Hello all,

I've been using my 1991 XJ-S more often these days, and I am trying to build my confidence in my car's capability to maintain a proper operating temperature. When I first got this car, it was in need of many fixes, and I installed, among other things, a NOS rad, new fan, fan clutch, and thermostats. I also rigged the auxiliary fan to turn on with the key. I hadn't really used the car since the repairs, but it's gotten a lot more use in the past month. I heard that the temp gauges on these cars can vary from car to car, in that N could mean one temperature in one car and a different one in another. I decided, after bleeding the cooling system today, to let the car come up to temp, wherever it decided to stop on the temp gauge, and use a handheld infrared thermometer on the driver's and passanger's side hoses running to/from the radiator, This was more difficult than one might think, as the thermometer would read ever increasing temperatures at the driver's side hose, likely due to the thermometer being heated by the hot air. Walk away for a few minutes, and the temp on the thermometer would go down again to the same starting point and rise steadily like before. All this is to say that I have a range of temps, but a consistent range of temps, taken over about five or so seconds.

So, today at ~29 F outside, when the top of the white line settled just above the top of the N the thermometer read an average of around 65-70 C on the driver side rad hose. It read similar on the other hose, and about 80-85 C in "death valley" between the distributor and the cam covers. The reason this confuses me so much is that, when trying this same test months ago in warm weather, I was getting around 80 C at the drivers hose at the same dash gauge position. I'm not sure what to believe anymore, especially with that strange thermometer temp increase. Another confusing thing is that it seems to get up to temp faster and faster with each passing week. The bonnet is usually tepid to the touch, except in areas above the gap between the rad and engine and along the sides where the hot air gets out. The metal part of the gear shift and inside the cigar lighter have always gotten warm after extended driving, though it's hard to gauge if it's more than usual due to the heater making the whole area warm.

The car scared the hell out of me a few weeks ago, as, during a 20 minute drive to show off the car with the aux fan connected back to stock configuration, the temp gauge got well above the N. However, the car was operating normally and there was no great blast of heat from under the bonnet. Of course, I returned home quickly, but it seems like all was normal. Could my temp gauge be off? And, moreover, could it be getting more off? I was reading 50 psi of oil pressure this morning, and, after a long drive, I can still expect around that much. I would think if things were really heating up I would have lower oil pressure. I also doubt that the water pump is bad, as coolant will be pushed out of the open fillers from dead cold with the engine running. Further, I hear no grinding or other noises that would suggest that the pump has gone.

I would appreciate opinions on the situation, as well as other tests to determine the actual operating temp of the car. I enjoy using this car, but I am reluctant to do so until I determine conclusively whether it is operating at safe temperatures. The last thing I want to do is overheat this car.

 

You still have air in the system. 70C is bellow where the thermostat will open. You should have either 82C or 88C thermostats. The 74C ones are for carbureted cars only. But 70 is less than all of those. The dash gauge rises rapidly because the air in the system gets hot.

 

The rubber hose isn't the best location, due to rubber not being a good heat conductor. It can be 20 or more degrees cooler than water temp. For example, you can grab and hold a radiator hose for a bit where touching the metal surface will burn you.

A flat or matte black metal surface on the radiator inlet is better if you can get a good reading on it and not get caught in the fan.

On other cars with aluminum motor water outlets, i have sprayed a bit of matte black paint on the outlet where a good straight shot with the IR gun is available.

Most IR guns have a ~ .95 emissivity factor built in, which is about what a matte black metal surface radiates at.

Doug

 

I agree that you probably still have air in the system.

IMO, lack of coolant flow to all areas of the engine is a bigger worry than coolant temperature.....up to a point of course.

Cheers
DD

 

Also

Instrument cluster earth issues. Bd earth path, gauges do weird things. Add a dedicated earth wire.

Thermostat length. There was a batch, for many years, of stats being too short, thus not closing the by-pass ports. This means 30%+ of the coo;ant never sees teh radiator, oops/

Have a read of what I wrote a good many years ago.
Also the cooling system sticky at the top of this section covers a heap of stuff.

