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        General >> Problems >> Pressure build up in fuel tank
        
(Message started by: Gilly on Mar 18^th, 2010, 12:02pm)

Title: Pressure build up in fuel tank
Post by Gilly on Mar 18^th, 2010, 12:02pm

Hi
My 98 Scorpio 24v has got a problem with pressure building up in the fuel tank.

After driving for just a few miles the engine dies but starts up again first turn of the key. If i open the filler cap slightly, pressure is released from the tank for up to 20 seconds or so and if I undo the cap completely it gets pushed gently into my hand. There is also a strong smell of petrol as the pressure is released.

If I leave the cap loosely fitted the car drives fine but there is a smell of petrol as i'm driving.

I have recently had a new tank fitted at my local garage because the old one had corroded around the filler neck but I don't know if this problem is new or if the pressure was just escaping from the old tank.

There is a pipe on top of the tank that i think is a breather pipe and it goes to a black plastic box just in front of the tank ( I don't know what the box is for )

My first thoughts were that the breather must be blocked but thinking about it, I would ecpect a vacuum to be created in the tank as fuel is drawn off, not a build up of pressure altough if the breather was not blocked then pressure would escape unless there is a one way valve in the black box that lets air in but not fumes out.

Or is the black box nothing to do with venting the tank. Is there a breather on the filler pipe somewhere.

As you can probably tell, I'm flumoxed and any help would be much appreciated.

Thanks in advance

Ean

Title: Re: Pressure build up in fuel tank
Post by Highlander on Mar 18^th, 2010, 12:13pm

Hi Ean and welcome to the site :)

The pressure in the tank is supposed to be there, dont worry about it :)

"An Evaporative Emissions System draws petrol fumes into a charcoal storage device that is regularly purged by submitting it to engine vacuum, and this system is the cause of the hiss when the petrol cap is opened".

http://www.fordscorpio.co.uk/fuelsystem.htm

Title: Re: Pressure build up in fuel tank
Post by Gilly on Mar 18^th, 2010, 12:32pm

Thanks for the quick reply Highlander and for the welcome to the site.

I have been a member for years but fortunately I haven't had to trouble anybody very often.

I have owned the car for 5 years or so and before that i had a 24v Granada and before that a 2.4 Granada Ghia. I also own another 24v Granada that has been bored out to 3.7L and produces 300 bhp but that's another story.

I've never noticed this build up of pressure on any of my other cars and this one has only just started doing it.

I wouldn't be worried if the car didn't cut out after a few miles with the cap tightly screwed on and run fine with the cap loosely fitted.

I know that fuel expands when the sun is shining but I doubt that its got anything to do with that because it also happens at night when it's cold.

The emissions system you mentioned uses engine "vacuum" and i appreciate that you would get a hiss as air is drawn in to the tank but on my car air is being pushed out.

I haven't a clue where the pressure could come from.

Any help would be much appreciated

Kind regards

Ean

Title: Re: Pressure build up in fuel tank
Post by Highlander on Mar 18^th, 2010, 12:54pm

Ahhh my apologies, I just skimmed over your first post, i didnt spot the engine was cutting out...

I dont suppose they fitted the vaccum pipes on back to front when replacing the tank?

Title: Re: Pressure build up in fuel tank
Post by Gilly on Mar 18^th, 2010, 1:20pm

Thanks

As for as I know there's only one breather pipe on top of the tank. There are two fuel pipes (supply and return). I dont know what would happen if the fuel pipes were fitted the wrong way round but they are colour coded so it's unlikely that they are. There is a pressure regulator on the fuel rail and i think if the fuel pipes were the wrong way round i would be getting all sorts of running problems.

I've just been out and stretched a rubber glove over the filler tube and the glove inflated quite a lot in a couple of minutes with the engine ticking over. I think that it would have blown off if i left it any longer

kind regards

Ean

Title: Re: Pressure build up in fuel tank
Post by Highlander on Mar 18^th, 2010, 3:34pm

Didnt mean the fuel pipes round the wrong way, it wouldnt start in that case :)

I meant the pipes in respect of the vacuum system

"An Evaporative Emissions System draws petrol fumes into a charcoal storage device that is regularly purged by submitting it to engine vacuum, and this system is the cause of the hiss when the petrol cap is opened".

The "engine vacuum" part may be work investigating?

Can we assume the pressure reaches a certain level and cuts the engine out then?

