Positive pressure in GTC fuel tank

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Positive pressure in GTC fuel tank

Post by Fred Warwick » Sat Oct 13, 2018 9:03 am

Good morning All

I recently started up my GTC for the first time in over 2 years and ran it for about ten minutes, maximum (until the water pump started leaking!).

When I went back into the garage a few minutes later I was greeted by a puddle of petrol, it was coming from around the filter on the tank at quite a rate. Out of instinct I opened the petrol cap there was a whoosh (positive pressure) and the leaking stopped.

Obviously I need to look at the filter and connections but my question is how did the positive pressure build up? I've had negative in tanks in the past but not positive.

Thanks in advance

Fred



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Positive pressure in GTC fuel tank

Post by Oldconn » Sat Oct 13, 2018 9:22 am

+++This sounde like the same problem we have been discussing on the FB page. The cause is probably the vent-hole in the filler cap being blocked, and the fuel return pipe from the carburettor continually pumping as long as the engine runs.


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Positive pressure in GTC fuel tank

Post by Fred Warwick » Sat Oct 13, 2018 10:22 am

Thanks Oldconn, yes it is the same query, I am just trying to get the widest view possible. I am still intrigued about the differential of fuel sucked out/returned.



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Positive pressure in GTC fuel tank

Post by Roger Pennington » Sat Oct 13, 2018 12:02 pm

Surely, the fuel return can't possibly return more fuel than has already been removed (if it is, then patent it quickly, you could make your fortune!) - so it shouldn't cause any increase in the tank pressure? (i.e. with the engine running, there is always going to be a net reduction in tank contents and hence pressure)
More likely that if the vent is partly or fully blocked, then it may not respond to ambient temperature changes very quickly - it's been quite warm over the last few days?


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Positive pressure in GTC fuel tank

Post by peter freeman » Sat Oct 13, 2018 12:24 pm

The cap has a one way valve which allows air in but not out - try sucking and blowing the little hole in the cap for confirmation - My 6A and my GTC did the same "blow out" when I went for fuel - not noticed the same effect with the MB though but that has a different fuel system - Injection.



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Positive pressure in GTC fuel tank

Post by Fred Warwick » Sat Oct 13, 2018 12:41 pm

Roger Pennington wrote:
Sat Oct 13, 2018 12:02 pm
Surely, the fuel return can't possibly return more fuel than has already been removed (if it is, then patent it quickly, you could make your fortune!) - so it shouldn't cause any increase in the tank pressure? (i.e. with the engine running, there is always going to be a net reduction in tank contents and hence pressure)
More likely that if the vent is partly or fully blocked, then it may not respond to ambient temperature changes very quickly - it's been quite warm over the last few days?
My thoughts exactly Roger, although the car has sat there all through the scorching summer without a leak.



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Positive pressure in GTC fuel tank

Post by Fred Warwick » Sat Oct 13, 2018 12:43 pm

peter freeman wrote:
Sat Oct 13, 2018 12:24 pm
The cap has a one way valve which allows air in but not out - try sucking and blowing the little hole in the cap for confirmation - My 6A and my GTC did the same "blow out" when I went for fuel - not noticed the same effect with the MB though but that has a different fuel system - Injection.
Peter I also thought that the vent should let air in but not fumes out, if that is the case then there could still be a pressure build up, but for what reason??



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Positive pressure in GTC fuel tank

Post by Junwin » Sat Oct 13, 2018 12:59 pm

There is an article on the Middlebridge website that deals with this issue.

John



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Positive pressure in GTC fuel tank

Post by peter freeman » Sat Oct 13, 2018 1:16 pm

The fuel pump will create pressure - that's what it does for a living Fred - plus the returned fuel will have expanded due to the engine heat - try the suck blow test on your cap.



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Positive pressure in GTC fuel tank

Post by Fred Warwick » Sat Oct 13, 2018 2:43 pm

peter freeman wrote:
Sat Oct 13, 2018 1:16 pm
The fuel pump will create pressure - that's what it does for a living Fred - plus the returned fuel will have expanded due to the engine heat - try the suck blow test on your cap.
Hi Peter, I can see the logic but thought the burnt fuel would more than compensate for that.

The vent in the cap is indeed one way allowing air in but not relieving positive pressure. If the fuel continues to expand with time it would suggest that the pressure would continue to build up - surely not a good situation. Is the tank and whole system designed for this do you know?



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Positive pressure in GTC fuel tank

Post by peter freeman » Sat Oct 13, 2018 4:42 pm

Never heard of a tank being deformed or going ban with pressure - I assume it helps fuel going out of the tank to the carb.



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Positive pressure in GTC fuel tank

Post by demonbluedays » Sat Oct 13, 2018 7:02 pm

Think of it as a bottle of coke...with a hole in to let excess pressure out, if you block it up pressure builds until it's got somewhere to go or the pressure "stops" any more going in



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Positive pressure in GTC fuel tank

Post by scimjim » Sat Oct 13, 2018 8:23 pm

Who would design a tank that couldn’t let pressure out?

The cap has a one way valve to allow air in - it also has a spring loaded seal to allow excess pressure out. As roger says, you can’t pressurise a tank by taking fuel out, burning half of it and sending a bit back :D


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Positive pressure in GTC fuel tank

Post by demonbluedays » Sat Oct 13, 2018 8:33 pm

Best analogy I could use, my Corsa used to on and off do the same, every time i went to fuel or it was running like a bag of spanners in a tumble dryer, undidnthe fuel cap a huge hiss of escaping...something a stink of fuel and then fine. Never found out what it was as the clutch went boom not long after wrecked the gearbox (or it was almost gone on its own) so the car went to bucklands


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Positive pressure in GTC fuel tank

Post by scimjim » Sat Oct 13, 2018 9:07 pm

Guessing the vent was blocked then?


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