X308 XJR Thermostat Replacement
Discussion
A few medium trips in the current hot weather have resulted in my fans kicking in fairly early and staying on for a good 5 mins after I turn the engine off. My mechanic suggested I replace the thermostat. Do I need to drain the coolant completely before doing so or can I just replace the thermostat and top up the coolant to replace any that is lost in the process?
It's normal for XJ fans to work hard in this ambient temperature - especially if you are using the a/c a lot.
You may have a lazy thermostst, and the easiest way to check is replace it. You can do so without draining all the coolant. Spill what you have to and then top up with the same colour coolant diluted 50:50 with tap water. The thermostat housing and filler cap on top is made of brittle plastic so handle carefully.
The other possibility on a pre 2000 car is the waterpump is loosing efficency because some of the impellor vanes have fallen off. With the engine stone cold, remove the the header tank cap, start the engine and look inside the tank whilst you gently increase the RPM. You should see a steady strong stream of coolant into the tank from the return hose at the top of the radiator. If you don't then chances are the pump is knackered and needs replacing with a later design one with metal not plastic vanes.
The temperature gauge on an X308 is driven by a software programme and does not directly provide a linear indication of actual coolant temperature. Any suggestion of low coolant levels or hint of overheating needs attention as the next stop down the engine running too hot road is blown headgaskets.
You may have a lazy thermostst, and the easiest way to check is replace it. You can do so without draining all the coolant. Spill what you have to and then top up with the same colour coolant diluted 50:50 with tap water. The thermostat housing and filler cap on top is made of brittle plastic so handle carefully.
The other possibility on a pre 2000 car is the waterpump is loosing efficency because some of the impellor vanes have fallen off. With the engine stone cold, remove the the header tank cap, start the engine and look inside the tank whilst you gently increase the RPM. You should see a steady strong stream of coolant into the tank from the return hose at the top of the radiator. If you don't then chances are the pump is knackered and needs replacing with a later design one with metal not plastic vanes.
The temperature gauge on an X308 is driven by a software programme and does not directly provide a linear indication of actual coolant temperature. Any suggestion of low coolant levels or hint of overheating needs attention as the next stop down the engine running too hot road is blown headgaskets.
There is an aftermarket aluminium replacement for the thermostat housing.
You don't say what year your car is - if it's a pre 2000 model with a VIN ending in 6 numbers then it may have the original design of water pump fitted which has plastic impellor vanes which can disintigrate leading to reduction in efficency. A 2000 and on car with a VIN ending in 5 numbers should have the later design pump with a metal impellor. Later cars don't have the same problems with coolant flow.
It's difficult to diagnose cooling issues with the software driven temperature gauge but I suspect you don't actually have a problem though. Cooling fans on a hot engine will be running almost continuously at low road speeds in these ambient temperatures, especially if the a/c is on too.
You don't say what year your car is - if it's a pre 2000 model with a VIN ending in 6 numbers then it may have the original design of water pump fitted which has plastic impellor vanes which can disintigrate leading to reduction in efficency. A 2000 and on car with a VIN ending in 5 numbers should have the later design pump with a metal impellor. Later cars don't have the same problems with coolant flow.
It's difficult to diagnose cooling issues with the software driven temperature gauge but I suspect you don't actually have a problem though. Cooling fans on a hot engine will be running almost continuously at low road speeds in these ambient temperatures, especially if the a/c is on too.
Strange..... I thought that the later water pump still had polymer vanes ? The only metal vaned pump was for the Mercury in the States.
If you are in a hard water area do NOT use tap water. I did and ended up with a fine white coating on the thermostat 
Filtered rain water if you can't find cheap deionised
If you are in a hard water area do NOT use tap water. I did and ended up with a fine white coating on the thermostat Filtered rain water if you can't find cheap deionisedP700DEE said:
Strange..... I thought that the later water pump still had polymer vanes ? The only metal vaned pump was for the Mercury in the States. If you are in a hard water area do NOT use tap water. I did and ended up with a fine white coating on the thermostat Filtered rain water if you can't find cheap deionised
Best information I have is this is the difference. I've seen a Jaguar V8 OE pump in a box that had a pressed stainless steel impellor, but not seen both old and new pumps side by side. There was an article in a 2004 Jaguar World magazine that said this was the case and it's come up from time to time in the technicial information published in the JEC magazine too. If you are in a hard water area do NOT use tap water. I did and ended up with a fine white coating on the thermostat Filtered rain water if you can't find cheap deionisedThe main thing of course is - does it work and not what is it made of and you can find that out by simply looking at the coolant return flow to the header tank.
If you can show this isn't the case 'tho, in the interest of accuracy I'll happily change the buyers guide
Hi Steve
I was told the original pumps had a white/gray polymer fins and used a "paper/card" gasket. Later genuine pumps used a metal gasket and have black polymer fins
Metal pumps are available in the States and other types from alternate suppliers
The polymer one is from a pump removed from my XKR the metal one from an e-bay vendor
As you say the important thing is they work and do not suffer failure due to the fins breaking up.
I was told the original pumps had a white/gray polymer fins and used a "paper/card" gasket. Later genuine pumps used a metal gasket and have black polymer fins
Metal pumps are available in the States and other types from alternate suppliers
The polymer one is from a pump removed from my XKR the metal one from an e-bay vendor
As you say the important thing is they work and do not suffer failure due to the fins breaking up.
Edited by P700DEE on Wednesday 24th July 15:06
Hi, running a 1998 XJ8 3.2l Jaguar; it runs sweet and pulls greeat.
Missus had it overheat but not a cooking- she noticed gauge was rising and stopped. It pumped water out via header tank overflow pipe. I have since checked thermostat and it opens fine in hot water. NO oil in water nor vice-versa. Oil is clean golden.
Tested today after refitting stat. Seemed fine- ran sweet and mid-temp on gauge. Test run on road fast, then slow, then traffic. Great - all warm, nice heater in cabin. OOPS! Suddenly gauge climbed. Pulled in and watched water ejecting from overflow again. Topped it up, started engine after ten mins. All good- and she drove home fine! Wondering if the water pump is culprit- rads all clean, pipes all good. Power is great. It is just water in system- would proper coolant help?
Cheers for any help!
Missus had it overheat but not a cooking- she noticed gauge was rising and stopped. It pumped water out via header tank overflow pipe. I have since checked thermostat and it opens fine in hot water. NO oil in water nor vice-versa. Oil is clean golden.
Tested today after refitting stat. Seemed fine- ran sweet and mid-temp on gauge. Test run on road fast, then slow, then traffic. Great - all warm, nice heater in cabin. OOPS! Suddenly gauge climbed. Pulled in and watched water ejecting from overflow again. Topped it up, started engine after ten mins. All good- and she drove home fine! Wondering if the water pump is culprit- rads all clean, pipes all good. Power is great. It is just water in system- would proper coolant help?
Cheers for any help!
CAPNGEOFF said:
It is just water in system- would proper coolant help?
No, water is more efficient at heat transfer than coolant.You've clearly got a problem, you'll have to do a bit of fault finding to find out when (and why) the car suddenly overheats. If it's fine on a run and then overheats some time after slowing down, traffic, etc, are the fans coming on?
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