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Fuel tank vacuum

JBAV6Tony

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Hi all,

Since I've had JBA (for newcomers it runs a V6 3800cc Buick engine EFI) everytime I've unlocked the fuel cap a rush of air enters the tank. This vacuum I'm suspicious has caused me a few issues in summertime.

In summer if having filled the tank and driven say 300km, the car runs well until a long hill or heavy slow traffic comes along. The engine starts to miss, wont rev well and eventually dies. Then its hard to restart.

As the engine has had coil, plugs, leads etc all replaced as well as all sensors, I'm suspicious the tank isnt allowing air to enter via the fuel cap. I've tested the Ford XE cap, i.e. blew air through the keyhole and sucked through it and some air gets through both ways but it might not when hot?

So, I've drill a few small holes in the rubber gasket to help this senario.

I might point out that in a standard Holden Commodore, where the engine came from, the tank normally has a venting pipe from the tank to a carbon canister for anti pollution reasons and is vented to the atmosphere or the engine, I have no such canister.

So my theory is that in hot weather the tank has a vacuum to the point whereby the exterior (12 months old)electric pump can no longer pump fuel to the engine , the engine gets less and less fuel and begins to starve itself.
What are your thoughts? Is it just a case of ensuring the tank has the ability to replace the used fuel with air and all will be good?
 

MikeC

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The tank needs to be vented or a vacuum will form as the fuel is pumped out. On a hot day the vapour pressure of the petrol l rises and if any part of the pump suction line is below the petrol vapour pressure the fuel will flash into vapour stopping the pumping. As you say, not venting the tank pulls the whole pump suction line down closer to or below to the fuel vapour pressure.

No problem with a small vent hole to the tank- that was standard practice once and my 1969 TR6 has had this type of vent for 50 years..
 

JBAV6Tony

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The tank needs to be vented or a vacuum will form as the fuel is pumped out. On a hot day the vapour pressure of the petrol l rises and if any part of the pump suction line is below the petrol vapour pressure the fuel will flash into vapour stopping the pumping. As you say, not venting the tank pulls the whole pump suction line down closer to or below to the fuel vapour pressure.

No problem with a small vent hole to the tank- that was standard practice once and my 1969 TR6 has had this type of vent for 50 years..
Thanks Mike, I needed confirmation.
Love the TRS. TR6 my fav. Cheers
 

losh1971

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Mine has vacuum yet it doesn't affect anything.
 

shane_3800

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As I understand registration that engine no matter the car it's fitted to should have a charcoal canister.
Is there a vented cap avaliable? I just cut my VS tank open and the tank vents have floats to stop fuel going through the line to the charcoal canister so I don't think you can just fit one.
 

HarryHoudini

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Mine has vacuum yet it doesn't affect anything.
Yes,mine as well,always had it,not sure if it was part of the sealed system or not but as it never affected anything i just didn't worry about it.
 
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