Where's the water going?

Fellow Jaguarist …

2 years ago on my 1989 XJ40 I keep losing coolant. It turned out that I had a failed head gasket that was leaking between the coolant gallery and the number 1 cylinder. I pulled the head, had it remachined, then back together with a new gasket and all was fine.

Now I’m slowly losing coolant again. Tried to pressurized the coolant system with a hand pump and it wouldn’t hold pressure but I did find several hoses weeping at the thermostat manifold. Replaced all the hoses and no leaks, but it still wouldn’t hold pressure. Aha, I found another leak at the back of the header tank where the low coolant sensor connects. Fixed that and no more leaks that I can find. But I’m still losing coolant … However now I can pump up the system and it holds pressure (a check online says the system should hold pressure for at least 2 minutes. Mine now held pressure for 20 minutes with no drop).

Being suspicious of another head gasket problem I removed all the spark plugs and they were a uniformly tan color. I also looked in the cylinders with my borescope and everything looked normal with no stem cleaning evident like I had with my failed head gasket. While I had the plugs out I pressurized the coolant system and listened at each cylinder for escaping air … all was quite. So I’m comfortable in ruling out a head gasket problem.

I can’t smell any coolant from the A/C vents so that would eliminate the heater core leaking. I carefully checked around the water pump and see no signs of a leak. Two days of driving and I’m losing about half the coolant out of the header tank. The engine (3.6L) is running smooth as silk with absolutely no overheating even with the temperatures above 100F here in Dallas.

I’m scratching my head how the system can hold pressure and still be losing coolant. The only thing I can think of is the coolant recovery system somehow not functioning properly. Before I pull out the inside fender lining to get at the plastic recovery tank I’m wondering if I missed something else?

My “undetectable” leak was at the water rail, rear. Turned out to be a nightmare as several bolts sheared off in the block. Broken bolts had to be drilled out and helicoils installed to reinstall the rail. Hope your situation is not the water rail. Probably the worst job I ever had to do on the Jag.

I am not familiar with the XJ40, but if the same theoritical system is like the XJS, then I am wondering if you have actually overfilled the system, so that what you are losing is actually what is going out the overflow tank - but once you check the overflow tank in the fenderwell for any leaks, make sure that it is only half full when cold, and after you have correctly filled the coolant to level requirement, with the correct pressure caps(s) installed, then I would think that you will find that you are no longer losing coolant - that is, if you fo not have a minute leak at the rail as mentioned above - just a suggestion - Tex.

Larry …

Sheared bolts trying to remove the water rail huh. Been there done that,

When I rebuilt my head with everything out of the way the water rail was right there all in the open. My thought process went something like this … “It hasn’t leaked in 30 years so leave it alone — but on the other hand it’s setting RIGHT THERE and I can put new gaskets on it so it won’t leak for another 30 years”

Can you guess which way I went … :smiling_face_with_tear:

Sheared bolts

I’ll crawl under the car tomorrow to look up and see if I can detect a leak there, fingers crossed it’s not that.

Adna_Terry … That makes sense and I’m leaning in the overfill recovery bottle direction. I think I’ll dig it out (I’ve never removed it in 30 years) and see what kind of shape it’s in.


Have you considered putting some dye in the system and using a UV (“black”) light to spot a leak? If so, and if you have previously used dye to troubleshoot any other system, have a look with the UV light before you run the cooling system with dye in it to see if you have anything showing up from previous test(s). Don’t need any red herrings.


Well I jacked up my car this morning and crawled underneath for a close inspection. And it looks like my problem may have turned out to be the usual suspect … the water pump.

I definitely have a very small amount of coolant coming out of the water pump weep hole, but not enough to drip on the floor. So I’ll replace that and see if it solves my problem.

While I was under there I checked the water rail connections and both of them looked dry … thank goodness.

So as usual it’s never one thing … hoses leaking, header tank leaking, and finally the water pump.

Good news, at least do-able without too much stress

Hi Grooveman, I realise this post is now 8 days old, I’ve only just seen it as I’ve been away for a week and I don’t use a smart phone with on-line capability.
I hope you have resolved the water loss mystery by now, but in case you haven’t one thought springs to mind, is the rubber seal on the filler cap on the header tank in good condition ?
I’m assuming that when you performed the pressure test, the device you used fastens to the filler of the header tank in place of the cap so you wouldn’t be able to detect if the cap was losing pressure and allowing steam to escape while you were driving.

So glad to hear the water rail was fine, I remember reading your posts when you did the head gasket and thinking changing that water rail gasket is exactly what I’d have done in your position.

Hi all …

I just finished installing my replacement water pump last night. I’ve done it before and don’t recall the job being such a pain in the butt. Of course it was 100F in the garage and also I forgot to label where each bolt goes on the pump. 3 different bolt lengths with 2 different head sizes … a lot of trial and error figuring out where each went. VERY poor planning there.

I thought about using the paper gasket that came with the pump but decided instead to just use Permatex water pump sealant. Only down side is the 24 hour curing time. So tonight I shall see what I shall see.

Also gave me a chance to install my new black radiator cooling fan replacing the jaguar yellow one.

All seems to be good …

Well it’s been over a month now and the coolant level has remained constant at about an inch below the filler.
I was concerned when the level dropped that much in a day or two from completely full. But strangely enough it has now remained there. Same driving conditions and outside temperature.