1998 XJ8 coolant warning

[spreadsheet monkey]

Jaguar steve said:

You cannot change the volume of a liquid but you can of a gas.

If you're actually seeing a drop in coolant level with an increase in engine RPM and a rise when the revs fall that indicates there must be some trapped air within the system being compressed as you speed the engine up. The volume of the air is being reduced by the increased flow rate from the water pump when running at higher speeds and that reduction in air volume is what allows the coolant level to drop.

The V8 won't tolerate overheating and replacement head gaskets are expensive. I'd investigate further to find and solve the underlying problem and start by making absolutely sure the system is completely free of trapped air rather than just try to work your way round it.

Thanks for the advice. Coolant flush/replacement was done as part of the last service (800 miles ago). I'll keep an eye on the level from now on.

What's the normal operating temperature for the AJ V8 engine? According to my OBD2 gauge, mine runs at 89-93C. Is that a couple of degrees hotter than it should be?

[RingSpanner]

Jaguar steve said:

....
The V8 won't tolerate overheating and replacement head gaskets are expensive.

Can one assume that a pressure-test on the system would highlight any likely damage to a head gasket; or, is that expecting too much?

[Jaguar steve]

spreadsheet monkey said:

Jaguar steve said:

You cannot change the volume of a liquid but you can of a gas.

If you're actually seeing a drop in coolant level with an increase in engine RPM and a rise when the revs fall that indicates there must be some trapped air within the system being compressed as you speed the engine up. The volume of the air is being reduced by the increased flow rate from the water pump when running at higher speeds and that reduction in air volume is what allows the coolant level to drop.

The V8 won't tolerate overheating and replacement head gaskets are expensive. I'd investigate further to find and solve the underlying problem and start by making absolutely sure the system is completely free of trapped air rather than just try to work your way round it.

Thanks for the advice. Coolant flush/replacement was done as part of the last service (800 miles ago). I'll keep an eye on the level from now on.

What's the normal operating temperature for the AJ V8 engine? According to my OBD2 gauge, mine runs at 89-93C. Is that a couple of degrees hotter than it should be?

I don't know for sure but that seems about right. If you can find out what the opening temperature of the thermostat is that should help establish a figure that in the workshop or under moderate use you should see.

[Jaguar steve]

RingSpanner said:

Jaguar steve said:

....
The V8 won't tolerate overheating and replacement head gaskets are expensive.

Can one assume that a pressure-test on the system would highlight any likely damage to a head gasket; or, is that expecting too much?

That'll depend on how the gasket has failed. Cylinder to cylinder or into an oil gallery won't show anything via a coolant pressure test and even if you do see a drop on test then there can be other causes such as a hidden leak inside a heater matrix for example.

Coolant samples can be analysed for traces of combustion gasses which is a robust test.

[spreadsheet monkey]

Jaguar steve said:

I don't know for sure but that seems about right. If you can find out what the opening temperature of the thermostat is that should help establish a figure that in the workshop or under moderate use you should see.

Thanks for the advice.

[New Relic]

Jaguar steve said:

That'll depend on how the gasket has failed. Cylinder to cylinder or into an oil gallery won't show anything via a coolant pressure test and even if you do see a drop on test then there can be other causes ..

Going off topic a touch but, if it were cylinder-to-cylinder gasket leak - the engine would run as rough as a badger's ass?

And if the gasket had gone between cylinder and oil gallery, then the blowback would probably have oil leaking from the filler cap?

[Jaguar steve]

New Relic said:

Jaguar steve said:

That'll depend on how the gasket has failed. Cylinder to cylinder or into an oil gallery won't show anything via a coolant pressure test and even if you do see a drop on test then there can be other causes ..

Going off topic a touch but, if it were cylinder-to-cylinder gasket leak - the engine would run as rough as a badger's ass?

And if the gasket had gone between cylinder and oil gallery, then the blowback would probably have oil leaking from the filler cap?

Very probably on both - but as I said neither would show on a cooling system pressure test

[fisherdj1]

Hi, I have just bought a 1998 Jaguar XJ8 Sovereign. I have a continuing loss of coolant and now external indication of a leak. One straight forward question I have is what level I should gill and keep the coolant at? The manual says to the bottom of the filler cap when the engine is cold - is this correct? A new water pump was fitted by the garage of the previous owner 3 months ago.

[Jaguar steve]

fisherdj1 said:

Hi, I have just bought a 1998 Jaguar XJ8 Sovereign. I have a continuing loss of coolant and now external indication of a leak. One straight forward question I have is what level I should gill and keep the coolant at? The manual says to the bottom of the filler cap when the engine is cold - is this correct? A new water pump was fitted by the garage of the previous owner 3 months ago.

That's right.

Two coolant types used in the XJ8, find out which colour one you currently are filled with and don't mix the two. Don't top up with plain water either, use a 50:50 of coolant and water mix and get the leak sorted soonest or you might find yourself having a unpleasant phone call with the local Scrapman.

[P700DEE]

No external sign! Most likely you have a leak on the valley pipes, check immediately as when they go you can empty the engine of all its water very quickly. The worst case is that you have head gasket failure but whilst not unusual you usually have other symptoms than just dropping fluid level. Leaks are common and not always obvious. Valley pipes leak in the V8 valley and also sometimes down the back of the gearbox