My 2000 Xk8 runs well for 20 km’s with water temp sitting where it always does and then in 30 seconds water temp will go to over temp. No low Low coolant light. The first time it did it coolant was venting out of the filler tank. Oil doesn’t appear milky

 

'Venting out' means blowing by the reservoir cap or leaking from some part of the tank?

 

When was the last time the the coolant system was flushed?

 

Always diagnose in the order of cheapest/simplest first to most expensive/difficult last. The first thing to check is that the cap still holds pressure. Its got a big o-ring that after 20 years isn't very rubbery anymore. Some shops may have the adapter to test the cap, but its cheap enough to just replace it and see if that works.

 

How old is your thermostat?

 

The weird thing is the car was venting coolant from the RH side in line with the coolant resevoir. Not from the Overflow. I have orderd a thermostate. If the thermostate was US why would it maintain the temp to mid scale for 20 km and then go to overtemp in 15 secs? Has anyone got the coolant schematic for the XK8 non turbo?

 

To clarify, where was the coolant coming from? Do you have a picture? Was it leaking from two separate places at different times?

Also, the XK8 temperature gauge is a bit misleading (by design). it sits in the middle of the gauge for a wide range of temperatures, only increasing towards the top once potentially dangerous temperatures are reached to prevent people being alarmed by a normal temperature variation.

However, it could well go from the middle to the red in a short time depending on engine load, air flow, air conditioner, coolant flow, external temperature, etc.

 

If it's venting from the right wheel well, not engine compartment, that's the atmospheric overflow. I am rather certain the thermostat is stuck closed, a common failure mode for the XK8.

 

Is your cooling fan working? The relay malfunctioned on mine and had to be replaced. Does it overheat while the car is moving, or while sitting, which is when the fan is needed?

 

Ditto on what PDUPLER recommended. Highly recommend replacing the reservoir cap along with the thermostat if it's several years old. I had my reservoir, thermostat, water pump, serpentine belt, and upper and lower radiator hoses replaced by my local indy all at the same time due to increasing overheating symptoms out here in Las Vegas (@ 89,000 miles). The only flaw was that one of the mechanics accidentally put the old cap back on the reservoir, which was leaking imperceptibly around the o-ring, and caused loss of pressure, coolant blow-by to the wheel well holding tank, and an increase in water temp (although I've never had the water temp go above 218 degrees Fahrenheit even in 117 degrees F ambient, driving from Las Vegas to Los Angeles in the summer) (I monitor real-time coolant temp with an inexpensive engine OBD scanner and an app on my cellphone). Went back to the shop, got the new cap, and now at 105,000 miles, I haven't seen coolant temps rise above 205 F even in stop and go city traffic...

 

After replacing water pump and therostat problem remained. Sniffed for exhaust gas in radiator. No problems. Hoses were always sucked flat when enginie cooled. Plugged into monitor and once coolant temp moved from centre 113 degree C to redine at 118.Replaced radiator cap and problem solved. Thankyou all for your help. I do enjoy learning

 

The coolant reservoir caps these days do not seem to function properly for more than a couple of years. I believe my wife's 2006 XK8 is currently on its fourth cap (while only on its second coolant reservoir)....