Question about 1998 Jaguar XK8

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Coolant light will not clear after filling with coolant

Posted by Robert Buth on


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Duff Howell

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Hi Robert.

The most likely issue is that the coolant may not have been bled completely (or at all) for air bubbles. Draining a system can introduce air in various places, including between the blades/fins of the water pump, and re-filling it won't always force all the air back up to the top during the fill - the overflow / fill tank fill lip is often lower than the top bleed valve. If the system is still sensing a lack of coolant, say from an air bubble on/by the sensor, then it'll trip the coolant sensor.

The forum I found here Thread XJ8 cooling system bleeding procedure Tweet actually just emphasizes adding coolant, running the engine for 20 minutes or so, then re-topping-off the coolant, possibly repeating the process once or twice. A very important step there is that, during the process, you turn on the cabin heater full-bore and run it at full air for some/all of one of those 20 minute stretches. This is because the cabin heater is like a small radiator inside the dashboard that you flow air past. If you open coolant flow to this small radiator, it'll get hot, and that heats the air flowing by. (It's also why there will be advisories when driving up a long steep grade in the summer to turn the heater on full bore - it's an auxiliary radiator that can help keep the engine cooler.) It's easy to get air into this heater core during a coolant change or top-off, and it needs to be set to full-high on heat to allow coolant to flow through and chase out any air bubbles.

At the end of it, the coolant alarm may also just need to be re-set. It'll probably show up on an Engine Scanner (you can look for yourself with an OBD reader)

Finally, this thread ( ) notes that there is a float in the coolant tank that can sometimes get stuck during re-filling of the coolant. They have some suggestions about how to get it un-stuck.

Hoping this helps, and Happy Holidays!

Posted on Dec 09, 2015



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Are you sure it is the coolant light? The coolant system is a sealed unit and should not need topping up unless there is a leak!

Posted on Dec 09, 2015


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