hi

 

I´m still trying to sort out the problem I’m having with the petrol tank.

 

I know all about the petrol tank causing a slight vacuum through pumping the fuel and the air not being replaced entirely until the vacuum pressure opens the valve on the petrol cap to allow air in but mine is doing the opposite, when I open the fuel cap air isn´t being sucked in it’s actually being blown out, so much so that if I jam the small valve on the petrol cap open it starts to spit petrol out. How can the petrol tank be under pressure like that? :confused: I have to open the petrol cap every 20-30 km to release the pressure otherwise the pump starts to fail.

 

Any ideas would be greatly appreciated.

 

Maz

could it be a problem with the carbon canister not absorbing evaporative gases which are produced in the petrol tank and thusbuilding up until released by opening the patrol cap ?

anybody ??

 

could it have anything to do with the Walbro pump not having that funny disc shaped membrane thing on it that the OEM pump has ? ( still haven´t figured out what it´s for i.e what it´s function is ) could it be the fuel flowing back to the tank from the fuel pressure regulators ? I´m at a total loss here as I can´t even get my head around the basic physics of it on how you can pump out liquid from a container and get outward pressure as opposed to a vacuum.

 

Maz

No don't try that you could cause a lot of damage:headvswal

I think most people are under the impression that the petrol tank is under a vacuum, but I think it is under pressure, if I put my hand by the cap as I open it I feel air coming out of the tank. I don't think it would work well with a vacuum as this would counteract work down by the fuel pump!

I thought fuel-injected engines used a pressurised system anyway :confused:

(Mind you, I know nowt about engines LOL).

Topless I thought the pressurized system as such doesn´t start until after the petrol has passed through the pump on it´s way to the fuel pressure regulator ...the vaccum in the tank is caused by the petrol being pumped out and the empty space requiring air to fill it. The air is sucked in through the two way valve system on the petrol cap....how is pressure then obtained within the tank ..enough to force the petrol up the filler neck ?

I presume it is on a full tank when it is spitting petrol out, if the engine is runnning it is returning fuel and vapour to the tank and this pressure is being released through the vent you have made in the petrol cap.

mmm, does she suck or blow lol

 

will have to see if i can tell next time I take the cap off

try hooking up an air compressor to the tank build up loads of pressure and see if it unblocks something.

Lol :slap::rofl::rofl:

Lol :slap::rofl::rofl:

 

 

:D:D:D

have a fag while your at it too :D

:D:D:D

have a fag while your at it too :D

 

 

and use your lighter to shine some light in to see were its blocked:x::slap:

OK, I just read the manual ! The diagram on EF & EC-189 (in my manual) shows the fuel filler cap having a vacuum relief valve - therefore I think we can assume it's supposed to be a vacuum in the tank. The separate vapour vent line, with a one way valve to prevent fuel escaping goes to the carbon canister. The canister is open to atmosphere (it looks like) and the vapour is only drawn through to the intake manifold when the engine is running. In theory there should be no pressure in the tank, the only way I can see there being some is if pressure (under boost) from the intake manifold is directed to the tank - has your carbon canister been messed with/bypassed/connected incorrectly ?

 

Andy, I think there may be another way of looking at it? Vacuum in a petrol is potentially detirmental in terms that it can collapse the tank and impede fuel flow to the engine, so that is maybe why there is a vacuum relief valve in the cap?

 

Also it is a vacuum relief valve, means it is getting rid of vacuum or a certain level of vacuum, if it was a pressure relief valve then we could presume the tank being pressurised was a bad thing?

Andy

 

the carbon canister and EVAP system is stock, nothing changed or tampered with. I looked at the same diagram and concluded there are only 2 ways pressure could possibly enter the tank and that is:

 

A: via the vapour vent line as the fuel check valve runs freely in direction of the petrol tank but i can´t see how it could pressurize the tank..i.e force air into it.

 

and

 

B: through the fuel return pipe

 

these are the only fwo in going pipes, I changed the OEM pump for a Walbro when it died...wondering if that could be causing it.

topless

 

it does the same . i.e spit the petrol out even on half a tank mate

topless

 

it does the same . i.e spit the petrol out even on half a tank mate

 

Yes but how much, just a few drips could be expected?

 

Did you plumb in your warlbro in exactly the same way as the original pump?

petrol cap off = a 12 inch wide cascade of petrol down the rear wing after about 10 km

 

valve open on cap = little puddle of petrol beneath the cap when you open the little door thing and the cap is soaked.

 

yep..pplumped the walbro in exactly the same way and place the OEM one was mounted.

petrol cap off = a 12 inch wide cascade of petrol down the rear wing after about 10 km

 

valve open on cap = little puddle of petrol beneath the cap when you open the little door thing and the cap is soaked.

 

yep..pplumped the walbro in exactly the same way and place the OEM one was mounted.

 

That is what I would expect if everything was normal.

but after about 100 -150 km the pressure is so much that the pump starts to fail. open the cap, let the pressure out and she runs fine again...that can´t be normal. it didn´t do that for the past 4 years..so something must be wrong.

So why did you change to a warlbro, as they can go faulty?

standard are good for 600bhp.

I see from your previous posts that you had issues with the orginal fuel pump, do you think it may have been an issue with wiring/fuel system and putting in a warlbro has just highlighted the problem again?

because my OEM pump failed and I needed a replacement quick as I was driving to France the next weekend. quickest option was the Walbro.The pump has been in about 6 months though and this problem is only since last weekend. The walbro pump never felt as good as the OEM one though ..plus i kept wondering what the purpose of the round metal diaphram on the OEM pump is:

 

http://www.mkiv.com/techarticles/fuel_pumps_test_2/nismopump3.jpg

 

the army green coloured round thing pointing to 1 oclock

i´m not sure topless...the OEM fuel pump worked fine for 4 years and then it died....baring in mind the car has done over 250 000 km i think it was time the OEM pump died. had no issues with the petrol tank in that time , wiring and fuel system worked fine and did for about 6 months on the Walbro mate.

Excess fuel is always returned to the tank, therefore if air can't vent though the filler cap and if the fuel check valve is blocked then pressure could build up if air can still be drawn into the tank. Otherwise the volume of fuel returned will always be less than that removed.

 

If the fuel check valve isn't blocked, then surely you are likely to push fuel into the carbon canister, not just vapour - if it blows out the filler cap, it must blow the other way too.