Cooling issues v12

My 1989 jaguar xjs v12 is reading hot at the gauge. I’ve checked temperatures all the way around the engine and radiator it’s normal. At the temperature sensor on the passenger side it reads 280 plus when warmed up but just in front of the sensor it’s normal temperature. I’m using a laser thermometer. It’s just hot at each end of the tube. Both sensors are new Both thermostats are new completely flushed system and a new water pump was installed before I purchased the car. Been using it as my daily for about a month now with no issues. Just can’t seem to find why there’s so much more heat on those two points and nowhere else. It’s driving me nuts.

what are the readings you say are normal, is there a temp rating on the sensor, also can you take a temp reading of the water in the expansion tank? 280 on a brass sensor is either faulty sensors or wrong parts…i use a laser as well…handy items but i use a thermostat pickup in the top hose with a temp dial on the control…set it to what ever is best for each season

190 to 195 on everything on the engine 140 to 160 on radiator. 250 to 300 at the end of the tube where the sensor is but just on the other side of the sensor back to the thermostat temperature goes back to normal.

well logically water can’t get that hot…250 F is 120C , the other temps make sense, and appears thermostats working ok…any chance of a blockage near the sensors

No I took it all apart today and made sure there were no obstructions in the line .yes that temperature would boil everything in the line that’s why it’s driving me nuts.

so you get this abnormal reading on the sensor but how far either side of the sensor does it read 250+, 1 -2 -3 inch?

Only behind the sensor just in front of the sensor temperature drops back to what I call normal. And the temperature gauge starts out on the cold setting and moves up gradually with the car running.

I’m going to take a short trip in the morning maybe 10 mile lap around the back roads here and recheck everything again. And check coolant levels before I leave

is the temp gauge showing normal or responding to the sensor high output…do you have electric fans or viscos fan

To understand why the cooling system is pressurized, consider that for every 1 psi over atmospheric pressure, a liquid’s boiling point is raised 3°F. Whereas water boils at around 212°F. at atmospheric pressure, if exposed to 15 psi (a common radiator pressure cap), the boiling point will now be 45°F. higher (3 psi times 15 psi cap). Under this pressure, water boils at 257°F

jim…it’s only showing a high reading at the sensor…not anywhere else.

jim…a suggestion>> if you copy /paste something from a quote somewhere else, you say where it came from ie:

Responding to the sensor . The car has factory fans and I’ve already checked the fan clutch it is operating as it should.

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Well, agriculture or Motor Trend. It doesn’t matter. Science is science and facts is facts. :slight_smile:
We all know pressurized cooling system operates at higher Temps. I didn’t know how much one psi changed boiling point, so I Googled. Didn’t check what the source was for a “proper” DNA.

Tom, you are correct that I should have indicated the website from where I pulled that information. Sorry for not documenting my source.
I am baffled by the temp change from one side of sensor to other.
But I did want to point out that the temp is attainable… but would not like my system to be at that temp


First thing would be to check the sensor Resistance vs Temperature.
These are the values from @SD_Faircloth, but as far as I know there are two sensor part numbers, so it’s a bit confusing, hope others have more info.
What I know is that when I replaced mine, the new sensor was reading higher temps, so I reverted back to the old one.

0°C 5.9 kOhm
10°C 3.7 kOhm
20°C 2.5 kOhm
30°C 1.7 kOhm
40°C 1.18 kOhm
50°C 840 Ohm
60°C 600 Ohm
70°C 435 Ohm
80°C 325 Ohm
90°C 250 Ohm

Did you check if the new thermostats have sufficient travel to completely close the bypass?
Some don’t… Travel (fully open) should be min. 41mm from top seat to bottom disc.

If your engine is indeed overheating and if the clutch is indeed working as it should, the fan should be engaged all the time. If not it means your radiator is compromised. Has it ever been cleaned/re-cored/replaced?

And did you ever check if the front of the radiator is blocked with leaves and crud? The XJS has this bad habit.

I would probably add to that, check the banjo bolts as well but a simple test for the radiator would be , check the top radiator hose temp and the lower radiator hose temp to see if the radiator is capable or doing it’s job.
You might like to see a 15 degrees difference (approx.) between the two.

I have read on the forum that incorrect length thermostats can create issues, could that be your problem?

Tom Thomas my son and I gutted the secondary cats and opened the others for a less restricted flow. So far it seems to have helped. I know the purists will say no but I plan on redoing the exhaust system to a more modified setup anyway. Thank you to everyone for the comments.

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