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Build up of pressure in TR6 fuel tank


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Yesterday I filled up the TR6 with fuel. The weather was very warm here in Essex. On opening the fuel cap, there was a significant release of pressure. I probably had only 5 litres of fuel so as you can imagine, plenty of vapour! My question is - is this normal or shouldn’t there be a vent to balance the pressure with the atmosphere through the cap maybe? 

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The cap should include a vent on all pi cars

perhaps a po changed the cap to a non vented one, or the vent is blocked ?

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Thanks Steve I’ll check. It’s original I think, hopefully just blocked. Trevor 

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The original TR6 cap has a small hole, 1-2 mm diameter, slightly off-centre. Try prodding it with some wire to clear it.

Had the same problem with my Scimitar recently and easily fixed with a bit of wire to clean the vent hole.

Nigel

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Interestingly, the increased pressure in the tank (which is depending on the vapor pressure of the volatile components in the fuel) can reduce cavitation at the pump-inlet. The increase in pressure as an absolute number is small, bit it equals a certain static height of fuel so the “fuel level” in the tank goes up. By how much? That is depending on the fuel temperature and fuel composition.

If fuel is drawn from the tank (like when driving) , or if the fuel temperature lowers,  this “over pressure” is reduced or even turns negative in case the tank cannot vent. But this is all in an uncontrolled manner. This is why a vented tank in our cars is important. Uncontrolled over pressure can result in leaks.

So a vent is needed in our old cars, although modern cars can be equiped with vapor pressure control systems. 

Waldi

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With only 5 litres in the tank the fuel will heat up, depending upon how the car is driven. If it is poddling along using little fuel almost all the pump's output ( roughly 80% at 40mph cruise) is returned to the tank by the PRV.  So heat is added to the tank by the pump. unless you drive flat out, when the prv barely opens and all the pumps output is burned. Its a good idea in hot weather to refill the tank when its half-full. The heat transfer is the same but the temperature rise and gassing will be much less.

Peter

Edited by Peter Cobbold
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Hi again, just reviewed my fuel cap (only had the car for 24 years!) and I cannot see a vent hole. I confess I have experienced this over pressure before, probably more so after I replaced the sender unit gasket two years ago. (See my posting on that story!). It appears I have no vent hole and the rubber seal has none either. The Moss catalogue only refers to a vented seal. This would explain the build up of vapour pressure. See my photos. My next question if I replace just the seal for a vented one, (mine is breaking down on the edges, not visible in the photo) - is there a path for the vapour through the seal and its support which floats on a screw and spring.

Thanks for the comments so far. Regards Trevor

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IMG_3763.JPG

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I had a problem many years ago with my TR6 ,after a short run if I accelerated hard the engine would

start to misfire, when I checked my new cap seal ( no vent hole ), problem was a vacuum build up

in the tank reducing fuel flow to pump. Punched hole in seal as original one, problem solved. It appears

many new seals are produced with no vent hole ?

                                                        Harvey S. Maitland

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Thanks Harvey, I must admit my seal is probably a few years old now. (10+ maybe). I bought it from Moss. I've also just asked Moss the question ref the vented seal on their website - Is it sufficient?

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Trevor if you buy a new seal just punch a hole in the centre, with the old one off you can see

a vent hole in the cap itself.

                          Harvey S. Maitland

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Thanks Harvey I'll do that as a starter for ten. If you see the next photo under the seal. I think I may drill a hole in the fitting to guarantee a pathway. was yours like this?

Regards Trevor

IMG_3755.JPG

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Yes same as mine, the centre screw is spring loaded, the vent opens as the cap is closed.

                                             good luck

                                                           Harvey

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Isn't this is a negative pressure, (ie vacuum as Harvey says), not a positive pressure? Because the cap vent is not working, no air is getting in to replace the fuel that is going out then when the cap is opened there is a massive "breathe in"

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That's an excellent point Sean. Maybe I was wrong with the 'release' observation. Either way a clear pathway will solve my problem. Thanks again Trevor

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You guys with the vented caps are lucky.  On State-side cars, at least the later ones, the cap was sealed, and there was considerable plumbing and a charcoal canister to deal with.

Ed

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On 7/18/2019 at 2:12 PM, HSM said:

I had a problem many years ago with my TR6 ,after a short run if I accelerated hard the engine would

start to misfire, when I checked my new cap seal ( no vent hole ), problem was a vacuum build up

in the tank reducing fuel flow to pump. Punched hole in seal as original one, problem solved. It appears

many new seals are produced with no vent hole ?

                                                        Harvey S. Maitland

I brought this problem up with Moss over 10 years ago, i.e. no vent hole! Moss poo pooed it as not necessary? I vented mine by heating up a panel  pin nail tip on our gas hob and just pushing it through. Makes a very clean hole!  Roger: I think that you need to have Moss's stock checked on one of your visits? It could be another lay shaft problem that has gone on,for years!

Bruce. 

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A low tank pressure, as experienced by Harvey above, is primarily a reliability issue.

A high tank pressure is also a safety concern! This should be taken serious by our suppliers.

Waldi

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Here`s photos of my filler cap, with a small hole that seems to vent to the to the outside via the "opening lever?

Don`t know what make/model it is, nor whether it`s an original Triumph TR6 cap?

 

Colin.

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IMG_4039.JPG

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Yes John, TR6 LHD with carbs etc, imported from California circa 1998. Was then converted to Injection, RHD with lots of other modifications by the late Alan Wadley of TR Workshop in Cirencester.

So I shouldn`t suffer from fuel starvation - correct?

Colin.

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I’ve had this response from Moss Technical Support, 

‘I have checked the rubber seal 718738 and if you stretch it apart a bit there is a small vent hole in the middle. You could try putting a tiny hole in the centre of your rubber seal and see if that cures it. ‘

I have made an incision in mine, but given it’s perished on the outer edges, I’ve ordered a new one. 

Thanks for all the responses. Best regards Trevor 


 

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I received the rubber seal from Moss today; only ordered it yesterday standard service! It’s as described but a surprisingly small hole visible  only on serious interrogation! I’ll install as given and monitor accordingly. Regards Trevor 

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  • 1 month later...

Hi, just thought I'd give an update on the fuel tank filler seal. The new one from Moss was fitted, I had said the hole was hardly perceivable and on a run out a few weeks ago and on entering a garage, there was over pressure in the tank!! still quite significant.

Just last week I took the advice of previous contributors and took a very hot skewer (say 2mm dia) and enlarged the hole; a neat job I must say.

Next few opportunities to review the tank, no over pressure, so job done!

Thanks for all your help on this one.

Regards Trevor

 

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