All mine have always been daily drivers, and summer here is 40C ++, and no issues in any of them ever.

 

The only way to accurately measure temperatures on a V12 is with a twin channel K type thermometer with the flexible probes taped to the thing being measured. Then you can get reliable measurements in real time, running the probes into the cabin through the door/rubber seal join.
https://www.ebay.com/itm/23423044564...3ABFBMstGWradj

 

Water doesn’t boil until 212 degrees. Add 50% coolant, , now you are talking 235 degrees plus the pressure cap? 250!!
As long as the coolant isn’t boiling the radiator is doing its job.

jJust as being too hot is bad, being too cool is also bad. You need heat to boil off water in the oil. ( it enters because of humidity through the engine breather) Etc etc. if you start the car, drive it at least 30 minutes before you shut it off. ( nope idling doesn’t count). Periodically if most of your driving is around town once a month do an Italian tune up.
The reason there aren’t numbers on the gauge is people like you. Believe it or not people obsess about things like this. Plus a gauge isn’t calibrated. It’s an indicator not a scientific instrument.
I like that you use an infrared sensor. Measure 2 place on the top water the water hose goes into the radiator. ( measure the radiator not the rubber) and the bottom where the coolant goes into the engine. ( radiator again) above 150 degrees it’s OK to drive below 240 you’re safe.

 

Air and bad grounds could indeed be giving me false readings. With advice taken on where to take temperatures, I let the car idle at ~1300 rpm in park for ~30 minutes with the aux fan connected back to stock and used the handheld thermometer to measure at the metal of the rad next to the driver's side upper hose, the fins of the rad behind the aux fan (with the fan off of course), and the lower driver's side hose. I couldn't get the thermometer down to test the metal of the rad at the lower hose; so, the rubber had to suffice. I got ~86 C at the upper hose, ~93-98 C behind the fan, and ~86 C at the lower hose. The aux fan didn't come on, but I have seen it come on in the past on hot days. The funny thing is that the temperature of the rad behind the fan seemed to decrease from 98 C when measured at about 20 minutes in to 93 C at the full thirty minutes. The needle remained at the middle of the N today, rising and just beginning to pass the N at the end. Though that was funny too, as the needle was higher yesterday when I was reading lower temperatures. Funnier still is that the needle seemed to go up at the end with the bonnet latch in the open position and the door open. I tested the thermometer on a person's forehead this morning before testing the car, and it read about 90 F, which seems about right.

 

You're now seeing why Jaguar....and so many others....eventually went to "dummy gauges"

FWIW I take the IF measurements at the thermostat housings

Offhand I don't see anything worrisome with the temps you've listed.

Ideally the needle will be about middle-N with coolant temp of about 90ºC but the gauges are flakey, as you know

If the cooling system is in good shape it should be able to maintain that in all mild and moderate driving conditions. If not, investigation is needed

City traffic or long upgrades on hot days might cause the needle to rise a bit. Some guys freak out over this. Some don't. Personally, I don't.

If your temp keeps going up-up-up with no end in sight....that's a reason to worry.

A normal/normalish temp reading that suddenly dips up/down for no apparent reason is almost always air in the system. It is important to properly bleed the system....sometimes it takes a repeat effort.....to make sure coolant is flowing thru all parts of the engine. Cylinder heads, most importantly

Cheers
DD

 

Well said.
You do understand the correct coolant system bleeding process? It’s not straight forward.
If not please read the subject listing at the top

 

I have bled the system twice in the past three days. I know the proper procedure of jacking the front, opening the caps and bleed bolt, filling from the center filler until coolant comes out the bleeder, closing the bleeder, then the same at the expansion tank, finally topping off at the center filler. I also cracked the bleeder when running to check for air bubbles.

This gauge has read increasingly higher average temps each time the car was used this past month, and I doubt the cooling system is somehow failing just a bit more each day. I also find it strange that the coolant inlets would read lower temps than the radiator fins. I'll keep the thermometer on hand next time I commute to check temps when I reach my destination.

