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Obviously a very temporary measure, but perhaps try getting a set of hose pinch off pliers and simply squash the bypass hose closed? It's the hose that runs from the thermostat housing down to the waterpump. The pliers look something like this: https://www.princessauto.com/en/hose...t/PA0008211641

Zip tie them in place, so the weight isn't hanging on the hose all the time and see if that helps?

 

It sounds to me like the thermostat may be missing.
Even with the wrong one in there and fully open, there will be enough coolant flow to cool the engine, the amount that gets into the bypass isn't that much, it's just a narrow slit.

I ran a standard thermostat for years and the car never over heated, and when I installed the correct one, it made very little difference.
The correct one doesn't block the bypass off with a perfect seal, it's pretty sloppy over-all.

Also check your old one with a new one with a pot of water on the stove, but don't let it boil.
Tired thermostats have a tendency not to open far enough.

A partially blocked rad will also cause what's going on _ can't remember if it was mentioned anywhere in the thread if it was replaced, and I'm too lazy to sift through it all.

EDIT:
So I did go through the thread, the rad never came into play, consider a partially blocked rad.
And from post #36, if the thermostat was stuck right shut, it would overheat right away with in 5 or 10 blocks of your house.

 

The radiator is very much the likely culprit unless the waterpump impeller is no longer effective. Impellers fail due to corrosion etc.
The correct shrouded thermostat actually works very well. Water does not like changing direction & the approx 2mm to 3mm gap is of no consequence. It works & very well. You need a borderline cooling car to show how well. This car is not borderline. It is overheating & runs too hot period. A car with excess cooling capacity like mine you can get away with the wrong thermostat. The shrouded thermostat is correct. Jaguar used it for good reason in it's different configurations depending on position & size of the bypass port which varied by head design.

 

Another thought I had is the spring loaded tensioner on the fan belt can become ineffective with age, I wonder if the belt doesn't have enough tension to turn the water pump and fan adequately?

 

Good thought JB but I would expect noise with it.

 

update...

have crossed the sierras and the rockies. highest elevation about 12,000 ft. car is constantly running very hot (and i am beginning to suspect the accuracy of the temp gauge because; while the coolant was boiling the other day in the upper return hose, the temp gauge was reading about 85C. i WAS at ~8000 ft though. looked in the radiator for the first time and notice something like baby poop all around the filler area. i suspected it was boiled rust that had been accumulating for about 1000 miles. so i stopped in denver for a radiator flush, and the mech said the car had a blown head gasket. he showed me something that looked wet in and around the head. i wasn't convinced in that the car has been spewing a LOT of raw gasoline into the engine compartment since i've owned it once really warmed up (takes about a 100 miles or so) and i suspect THAT was what he saw. they eventually declined doing the flush, saying that i shouldn't drive the car, and to have it sent back to california and have the head gasket replaced. under different circumstances i might have agreed with them. i asked how the head gasket could have any serious leak inasmuch as i only added about two pints of coolant to it the entire time that i've owned the car, and it may well have been two pints down when i bought it, AND the oil was free of any trace of water. ....they couldn't give me an answer. so i'm continuing on.

the car, as i've mentioned before, may well be suffering the ravages of overheating due to a thermostat that was NOT designed to cut off the radiator bypass when it opens. that would just about explain all my problems; the over heating the spewing of gasoline and the intermittent loss of power. anyway....i'm on flat land now all the way to ohio and my sister has scheduled an appointment with a jag savvy mech in delaware, oh. so i'll talk with him and maybe suggest removing the thermostat altogether and plugging up the bypass to some degree...maybe all of it. i would LOVE to experience the discomfort meted out by an engine that is slow to warm up to operating temperature.

 

That could be a stop leak additive. I don't like those as a rule, because they coat the inside of everything and reduce the heat transfer.

The XK engine does typically need a head gasket every 90-100,000 miles or so. The usual test is a CO detector in the coolant, the present of that shows combustion gas is touching the coolant.

 

 

lots of good thoughts, i can't tell you how much i appreciate the time and effort that has gone into helping me out.

looks like i may know for sure what's going on come this next weekend. one way or another the thermostat housing is coming off. either the mech that my sister has set me up with will do it, or i will. that's for sure!

this evening, after having checked into a motel here in limon, co. and after about four solid hours of sitting out in the parking lot of the motel in 65-70F temps that thermostat housing is still hot to the touch, the rest of the engine has cooled down, yet that part is still hot. ...strange.

 

If the thermostat housing is still hot & the rest cool the temperature sensor is not seeing a true reading. It's right there.

If this were my car I would clean the cooling system. Fit the correct shrouded thermostat P/N C3731 & have the radiator re-cored with a tropical core.

Once done you can trouble shoot other issues.

Otherwise you are just going around in circles.

 

I agree with Glyn. If you are getting a high temp reading but the engine itself appears to be OK then it might be that the thermostat is not functioning correctly as the temp sensor is right by the thermostat.

The picture below shows the thermostat housing with the temp sensor indicated by the arrow on the left and the Otter switch indicated by the arrow on the right.