I wonder if its possibly some sort of fault in the fuel pressure regulator?

something to do with the excess fuel being returned to the tank?

Not really had any problems in that area before so cant really think what would be doing it.

Title: Re: Pressure build up in fuel tank
Post by gozz on Mar 18^th, 2010, 3:41pm

Gilly.
As Highlander says the emission control system should periodically draw the fumes from your black box,it is a small electromagnetic valve controlled by the PCM,which admits the fumes to the intake to be burned in the engine.I would check the valve and the electrics to it first of all,it is located,I think, down behind the offside headlamp.
GOZZ.

Title: Re: Pressure build up in fuel tank
Post by Gilly on Mar 18^th, 2010, 9:49pm

Thanks for your input

It sounds like another day of dismantling stuff to find out how it works is on the cards

Whats puzzling me though is if the emission control system uses a vacuum, where is the positive pressure coming from? it seems logical that the vacuum would be taken from the inlet manifold but theres no way there can be positive pressure in there ( or can there ? ). Assuming that, the obvious way to control it would be via a solenoid valve and if that was faulty it would either be stuck open or closed meaning either a permanent vacuum or just nothing at all.

My understanding is that it is only the pump that supplies pressure to the fuel system, the pressure regulator just lets a certain amount of pressure through to the rail.

I've just had a thought. I wonder if the fuel is being heated as it passes through the pipes maybe the fuel pipes are touching something that they shouldn't. I'd better check it out tomorrow before the whole thing explodes !

thanks again for your thoughts

I'll keep you posted

Kind regards

Ean

Title: Re: Pressure build up in fuel tank
Post by Highlander on Mar 18^th, 2010, 10:52pm

Clogged fuel filter??

I wonder if the pump is working against a dirty filter and creating pressure?

Title: Re: Pressure build up in fuel tank
Post by Highlander on Mar 18^th, 2010, 11:11pm

on 03/18/10 at 21:49:50, Gilly wrote:

My understanding is that it is only the pump that supplies pressure to the fuel system, the pressure regulator just lets a certain amount of pressure through to the rail.

But theres a return pipe from the fuel rail, excess fuel is returned to the tank.

If there is some problem with the regulator or electrics,

eg the fuel pump is running constantly instead of cutting out when the correct fuel pressure is reached, then that would create positive pressure?

Just thinking out loud ;D

Title: Re: Pressure build up in fuel tank
Post by Jonnycab on Mar 19^th, 2010, 1:10am

As was said earlier, a bit of vacuum release (hiss) when opening the filler cap is normal....it shows that the EVAP system is working fine..... http://www.fordscorpio.co.uk/evapmonitor.htm

As Highlander suggested.....If it were the pump causing the problem by running constantly instead of cutting out when the correct pressure is reached, then personally, my first port of call (just to rule it out) would be the fuel pump relay, especially if it's stalling :)

Sorry if I'm going over old ground, but have you checked to see that the pipe that runs from the evap purge canister (black charcoal filled box behind tank) to the evap canister purge valve (drivers side lower engine bay) hasn't been pinched when the new tank was fitted.....or is it possible that the atmospheric pipe that comes out of the purge canister & goes to air has been pinched ? :-/.... :)

P.S....Evap canister purge valve position is shown at the bottom of this page.... http://www.fordscorpio.co.uk/24vcomponents.htm :)

Title: Re: Pressure build up in fuel tank
Post by Gilly on Mar 20^th, 2010, 8:38am

I found the problem with the evap system

The breather pipe from the top of the tank goes to the evap canister via a roll over shut off valve that is fastened to the filler pipe. The bracket was broken and the valve was cable tied to the filler pipe. I removed the valve and connected the pipes together and it has cured the problem.

I'm off to buy a new valve today because sod's law dictates that the car is bound to roll over if I don't.

I'm still curious about where the pressure is coming from though. The fuel system is a closed loop and the engine and evap system all use vacuum. The only thing I can think of is the fuel gets warmer and expands as it passes through the fuel rail which is bolted to the engine.
And why would the pressure cause the engine to cut out ????

Thanks again for your assistance

Gilly

Title: Re: Pressure build up in fuel tank
Post by Nunnspoet on Mar 20^th, 2010, 9:44am

:). Good news and well traced Gilly. I have no idea as to where or how the pressure is built up in the fuel tank, maybe the pressure is somehow related to the fuel return to the Tank line ???.

Mike