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Pi tank pressuring


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I recently purchased a 2.5 pi that has had the Lucas pump replaced with a Bosch one, which I am assured is much more reliable. The problem now is the pressure that builds up in the fuel tank. Does the pump need recalibration? Am I missing a vent pipe? I’m stumped right now, any advice much appreciated.

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53 minutes ago, Triker said:

The cap that’s on looks a bit like a rad cap except it says fuel. Should it be a vented cap?

It definitely should be, can you post a picture?

Stuart.

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2 hours ago, Triker said:

I recently purchased a 2.5 pi that has had the Lucas pump replaced with a Bosch one, which I am assured is much more reliable. The problem now is the pressure that builds up in the fuel tank. Does the pump need recalibration? Am I missing a vent pipe? I’m stumped right now, any advice much appreciated.

When you say you have a 2.5 pi, do you mean a saloon or a PI TR5/6 as I'm struggling to imagine a radiator type petrol cap  sitting on the rear deck of a PI TR?

cheers Rich

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And I'm strugging to imagine how a pump that is remeving fuel from the tank can be increasing the pressure insde it!   The evnet is to allow air to enter to replace te volume fo fle pumped out - else the pressure falls, the pump has to work against a negative pressure and fails.  The make of pump is irrelevant.

More dertails, pleae Triker, and pics!

Welcome, by the way!

JOhn

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Hi

I had the vent in my TR6 petrol cap block this summer, it caused a vacuum to develop which was enough to cause fuel starvation and the engine to misfire, especially when the tank was full. When I opened the cap you heard a noise when the air pressures balanced, and it could easily be mistaken for pressure rather than vacuum.

Dave

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My TR5 created a vacuum in the tank enough to stall the engine, found an unvented filler cap the cause. 

Paul

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The car in question is a 1971 triumph mk 2 2.5pi saloon. Good point John, is it pressure or vacuum? At the moment the car is at my friends garage having new drive plate and rebuilt starter fitted. As soon as it returns I will check. In the past I have had a few tc’s but this is my first pi. I will include photos when I get it back. Many thanks for all the useful points raised.

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Try checking the connections UPSTREAM of the new pump. Air may be being drawn in, some of which will end up in the tank via the return line and could cause pressure build up if the vent is either compromised or is acting "one-way", the wrong way.

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The car in question is a 1971 triumph mk 2 2.5pi saloon. Good point John, is it pressure or vacuum? At the moment the car is at my friends garage having new drive plate and rebuilt starter fitted. As soon as it returns I will check. In the past I have had a few tc’s but this is my first pi. I will include photos when I get it back. Many thanks for all the useful points raised.

 
.

 

Edited by Triker
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My PI saloon had a labyrinth seal under the filler cap. It wasn't very effective at keeping fuel in when the tank was full. It used to centrifuge out of the nearly horizonal filler tube when cornering hard, but I never had any problems with the tank pressurising.

A build up of pressure in hot conditions is not unusual due to the volatility of fuel. Just leave a square plastic 5 litre fuel can in the sun for an hour or so and see what shape it becomes!

Neil

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6 hours ago, spyder dryver said:

Try checking the connections UPSTREAM of the new pump. Air may be being drawn in, some of which will end up in the tank via the return line and could cause pressure build up if the vent is either compromised or is acting "one-way", the wrong way.

Upstream of the pump is at a pressure of something like 100psi, if the line had a break of some kind fuel would come out,  not air go in.

 

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19 hours ago, OldBob said:

Upstream of the pump is at a pressure of something like 100psi, if the line had a break of some kind fuel would come out,  not air go in.

 

You seem to be confusing Upstream/downstream Bob. Upstream is before the pump, downstream after.  Many moons ago we experienced this phenomenon on our TR7V8 racecar. It manifested itself as a high RPM misfire which took ages to diagnose and cost Mick (Richards)  his first victory. Snetterton 1990.

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2 hours ago, spyder dryver said:

You seem to be confusing Upstream/downstream Bob. Upstream is before the pump, downstream after.  Many moons ago we experienced this phenomenon on our TR7V8 racecar. It manifested itself as a high RPM misfire which took ages to diagnose and cost Mick (Richards)  his first victory. Snetterton 1990.

Remembered well...if not a little sadly, the misfire caused an off in practice by reducing the power at the back wheels needed to balance the understeer * dialled in on handling to allow the excess V8 power to be applied early and balance the car neutrally. It also entirely removed one half of the exhaust system when the car ran wide over the Russel kerb causing much blacksmithing (thanks Geoff) in the period between practice and race.

