350I swan neck filler cap

[SteveBikeandAuto]

Hi Folks

I have been checking the cooling system and I noticed a couple of on line discussions where people have said there should not be a pressure cap on the swan neck on the V8. My car has a 16psi pressure cap fitted, as well as a new 15psi cap on the expansion tank. Can anybody shed light on what issues this may cause and if I should be changing it?

Cheers

Steve

[mrzigazaga]

Hi mate...I have heard that there should be a pressurised cap on the swan neck by some well known TVR engine builders and then others saying it should be a blanking cap?...I have a blanking cap on mine...Now the hose coming off of the swan neck overflow is going to the expansion tank...It would make sense to have a pressurised cap there as the expansion tank is just that...Not like on the 280i whereas the overflow on the rad neck is actually part of the flow...The V8 seems to have no flow in the tank so is purely for expansion.

Sorry but i don't have a definitive answer...Hopefully someone can explain it as i would like to know as well...Not that I'm having any issues at present....Cheers...Ziga

[ElvisWedgely]

I have owned six V8 Wedges and not one of them had a pressure cap on the swan neck. Always a blanking cap. None of the cars have had a heating or water loss issue, so I have to say that the blanking cap works well. The proof of the cake is in the eating as they say.

Tony. TCB.

[SteveBikeandAuto]

Thanks guys, I must admit I was struggling to see the logic, the overflow from the swan neck on my system feeds back to the expansion tank, which also has the pressure cap. Always good to be able to check with people on this forum who have extensive knowledge and might just point something out which isn't obvious to a novice such as myself

[The Hatter]

I spent ages looking at wedge cooling systems before deciding what to do with mine. My conclusion was that TVR kept changing their minds and I'm not sure if they ever got the system to work properly.

But to answer the original question - definitely a blanking cap. the small pipe off the top of the swan neck is the highest point in the engine system and must vent back to the 'expansion tank'. If you put a pressure cap on the swan neck you don't get venting and the air can't escape from the engine.

And worse... If you put a pressure cap without a neck seal on the swan neck (many don't have a neck seal) then the small pipe from the swan neck to the expansion tank bypasses the expansion tank cap and vents the system to atmosphere - very bad!

[KKson]

Agree with the responses. Should be a blanking cap on the swan neck so air can get back to the expansion tank. When the expansion tank pressure relief lifts then this puts water into the overflow bottle. The pipe into this should be at the bottom of the overflow tank so when things cool down and a vacuum is created it is the surplus water in the overflow bottle that is drawn back in rather than air.

[SteveBikeandAuto]

Great info, now I get it. Does anybody know where to buy one?

[KKson]

Got a new one recently from my local motor factors. Flea has them for £6 also:

http://www.ebay.co.uk/itm/60-5mm-Dia-Non-Pressuriz...

[leorest]

SteveBikeandAuto said:

Hi Folks

I have been checking the cooling system and I noticed a couple of on line discussions where people have said there should not be a pressure cap on the swan neck on the V8. My car has a 16psi pressure cap fitted, as well as a new 15psi cap on the expansion tank. Can anybody shed light on what issues this may cause and if I should be changing it?

Cheers

Steve

Schoolboy physics says that the pressure in a closed system is the same everywhere (OK a bit different either side of the water pump but negligible considering it's a high flow low pressure pump) so a higher pressure cap on the swan neck would give the same result as a blanking cap e.g. expansion cap will always blow first unless the expansion tank cap sticks.
I have a blanking cap on mine which I believe to be correct and also works well but I don't see why your configuration would work any less so and is even fault tolerant, though I would like more than 1psi headroom for that to work properly.
So to answer your questions...
1 What issues this may cause? A.It could mask a sticking expansion cap such that you loose some fluid. but under a no fault condition none whatsoever.
2 should I change it? A.Only if you want to.

[Number 7]

definitely only use a blanking cap. I tried a pressure cap on a 350 years ago and for some reason the coolant got pushed out of that cap.

[adam quantrill]

A pressure cap seals on the bottom, whereas a blanking cap seals at the top only.

So you need a blanking cap as has been said, to allow free flow down the pipe to the expansion tank.

I seal mine up to with a smidge of blue stuff and never touch it again.

[mrzigazaga]

Also keep an eye on the cheaper blanking caps as the seal is nowhere near thick enough and will leak...Im sure the original fitment was XJS/6...Like these..
http://www.ebay.co.uk/itm/JAGUAR-XJ6-EXPANSION-TAN...

[Wedg1e]

If you consider the swan neck pipe as being solid, i.e. without the cap then the answer is obvious - the pressure cap goes on the expansion tank. IMHO the only reason there's a cap on the swan neck is to aid filling the coolant system - and it should be a blank!

[SteveBikeandAuto]

Thanks again to everyone who has taken the trouble to contribute to this topic. I have taken the general advice and ordered a blanking cap.

[B@man]

One of our locals had them the wrong way around and his 350i kept overheating !