What is the function of the header tank?

[Holly1638]

I have a 1991 V12 which has done 84,000 miles. On a short trip yesterday I noticed the temp gauge had crept well above 'N' which is not usual. This morning I checked the fluid levels and had to top up the header tank with about 1/2 to 1 litre of coolant. Is the absence of such a small quantity of coolant enough to raise the temperature so significantly? In fact is it necessary to fill the header tank at all , if it is acting as an expansion tank? Or is it acting as a reserve to the main reservoir of coolant?

When I tried to top up through the main filler cap that proved to be still full of coolant. Are there 2 reservoirs of coolant which are separate from each other? If the header tank needs to be refilled, does this indicate a leak, and where is the leak likely to be? The header tank was found to be rusted and replaced a year ago after the XJS overheated in a traffic jam in central London. The bottom radiator hose burst some weeks ago and was replaced; prior to that I was constantly topping up the main reservoir with coolant but it does not appear to be losing fluid any more. There seem to be constant problems with the cooling system and I simply don't trust the car, now never taking it more than a few miles from the house. Any advice appreciated as I am not clear why the temperature gauge rose above the normal level.

[HerefordMark]

Sorry to hear of these problems - we all know that an overheating V12 is no fun. I rebuilt my car's cooling system last year, and found it quite tricky to get right. My opinion, for what its worth, is that the header tank is not involved. Being down by a litre or so should not have such an effect on the temp.

I think its more likely to be connected to the recent loss of coolant via the burst bottom hose. You don't say who replaced it, but it is quite difficult to get this right, as I know through experience. A blocked rad will also cause these symptoms - was it flushed when the hose was replaced?

My advice would be to drop the coolant, and flush the engine and rad through. Be vigorous about this, flushing and backflushing until you have as much crud out of there as possible. Then get the front of the up on ramps, or on a hill, so that the rad cap is definitely the highest point.

Make up some new coolant - be sure to use 55% of a proper antifreeze with corrosion inhibitors in it. I know that quite a few non Jag specialists don't realise just how important the right mixture strength is, using the right antifreeze - it does have an effect. Dpon't be tempted to add any 'stop leak' type stuff - it kills V12s like Domestos kills germs.

Once you have the mix right, make sure you have the quantity specified by the book, and make sure it ALL goes into the engine. This is the tricky bit - there are lots of choke points, and it is very easy to get airlocks etc which make you think it is full. But until you have the whole amount in, keep going! I found squeezing various hoses etc helps, and once you have nearly all of it in (but not before or you might overheat the engine) starting the car for a few seconds will help, as the water pump will circulate it for you. Keep going until it is all in - this is not a quick job. I found I had to keep leaving it to settle and so on.

Once done, leave it overnight, then top up, and refit 2 new rad and header tank caps. Then get your high pressure hose out and clean the rad matrix and in between the rad and air con rad - this is important as much rubbish can accumulate in there. It won't do the job with total efficiency, but it will help.

Hopefully this will cure the problem for you, and although its lengthy and messy, its not too expensive. But if not, then I would start budgeting for a radiator recore.

Hope all this helps, let us know how you get on. PM me if any aspect is not clear - if you are anywhere near Hereford I'll be happy to help if I can.

HM