than CO2 detection in the coolant header tank
his procedure was flooring the throttle in low gear for 30s or more (road space allowing) on a uphill
any rapid rise in coolant temp made the head gasket suspect to him

A rise in temperature happens because the rate of heat generation exceeds the rate of cooling.

The usual Achilles heel for v12 is that the rate of cooling is very poor when at minimum rpm and minimum airflow, despite heat generation typically not being high in that scenario.

You are suffering when heat generation is high. Under this scenario, there ought to be plenty of heat dissipation capability, so the suggestion (assuming you are out on the open road) ought to be that you have insufficient water reaching the radiator.

This does need checking. Without it, you cannot harness all of the cooling capacity you have.

kind regards

Sorry missed that question, but as i said all 4 thermostats was boiled, measured, inspected, tested, sent to themostat laboratory to confirm they are breaking at about 82c, with juggle pins, moving at least 42mm and has a back plate diameter enough to close the seat.

to be precise i have pair of waxstat EBC3576W. and pair of Dayco DT18A

jiggle pin is on 12 oclock.

i have exactly what you described, while doing italian tuneup (which is exactly flooring in low gear) my temp was going right slowly, cant call it a rapid change, but a steady one for sure. Now thats what im trying to understand is it a head gasket or radiator or air in the system.

alright back to head gasket theory, sure this is what i was thinking about all that time, but i did combustion gas check with special fluid - negative, no white smoke , milky oil or other common signs, and i was pretty happy about it, also no loss of coolant, BUT here what im thinking right now.

Coolant loss - i was refilling, doing combustion gas checks, pressure tests all the time and coolant was all over the place, so i couldnt keep track if im loosing it or not, plus i didnt even do 100miles in those 2 years

i was trying to do some fill ups using different methods i saw on the internet, some of them was suggesting to start the engine with cap off (or bleed plug off) i cant say exactly, but what i remember that i never could do it their way, because as soon as i start the car coolant would come right through, however in the guide they were saying that it can take 5 mins to warm it up.
i didnt really bother about it, though that maybe i did something wrong, but now im thinking, could that be a head gasket?

I think you’re chasing a ghost assuming that the way you’ve filled your coolant is the problem. I try to keep to the book when I change coolant, have had to do it probably a dozen times since I’ve owned my car. Sometimes I’m spot on, sometimes I’m lazy and I’ve had to keep topping up the bypass for a few days until all the air is out and it’s fine. But during those days, I never saw the coolant temps go above normal.

I would run the car through a few cycles (warm, sit overnight) and top off the coolant each morning until it’s stable and doesn’t need topping off anymore. If it never stabilizes, then you have a leak. If the coolant level stabilizes, move on.

I don’t think you answered my question. Are you SURE you’re overheating?

Do you have an IR gun to check thermostat housing temps?

Are you still using the engine driven fan? If not, post a picture of what your electric fan setup looks like.

A little soap boxy from me on this subject, but a cycling electric fan that cuts in and out at relatively high temperatures is useless on an already heat soaked system that doesn’t have any bonnet vents, as the XJS lacks.

There is actually an engineering reason to always keep the engine driven fan, upgraded to a black unit, with an HD clutch, as it pulls air through the core, with the proper setback and shroud, during all operating temps. It’s critical, in my experience on an OE, standard depth core.

You can get away with a few electric fans on an aftermarket aluminum cross flow plumbed system, but you’ll never get the efficiency of the big engine driven fan on your current setup.

I tear them off any car that comes in my shop with cooling issues, and go back to proper manual unit, with the factory electric fan on an otter switch.

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Let’s assume best case. Try Water Wetter. Then do the tee at heater valve hose. Then radiator.

Aha, I know I took some abuse a while back about this.
I went to my Local Home Depot bought a Ryobi and checked the top hose temp, lower hose temp and look for a 15 degree temp change . Is your radiator working? Did you change your banjo bolts. Did you replace your aux electric fan temp switch?
Do you still have a yellow fan?
If all that you have stated has been done, starting with distributor, coolant passages, radiator , fans etc. , air in the system, fan shroud and all surrounding areas plugged with foam then you d covered your bases.
Then post a video and let’s all see whats going on here.
My two cents.
P.S. I don’t give up at this point , I drill down.

We may be giving you too much. But while we are, here’s another thing to check. Lol

Timing? Doesn’t a timing too retarded run hotter? Is the timing not advancing at high rpm?

.just a thought.

I thought about the timing and then asked what year and engine Timur had in his Jaguar. He said it was a 1992 XJ-S with 5.3L V12 so that means the Marelli ignition system (unless his car was modified). The timing in the Marelli cars is digitally controlled, there is no vacuum advance, and no way to set the timing. So I think we can eliminate the timing unless this car was modified or someone did something very unusual to his car.



this looks like normal behavior, but depends on the traffic conditions.
on our car, on the road the air flow to the rad allows the radiator and thermostat to regulated the temp without seeing the temp needle move from the “N” mark

the slight changed above N and back are visible in the traffic, as the electric fan is activated by the temp controlled fan switch