No, you don’t. The engine could be running quite nicely with the thermostats
holding the bypasses wide open so virtually none of the coolant is even
going through the radiator, but the temp would still be indicating that a LOT
of heat needs to be handled.
– Kirbert
There’s a 14 deg operating range for the older jags…82 deg thermostats… and a 5 deg range for the 88 deg thermostats.
So for my 82 thermostats they start to open at 80 ish and are fully open at about 95 deg C. This is from the Haynes manual. So…the flow increases as the temperature gets hotter up to max flow at 95 deg.
So as the water temperature leaving the heads flows thru the thermostat the flow rate is adjusted by the thermostat. That’s the control system . Crude but effective…and with a designed in range of 14 degrees. And presumably when the temperature gets on the high side the return temp of 85 deg C switches on the little electric fan. That’ll theoretical be about 100 deg C at the thermostats…still comfortably below the boiling point of the pressurised water of about 125 deg C.
If you are losing coolant, but very slowly, a small leak in the upper portion of the system can give these symptoms. You would be blowing off steam only (pressure and very little fluid) under higher loads. Under low loads the coolant is not hot enough to create steam, and the fluid does not readily pass through the very small leak.
Dear Matt,
Just because the thermostat takes 14’c to fully open doesn’t mean there is an expectation that temperature must vary by 14’c. (Neither is there any reason to believe that the flow split will be a linear function over that 14’c range.) It may be that the the car’s radiator is efficient enough to cool the water adequately long before the thermostat is fully open.
If you look at the bottom of the thermostat, then you’ll notice that the foot is spring loaded. This implies that it may already be fully open (with a fully closed bypass) before it has opened to its “14’c” amount. If 14’c range means anything, it is a cap on flow control.
Kirby’s statement makes no sense - if the indicated water temperature exiting the engine is higher than the thermostat spec, then we know that the thermostat must be open or opening. If it isn’t you’re going to overheat no matter when or how you chose to trigger the fan.
The temperature stabilises when the rate of heat production equals the rate of heat dissipation and moves when one exceeds the other. As Matt points out, the engine exit water temperature is an adequate proxy for the rate of heat production, whilst the radiator water exit temperature is the same for heat extraction, as truncated by the thermostat. There is more than one way to skin a cat.
kind regards
Marek
On reflection, as people don’t report any damage is caused, maybe there is a reasonable expectation that the cooling system will always easily cope with temperatures in the mid 90s, certainly for cars with 88’c thermostats. What surprises me is that the 82’c thermostat users would not expect to just take 6’c off of those numbers.
kind regards
Marek
That is one of the BEST accurate descriptions of what a T-stat truly is: I’ve given a similar spiel, over the years, but not as elegant as yours…thanks!
Hi Marek
Yes. I noted this in an earlier response to Kirby.
If you look at the length of the spring on the sealing flap you can see that it allows the main thermostat flow control to KEEP moving after the flap has shut off the recirculation of the flow. In the thread I deduced that the Engineers intended for this little valve to both moderate the amount of recirculation from full recirculation to zero recirculation…AND then modify the volume of cooling flow through the the radiator.
This is the heat rejection strategy from the 70’s Jaguar designers.
The only downside is it is a bit under damped and has a relatively wide control band. Modern systems with brushless DC speed controlled pumps and fans will just hold the temperature to within a few degrees of set point…and they will stop the engine before it gets damaged.
We have to keep all our senses in the game!!!
Regards
Matt
And as I pointed out, no it’s not. It’s supposed to be within the thermostat
control range regardless of heat production.
– Kirbert
Dear Kirby,
I’m not even sure what point you are replying to or what you are pointing out, so your argument here isn’t very well put. Phrases like “thermostat control range” worry me as they might be misinterpreted as referring to temperature, whereas the thermostat only responds to temperature, rather than control it. Whether you sit it in a pan of water or put it in the engine, it’ll open or close up depending on temperature. That’s all it does.
kind regards
Marek
Dropped valve seats is the “damage” that most people are concerned about and over the years we’ve somehow become convinced (incorrectly, IMO) that the only way to prevent it is to ensure that the temp gauge needle remains at or below the center of the gauge under all driving conditions, period. “Center of the N” has become the XJS nirvana… but it doesn’t guarantee that the valve seats will stay put.
Examples of hyper-focusing on coolant temp are abundant here and elsewhere among V12 owners. Our OP, for one, is obviously concerned that coolant temp is 93º- 96º when, as it turns out, that’s when 88º thermostats are designed to be open fully.
I may be off-base in my interpretations of original Haydar’s posting but, more broadly, when owners are concerned and asking about coolant temperature I think what they really want to know what they can do to prevent valve seats from dropping.
Cheers
DD
Im going to throw my 2 bits in to this discussion…
Right or wrong…i have always perceived the function of a thermostat to assist in warming a cold engine by slowing the flow of coolant thru the engine to help it reach a selected temperature where it will open and allow coolant to flow through the radiator. But the actual radiator cooling system in conjunction with air flow is what actually determines the final temperature of its operation. Now the question of the engine getting hotter under load ( when in thereory it should have adequate airflow to keep its operating temperature in check) is a bit puzzling. I suggest checking the harmonic balancer…i suggest a harmonic balancer which has suffered rubber isolation deterioation may allow its outter pulley to slip and not provide sufficient power to the water pump. But at lower rpms is sufficient to provide adequate coolant flow. Just a thought.
