So, after my car sat over the winter, did some topping off, etc., and I cannot seem to get the radiator cap on the crossover pipe to seal. Initially, after replacing the older cap (which I don't think was the stock one, but it had been functional), it would leak once the engine ran a bit and got some pressure in the coolant system, so I replaced it with a new 16 psi cap, but it leaks just the same. I've taken the damn thing off and gotten it back on as tightly as I could several times, but still the same deal.

Is there a "trick" to getting this particular cap to seal? Anyone had better luck with a specific brand/type?

The coolant light and major fault indicator light come on and stay on, even though the coolant temp sensor reads cold, and the levels seem fine. I'm betting there has to also be air trapped in there somewhere now, but until I can get this thing to seal, I think bleeding the system won't do much good.

 

I'd check the sealing edge of the pipe where the cap seal meets it and see if it is scored or nicked, maybe smooth it out with some steel wool.

 

I don't have the #'s in front of me, the the cap on the crossover should be of a higher PSI rating that then cap on the filler tank...you want the filler tank to always be the cap that gives way first. If the crossover pipe cap gives way first, that coolant is lost. If the filler tank gives way first, the lost coolant is recovered.

Hope this helps,

John
1987 XJ-S V12
64,000 miles

 

If my memory serves me, the higher presssure cap should be 20psi. (Based on memory only, no hard facts).

 

mine did that for years, messing up my u.s. made hood liner. I eventually found the bleed pipe was blocked that runs across the top of the top radiator panel, and also the banjo bolt holes
were bunged up with sludge, clean or replace the pipe, and I also run a drill through and made the little holes a little bigger in the banjo bolt, it hasn't pumped water out since,

 

done it yet?

 

Not yet, hopefully this weekend. Thanks for the pointers on the bleed pipe, definitely something to inspect while I'm in there.

I did look a bit closer at the replacement cap I got, and I see the inside rubber seal isn't quite as large as the one from the cap I took off, which could also explain some of the non-sealing. I'll see if I can find a 20 PSI one that has a bigger inner seal, as well.

 

the inner seal can be removed and added to the new cap


BB

 

when I had your problem it didn't make any difference even with silicone under a 20lb cap, you must remember if you stop the leakage the excess pressure that should be going to the expansion tank,
will have to go to the next weakest point, hose, water pump seal, heater core etc etc. until you check the bleed pipe be careful,

 

FWIW, I always lost a bit of water through the cross pipe cap, even with 100% plumbing to the rad bleed, until I fitted a 'solid' cap that I got from Car Builder Solutions. The pressure relief part of the cap on the cross pipe does not serve any purpose, but the top seal is always going to be iffy in that situation, I believe. The Great Palm deals with this point.

Greg

 

but only when you know the banjo bolt and pipe is clear.

 

100% agree RGP, but if it is blocked, while the system will not self-bleed, which obviously may lead to big problems, there is no danger of over-pressure as the wing header tank will relieve that aspect. In other words, the blocked purge system is bad news, whether or not the cap is a solid one or a relief type.

Greg

 

please show me photos so I can see how to fix my 1991 Jaguar xj6 sovereign. It’s leaking from the cap.

 

An XJ40 XJ6 system is quite different to an XJS V12 HE cooling system.

The XJ6 system is much simpler. If you've got a leak at the cap, it's very likely just the cap itself

Paul