Also remaking of every fuel pipe connection and smothering them with silicone sealer (Geoff again, took him a week to unstick his fingers !) between practice and the race, seemed to have cured the problem, allowing a 19th grid position to be converted into 2nd place at the end of the first lap (that was FUN !). 

However all was to nought, as the rocking engine loosened whichever connection was causing the problem, and around lap 8 the misfire returned with a vengeance causing me to lose 1st place and finish 2nd.

Much gnashing of teeth and renewed efforts, however it took another 2 races before fuel line problems were solved and a winning cycle was established. 

Mick Richards

* refer back to my various articles about the evil caused by the sole fitment of front anti roll bars. Also access to my article of "Bump steer and the art of Zen" which feeds into "How to make your car handle" now counting at 144 forum members who have the information to make their car impervious to bump steer. Yours if you want it just PM your e mail address to me.  

Mick Richards  

Edited by Motorsport Mickey
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I Just picked the car up, mechanic tells me it is definitely pressure not vacuum! The car does drive really well. Just tried uploading pics but it won’t do it, and I’m not out of memory. For now I’m going to drill a hole in a plastic fuel cap.

i guess I’ll just keep running it and see if anything else untoward happens. Is there any one in the leics/notts area that can either recommend someone who can look at my system, or is there a local club member who would check it over?

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I've no personal experience but TR Enterprises Mansfield I suspect closest and seem to have a good reputation.

Andy

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I had a mobile guy who said he was used to working on these systems and pronounced it fit and healthy. Said I should either use a vented filler cap or fit a tee piece in the vent pipe and run another vent outside the car. He said there was no swelling of the fuel pipes so pressure was about right. Just got to find a vented cap that fits, an6 suggestions gratefully received.

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Which is a bit like opening the chicken house door after the fox has been, to let all the feathers out.

There is absolutely no reason why a properly functioning Pi system should pressurise the tank.    Cause a vacuum, yes, unless you vent it, preferably with a one-way valve, so that fuel cannot get out, only air in.

If your tank is pressurising, Trike, then is the pump entraining air in some way?    But that would go to the M/u, and stop the engine briefly until the bubble has passed through and more fuel arrived.    I know!  My Pi, TR6 powered Vitesse Estate used to do that, just as I exited a corner.        If it really is pressurising, then a small balloon, attached to such a vent should expand.    I'd like to see that!

John

PS A 'mobile guy' (mechanic?) with experience on Pi?       Wow, how old is he?

Edited by john.r.davies
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I had a mobile guy who said he was used to working on these systems and pronounced it fit and healthy. Said I should either use a vented filler cap or fit a tee piece in the vent pipe and run another vent outside the car. He said there was no swelling of the fuel pipes so pressure was about right. Just got to find a vented cap that fits, an6 suggestions gratefully received.

 

 

have drilled small hole in petrol cap, seems to be working well, no pressure build up. Would still like to find a proper vented cap that fits. Alternative is to splice in to existing vent pipe and fit non return valve.

Mobile guy came with impeccable references, he wouldn’t have put one finger on my car without.

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Triker,

Did you ever THINK about what your mobile 'mechanic', amateur spanner twiddler said ?   When you are dealing with a Lucas Pi system?    Do you know what the pressure is in that system?  "no swelling of the fuel pipes"  when down stream of the fuel pump the hoses should see 100-110psi?  Thats six times normal atmospheric pressure!   And anyway,  "swelling of the fuel pipes" in any system should mean instant and urgent condemnation of the hoses and replacement!     You have an idiot advising you!

You have drilled a small hole in petrol cap?     I did that, for my highly modified Race Vitesse, that had a TR6 engine and Lucas Pi.  This was the result:

806397665_closeupfuelcapleak.jpg.8bce61f895792f90a22bc02fea70cf42.jpg

The jet of fuel out of the cap, on a left hand bend, is clearly seen!

John

Edited by john.r.davies
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5 minutes ago, Triker said:

I had a mobile guy who said he was used to working on these systems and pronounced it fit and healthy. Said I should either use a vented filler cap or fit a tee piece in the vent pipe and run another vent outside the car. He said there was no swelling of the fuel pipes so pressure was about right. Just got to find a vented cap that fits, an6 suggestions gratefully received.

 

 

have drilled small hole in petrol cap, seems to be working well, no pressure build up. Would still like to find a proper vented cap that fits. Alternative is to splice in to existing vent pipe and fit non return valve.

Mobile guy came with impeccable references, he wouldn’t have put one finger on my car without.

You need an original PI saloon filler cap then as they would be properly vented I would have thought.

Stuart.

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