You probably read my thread on changing the thermostat. Here is the sequal.
Prior to the change the engine always ran at 95 deg C ( I have a scan gauge fitted so have temp from the diagnostic socket)
When I fitted the new 84 deg stat the temp centred around 88 deg dropping to 86 on downhill no throttle and up to 90 when gassing away from trafic lights . So I tested the old 87 deg stat.you just hang it in a pan of water and raise the temp with a thermometer in the water.
It opens at 91deg so clearly out of spec . I discovered that the constant 95 deg running temp was because that is the temp the rad fans come on at half speed.
So I though best plan for those 80mph runs across France would be to bring the fans on "slow" with the ignition. this would help the rad and the oil coolers, particularly in traffic and keep the temp down.
So to the wiring diagram for the fans. The high/low is achieved with standard 3 relay switching giving series or parallel for the two motors
There is a white wire which switches the slow relay ON by giving it an earth. Now be careful here . The mass of wires going into the ECUs are all being switched to earth by transistors. These are easily buggered by fitting a non standard relay without suppression. ( resistor or diode across the coil) This applies to all ECU controlled circuits.
To


switch on the fans you need to earth the white wire . EM 13-15 .You have a choice of two places,At the relay pack by the left-hand horn which is very inaccessible or where it enters the ECU in the left hand bulkhead extension. If you do it at the relay pack join the white to separate earth on the chassis not to the black relay earth or you risk the ECU transistor carrying fan current
I chose the ECU. Photo shows the wire . You can be sure its correct as the next one is white blue.
Remove the 4 security screws with a wide-bladed screwdriver. I replace mine with Allen headed bolts for ease of removal. Plenty of bolts nearby to use for the earth.
The result is that the engine stays at 88 deg regulated by the thermostat and the run when you switch off still functions as does the fast fan speed.

 

To no one in particular, but perhaps nevertheless instructive to note:

a) that a thermostat is not an off and on device. Usually the marked temperature is where it begins to open. And is not fully open until 10-15 degrees F later. The “fully open” temperature is the temperature at which the engine are designed to perform best, not the opening temperature.

b) an engine coolant temperature close to or slightly above 212 F (100 C) is not overheating, but rather an ideal temp for most ICE’s. An engine that runs continuously at the thermostats opening setting, 84-88 C in our case, is not running hot enough burn off condensation in the oil, as well as not being hot enough to meet other performance related criteria.

I know these cars can suffer from catastrophic overheating. But that fact is no reason to overreact & desire them to run at a temperature which is too cold.

Just sayin’

Z

 

After 40 years in the motor trade I do understand the characteristics of the opening of the Stat but did not want to complicate the bit in the post about testing thermostats.
I have only done about 20 miles with the fans running so early days .Only one wire to cut if i change my mind.
To summarise
Faulty Tstat 95 deg fans on
New stat 88 deg +or - 2 ..
New Stat and fans 88deg + or - 2 but its not a hot Texas day !

Time will tell

 

My 2003's temperature stays around 194-197 even on hot days. This is too low to be optimal?

 

Thats fine . just be grateful its not overheating.

 

You’re asking for optimal ? No it’s not. The engine will wear less and have more power at 210-215.

but is it in an OK range ? Yes it is.

Z

 

This is why Jaguar didn't make the temperature gauge show the actual temperature when in the normal range - its much easier to say that if its in the middle its running fine. Here are the normal values for my 2001 XKR. The XK8 may be different.

The “normal” temperature range is 181°F - 230°F = 83°C to 110°C

Max on the factory gauge is 260°F = 127°C

The radiator fans come on in Slow mode at 90°C

The thermostat fully opens at 96°C

The fans switch to Fast mode at 97°C

 

On xk8 fans come on at 90 c
temp gauge reads normal from 80 c
thermostat opening depends on the thermostat you have ..my 84 c fully open at 88c

 

Jaguar published cooling information:








Z

 

Now it's running around 190. Probably a sticky thermostat. Suggests on best temp on replacement? Rock Auto shows one possible at 174 and second at 190. Thanks

 

190 F is nothing to worry about. In fact, 200 F is a good place for an aluminum block and heads V-8 to be for power and longevity.


I would give anything to have 190 F be my top coolant temperature. My ‘02 XKR routinely hits 210 F year round, and in 115 F summers it will hit 220 in traffic, but always cools down to the 200-210 F range once the cat is moving at 75 mph or better.

Z

 

So temp results are in on the 06 (Jag 1). Topping at 199 with the occasional nudge to 201. Fans operate correctly. It's an 85 degree day. The 06 runs hotter than the 03 (Jag 2). Also doesn't accelerate as well and slightly rough at idle. It's surprising that the def "worn around the edges" 03 (77k) runs cooler and is basically more fun to drive than the showroom perfect low mileage (43k) 06. Still getting both cars sorted out.

 

Have you performed the throttle cable adjustment procedure on the 2006 model? If not, be sure to do so before you try anything else. It will take you no more than a couple of minutes to do....

 

Thanks for the good suggest. I'll do it. It's only been driven 3k in the past 4 years (seller drove less than 500 mi in the year he owned), so non use might be a factor? Car is showroom condition (tan interior, Seafrost exterior). It will get driven regularly. Certainly different from my 2000 which was an ongoing project. Congrats on the VE. Very nice cars.