 

I suspect the thermostats. This is a classic symptom of a failing or incorrect thermostat on the V12.

 

I had the thermostats replaced a year and a half ago. I doubt they would go bad that fast.

 

I know this is going to sound wrong but I’m grasping at straws here.
The owners manual had a little note about oiling the distributor. In my book it’s page 65. I’m not sure all have it on that page but it is a critical thing.
What happens if the distributor isn’t oiled is the advance weight stop moving relative to each other.
That means the ignition timing is off. With the timing off overheating can and does happen typically as you are reporting.
It’s extremely hard to check the timing because the timing mark is on the bottom of the car. While the plugs are on the top. I don’t know of any timing light that will make that reach.
It’s actually easier to check the advance by taking the cap off the distributor and try to rotate the rotor by hand. If it slides smoothly and snaps back when you let it go, pull the rotor and drop of few drops of engine oil in the center right on the cotton insert. Put everything back together and you good to go. If the rotor doesn’t rotate or feels gravely or fails to snap back it’s time for some attention.

 

Being a 91, it be Marelli, so no oiling needed, as there is nothing IN the distributor.

I understand you replaced the stats lately. good, but are they the correct length when open, as in that scribe. I am suspect of them.

New NEVER means RIGHT. When I was with Jaguar Spare Parts, I lost count of the number of dud oil gauge sender units in unopened Jaguar packaging, just saying.

One of ,ine drove me nuts, almost to drink more. It was a Pre HE, and they be different to the HE. It was a lazy stat, A Bank, and a NEW pressure cap (they only have one cap) that was blowing off at 4psi, instead of the 13 rated on the cap. The cap was the main issue, the stat was just along for the ride.

 

Good point, I missed the 1991 part. I do agree that New does not equal right
I used to go by the local Jaguar dealer and periodically they would have a V 12 ( ( S or sedan) in the wholesale line, which because of the reputation/ market value, meant it would wind up in a salvage yard. Every once in a while my curiosity would get me and often as not it was something they” just can’t fix”
That was when the newest ones were only 15 years old.
Now that prices are on an upswing. That’s no longer happening.

 

Perhaps when my Jag man is done with my Fiat and RR I'll bring this car in for new thermostats and a block flush. I don't have a heated workspace.

One would think if there truly was an issue with cooling the temp would rise without the aux fan wired to always on and with the heater off. However, heater off or full blast makes no difference, and during the tests the aux fan wired stock made no difference compared to always on. It also feels like the temp gauge is sometimes lower with higher outside temps.

 

I had new Robert Shaw (good brand) T-stat that was bad from the start,and it got worse over several months. Not real bdd at first, the gauge read normal at cruise/slightly above normal on hills. Temps got hotter over time

I thought sure something was wrong with the cooling system or gauge (not a jag).

The T-stat finally failed closed. I had to replace a couple head gaskets because of that. (I choose to drive off the freeway to safe parking spot rather than shut the car down on a busy freeway).

Doug

 

Ok, well I'm convinced this temp gauge has multiple personality disorder. I just took the car for a drive for the first time in a week and a half and after those two bleed sessions. And... the gauge was much lower than normal. The middle of the needle was at the bottom of the N, and the bottom of the needle eventually rose to the bottom of the N only when idling at 1500 rpm for a minute while I opened my driveway gate and then a lot of reversing into said drive (a temp increase that always happens during said process). Immediately after I parked in my drive, the infrared thermometer read ~83.5-84 C at the thermostat and about the same on the rad behind the aux fan. So, problem solved? Maybe the air at the temp sensor got dislodged during the drive? Whatever the case is, the car seems to be running much cooler after the bleeds. I'll probably bleed it again now that the air might have gotten dislodged. I'll keep you all posted.

 

Well done, SIMPLE cars these V12's.

We forgot to tell you, ALL XJS a female (NO DISRESPECT TO ANYONE PLEASE), and then ET is usually residing somewhere in the loom behind the dash.

Aussie humour, old age, and waaaaay tooooo many Jags.

Drive it, enjoy it, they HATE sitting around, and most that sit have issues, oops.