 

Why did you wait until now to decide to remove the thermostat cover right in the middle of your 5000 mile road trip, that makes no sense ?
This should have been done at least a week or 2 before your trip.
You don't even have to remove the hose, once the two nuts are off the studs, the cover can be simply pulled away from the manifold.

Make sure and have a gasket handy, don't know what you're going to do about getting the proper shrouded thermostat to continue, but one with out is better than nothing.

Don't know what you're going to find in there, other than a faulty thermostat, and/or the wrong one, or none at all.
The car will over heat with out one up long grades as the coolant is pumped through the rad continuously and it has no time to cool.
Coolant is also pumped continuously through the by-pass as well diminishing the capacity to cool the engine.

It makes no sense to me why you would take a car that is right on the edge of severely over-heating on a 5000 mile road trip with out diagnosing and repairing the problem.

 

An infra red thermometer might be a good investment.

My experience of Smith's thermometers has been that sometimes with years the sensor/sender loses sensitivity and gives a low signal. In an old Austin in our family that coincided with the thermostat not opening fully so the gauge reading remained normal. We discovered something was wrong after the top hose exploded.

Is the engine an old 3.4? If so the head gasket would be fine close to indefinitely so long as you don't drive it long distances with the engine overheating... Other versions of the XK don't do so well. As Glyn wrote, XK gasket failures are often not very dramatic.

Like JB, my guess is the brown stuff is a leak stopper mixed with rust from the block. I realise Jaguar have in the past encouraged the use of Bars Leaks. I prefer to go without. If you feel you must, then once in 10 years followed by a complete system flush after 20 miles.

 

Jag recommend Bars Leaks because their Welch Plugs leak even when inserted with sealant. I have multiple IR thermometers. Even one for my Home Theatre to ensure amplifier biasing is correct.

 

I always considered Bars Leaks as a last resort we used in the previous century for leaky cooling systems in our old Fords and GM Holdens.
When installing Welch plugs in an early Jag motor I used a thin smear of Stag pipe sealer to get the ultimate seal. Note, this is a low-pressure system with Jaguar specifying a 4 PSI radiator cap for MK2s
I believe that hueyhoolihan needs to go back to basics.
Have the cooling system checked for combustion gases
Radiator out and cleaned/flushed. I suspect he is running on a bad radiator. I have had a few of these in MK1s and MK2s
Thermostat out and checked or replaced
Replace the temperature sender unit. I have seen them go out of "whack" (calibration.) as per Peter's experience
Until this sort of base line is established, I think it is going around in circles
.

 

On my S Type the gauge indicates correctly until 75°C and then it quits rising as the engine temperature keeps increasing (when I had the electric fan disconnected to test the cut in/out points).

 

utjust to be clear, i have not removed the thermostat cover as yet, however i intend to have it removed by a mech this coming weekend. my previous post may have been unclear. if so, my apologies.

i've added a bit more coolant. about an oz. is all. and i may not have mentioned it previously, but i had to add a couple ounces of brake fluid the other day due to the low-brake fluid light flashing on and off. but this morning i noticed fluid on the ground under the engine and suspiciously close to where the brake fluid reservoir would leak its contents. i put a couple more ounces of fluid in and checked it again this evening (i'm in junction city, ks. BTW) and all looks good. the first half of my journey will be over once i cover the remaining 750 miles. should take a couple of days, and then the car will be in the hands of someone, according to my sister and brother-in-law, that knows what they are doing. it will be a relief, i TELL ya.

i'm wondering if the gasoline is simply coming out of the float-bowl overflow tubes. except i would think that THAT would happen almost constantly, and its not.

and, yes, having the radiator, hoses, thermostat replaced or rebuilt would be a wonderful starting place to begin a more thorough debugging of this car's mechanics. when the car gets back to california i'll be booking a date with the local east bay jag guru (at this time he's booked solid for about four months) and have him go over the mechanics of the car. of course, the seller of this car, assured me that the car had been thoroughly tested for hundreds of miles in all conditions and that everything was in fine working order. he assured me that he won't sell a car that has not been through a vigorous testing.

OTOH, i'm not naive. i expected problems on this trip and thus far (fingers crossed) they have not been more severe than i anticipated. ....so far.

 

You may find that the bottom of your brake fluid reservoir is cracked and leaking fluid. Common fault on the old reservoirs as the plastic becomes brittle with age. The crack is caused because the nipple on the end where the outlet pipe is attached is not reinforced and cracks where it joins the flat base. The new one that I bought has a metal sleeve inserted in the outlet tube which strengthens the outlet and it no longer cracks.

First photo of the new one with the metal sleeve. Now been on the car for 5 years and no problems.
Second photo is the old one that I had repaired a couple of times with Araldite but after time it too cracked again.


New brake fluid reservoir with metal sleeve.

Old brake fluid reservoir with several repairs.

 

Hear hear regarding Bars Leaks. I'm told their new silver variety does not clog things up but I still see it as a last resort if stuck.
I replace the temp sender unit as a matter of course. As you say they become unreliable