Jaguar Repair: low coolant, head gasket leak, combustion chamber

Question
I travel about 35 miles to work one way.I will use about 1 pint of coolant at 63 miles a hour at 70 miles the low coolant light will come on at 35 miles and will need about 3 Quarts.1985 xj6 I have changed water pump,reservoir radiators pressure tested in shop help. Thanks.

Answer
Hi Isasc,
There are several things you need to look at. First the expansion tank is under the left front fender and the tank you add coolant to has a two way pressure cap so that when the coolant heats up and tries to overflow it is transferred to the tank in the fender and as the coolant cools and contracts it is drawn back in the filler tank if the cap is the correct kind of cap that allows coolant to be drawn back. If however, the wrong kind of cap is used or there is an air leak in the filler tank that will not allow a vacuum to form, then coolant will continue to be lost out into the expansion tank under the fender until it is full and then it will exhaust out on the road by an overflow line.
Another possible is that you have either a head gasket leak or an intake manifold leak at a water jacket. Thus burning the coolant in the combustion chamber. These can be tested by a coolant pressure test with the spark plugs out. Keep pressure on the system for a long time to see if it is maintained and that no coolant gets into any combustion chamber.
I take it that you have already pressure tested for external leaks.
Howard