Gary
Yes…the thermostat essentially responds to the temperature of the water flowing through it. When it gets hot enough it opens a bit and progresses to open until,it is fully open. With the V12, fully open is when the water temperature is about 96 degC.
But…the ability of the radiator to reject heat depends on the flow of the air through it, the temperature of the air flowing thru it, the temperature of the water entering it and the flow of the water through it.
If you are driving at a steady speed and start to climb a hill you increase the throttle to maintain speed. The engine is working harder but at the same speed. More heat needs to be rejected by the radiator but the air side of things is the same as before and the water pump speed is the same as before.
The first thing that happens is that the water leaving the engine gets hotter and the first thing it flows thru is the thermostat. The higher water temperature opens the thermostat which decreases the “blockage” in the pump circuit and this allows the pump to flow more water at the same speed. Higher flow in the radiator with all else the same will give you more heat rejection. And this variation in “blockage” of the thermostat controls the water flow through the radiator. With the original belt driven fan and the belt driven pump the engine speed directly affects the air flow through the radiator and the water flow thru the radiator so this a second method of adjusting the amount of heat rejection.
And if you select the fan correctly, and match the flow characteristics of the pumping circuit in the correct way to the flow characteristics of the opening of the thermostat then you have a crude but effective control system to reject heat from an engine.
You make a good point though…if the drive to the fan and pump is damaged…you are in trouble.
And fundamentally…I agree with Doug …that my concern is how high can I let my temperature gauge climb before I lose a valve seat.
I don’t think anyone knows…and nobody really wants to find out!!!
So I watch my temp gauge very closely when I am driving with my refreshed engine!!!
Regards
Matt
Gary, Waterpump drive is bolted directly to hub of damper so delamination of damper would not affect the Waterpump.
I guess this thread has provided some excellent data, and some agreement on what is the “desirable” running temperature for a V12 XJS.
I think we should try to cover as well, two different facts, that impact our dreaded “valve seat dropping” problem.
The first would be: Where do we measure temperature from? Is the stock setup enough? Should we add additional measuring points? If so, where?
The second: Seems that many instances of our valve problem is related to post-shutdown heat soak, rather than “running” problems. Would not this be one of main concerns, as well?
All I´m saying is that it would be fantastic if we could build a thread that gathers all knowledge on this subject.
Cheers.
Carlos
A good friend of mine taught me to pop open the bonnet when I get to where I am going, and let it rest on a small piece of wood placed under the aft end of the bonnet. Create about a 4-6 inch gap to help vent out heat - -prolong life of all rubber or rubber coated items, help the post-shutdown cooling, etc.
AND, an added benefit is that lots of people will stop by your office or cublicle and ask “What’s wrong with your car? I see you hood is open.” <:slight_smile>:
WHen I pull into my sub-division I release the bonnet , it pops open about 2 inches and when I pull into my garage I open it fully, Great garage warmer in the winter; makes the window a/c run like crazy in the summer.
I read thru most of the posts and did not see any reference to cooling systems holding the correct pressure. If there is even a small pressure/fluid leak, the temps will rise. Time to get out the pressure tester and see if the system is up to spec. Hope this helps!
Kirby, just to retake the conversation I left here; would the rad outlet hose be the big fat one at the rad under the expansion tank? (1986 v12 XJS)
Yep. That’s it. There are two radiator intakes at the top corners, one outlet at bottom left. The outlet leads directly to water pump inlet, which is where the OEM sensor is for controlling the electric fan.
I can think of 2 additional questions (as my goal is to install there the temp sensor for the large fan, independent of the smaller fan, which will be replaced by a more powerful one, but keep the factory connection and operation). These are:
1.- What would be the fan cut in temperature for the rad outlet? (meaning by that, at what temperature should the large fan start?
2.- It is a very thick hose. I´m not sure there are sensor “adaptors” for that size of hose. Is there an alternative place to place a temp sensor
Cheers.
Carlos
I think I’d want that fan to come on by the time the coolant exiting the radiator exceeds 160F. What temp does the OEM switch come on at? I’d want it slightly different than that, just so they come on at different times, and frankly I’d want the additional switch to come on a bit sooner because the OEM switch seemed to wait a bit too long for my taste.
When these cars were sold, you could buy a kit from Jaguar to install a block heater in the lower hose. In fact, it was two shorter hoses and a short pipe with a port for the heater. Some have used that kit to install a fan control sensor in the lower hose.
You might also want to know that I don’t recommend using the OEM lower hose. It’s molded and has a wire coil in it to prevent getting sucked flat. The coil gets corroded and crumbles and then the hose gets sucked flat and the engine overheats. Better idea: Buy and install a generic flex hose. Supposedly there is a generic flex hose that has a 90 degree bend at one end that fits the XJ-S perfectly. And in a flex hose, the coil is built into the hose itself where it cannot corrode. Even if it weren’t, the corrogated nature of the flex hose tends to prevent it getting sucked flat anyway.
If you’re good, you could buy two short flex hoses and a short section of pipe or tubing with a tee in it to install your fan sensor.