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DarkMatter

Original Poster:

1,167 posts

117 months

[news] 
Thursday 7th July 2011 quote quote all
Rather than hijack another thread titled 'Odd running and fuelling issue' I thought it better to start this one. In the other thread 'Loubaruch' replied saying....

Loubaruch said:
I stand corrected, but I thought that surplus fuel is pumped back to the tank from the plenum supply so there could even be positive pressure in the tank.

The fuel pump is taking in fuel at (Low) tank pressure, pressurising it for the injectors and then the surplus returns to the tank at arguably higher pressure than it left it.
I'm inclined to agree with you. I used to think that the banging of my fuel tank was due to negative pressure as the result of a faulty carbon canister not allowing sufficient air IN to the tank.

Having removed the carbon canister, blocked the tube to the inlet manifold, and allowed the tank to vent to the atmosphere I find that the tank still bangs and am now convinced that this is due to positive pressure due to the reasons you give above. I now also get the smell of fuel from that vent pipe when restarting a hot engine (the fuel lines are new and do not leak), previously the fuel smell will have been captured by the carbon canister.


I'm still a bit surprised about the positive pressure, it's as though the vent pipe is of insufficient bore or there is a restriction somewhere in the vent pipe between the fuel tank and the end of the pipe (where it used to connect to the carbon canister).

Other than the vented fuel cap suggestions what do others who have looked at the fuel tank know about the venting system?

FlipFlopGriff

5,650 posts

133 months

[news] 
Thursday 7th July 2011 quote quote all
I recall reading some timne ago that cutting a section of the rubber out from the filler cap solves the problem. Not getting the bang on mine but the whoosh. I think (hope) my problem is solved so I'll see if i'm still getting the whoosh.
FFG

haircutmike

10,407 posts

90 months

[news] 
Thursday 7th July 2011 quote quote all
After a run, if I release my fuel cap, there is always a big rush IN of air.

Something that is still on the "to do" list!

spend

12,468 posts

137 months

[news] 
Thursday 7th July 2011 quote quote all
DarkMatter said:
...it's as though the vent pipe is of insufficient bore or there is a restriction somewhere in the vent pipe between the fuel tank and the end of the pipe (where it used to connect to the carbon canister)
Typically the rubber return hose has been clamped by overzealous tightening of cable ties against the chassis. Replace with good quality larger bore hose to ensure proper venting & that the rollover valves etc have not gummed/rusted up.

The carbon cannister works fine IMHO, most of the problems are just poor / insufficient (read non-existent) maintenance. Even LR spec something like replacement every 5 years, and they use thin wall plastic pipe (ie no crushing or degradation into a 'valve' like rubber hose can go).

As a note the return is never going to be higher pressure than the feed at the tank (only in the pipes) as the pump is never going to return more than it sucks IYSWIM.

DarkMatter

Original Poster:

1,167 posts

117 months

[news] 
Thursday 7th July 2011 quote quote all
spend said:
Typically the rubber return hose has been clamped by overzealous tightening of cable ties against the chassis. Replace with good quality larger bore hose to ensure proper venting & that the rollover valves etc have not gummed/rusted up.

The carbon cannister works fine IMHO, most of the problems are just poor / insufficient (read non-existent) maintenance. Even LR spec something like replacement every 5 years, and they use thin wall plastic pipe (ie no crushing or degradation into a 'valve' like rubber hose can go).

As a note the return is never going to be higher pressure than the feed at the tank (only in the pipes) as the pump is never going to return more than it sucks IYSWIM.
Thanks spend.

Where on the fuel tank would I find the roll-over valve? If I could access that and disconnect the pipe I could rule in/out a pipe blockage/constriction.

Does maintenance of the carbon canister mean replacement, in which case I may need a part number (if it's not marked on the old one)? Any idea where to source a reasonably priced one?
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Pink_Floyd

784 posts

107 months

[news] 
Thursday 7th July 2011 quote quote all
DarkMatter said:
Where on the fuel tank would I find the roll-over valve? If I could access that and disconnect the pipe I could rule in/out a pipe blockage/constriction.
On mine it was just below the top of the tank and behind it by the filler neck. Mine was fitted upsidedown confused. I could not blow or suck anything from the tube connected to the canister. Now I can.smile

spend

12,468 posts

137 months

[news] 
Thursday 7th July 2011 quote quote all
Roll over valve is tucked behind the filler neck hose.

It's easier to check the vent pipe from the front, even though its a pain to get at in the wheelarch.

The cannisters are LR / Rover std items, early ones have a separate purge valve & the later ones are integral (as used on Rover 200's/400's...)

DarkMatter

Original Poster:

1,167 posts

117 months

[news] 
Thursday 7th July 2011 quote quote all
spend said:
The cannisters are LR / Rover std items, early ones have a separate purge valve & the later ones are integral (as used on Rover 200's/400's...)
Thanks. Is there any way of examining a canister to determine whether it is faulty or needs maintenance (what does maintenance entail)?

DarkMatter

Original Poster:

1,167 posts

117 months

[news] 
Thursday 7th July 2011 quote quote all
Pink_Floyd said:
On mine it was just below the top of the tank and behind it by the filler neck. Mine was fitted upsidedown confused. I could not blow or suck anything from the tube connected to the canister. Now I can.smile
Interesting. Should I be able to see/remove the valve by simply pulling back the carpet?

spend

12,468 posts

137 months

[news] 
Thursday 7th July 2011 quote quote all
Check pipes are all clear, purge valve is working....

CC should not smell too much and shouldn't be full of liquid...

I think after a period the carbon loses its ability to capture & release the petrol vapour, as well as getting clogged on the outside (it also filters incoming air as the tank empties). I actually prefer to route the vent from the cannister up above the headlight to avoid sucking in road muck, many are just hoovering the floor of the undertray I think (but thats a minor issue really).

Fairly simple devices really, similar systems used on all cars for last 15 years or so... Vent pipe from tank to carbon cannister, outlet to free air, purge valve to plenum. Basically its just a carbon 'mesh' to absorb crud in / fumes out as tank breathes, then under ECU control the purge valve opens occasionally to suck the encapsulated vapour into the engine. An excessively wet cannister suggests the purging has failed (ECU / valve / pipes) & high tank pressures either blocked pipes or cannister? OTOMH.

The valves are just a positive supply from the efi relays (main & fuel have been used at different times) and grounded by the ecu to activate under engine suction conditions theoretically frequent but brief periods. Nearly all cars have them & it should be very simple to adapt the plumbing & wiring to just about any other part from any car TBH.

DarkMatter

Original Poster:

1,167 posts

117 months

[news] 
Thursday 7th July 2011 quote quote all
spend said:
Check pipes are all clear, purge valve is working....

CC should not smell too much and shouldn't be full of liquid...

I think after a period the carbon loses its ability to capture & release the petrol vapour, as well as getting clogged on the outside (it also filters incoming air as the tank empties). I actually prefer to route the vent from the cannister up above the headlight to avoid sucking in road muck, many are just hoovering the floor of the undertray I think (but thats a minor issue really).

Fairly simple devices really, similar systems used on all cars for last 15 years or so... Vent pipe from tank to carbon cannister, outlet to free air, purge valve to plenum. Basically its just a carbon 'mesh' to absorb crud in / fumes out as tank breathes, then under ECU control the purge valve opens occasionally to suck the encapsulated vapour into the engine. An excessively wet cannister suggests the purging has failed (ECU / valve / pipes) & high tank pressures either blocked pipes or cannister? OTOMH.

The valves are just a positive supply from the efi relays (main & fuel have been used at different times) and grounded by the ecu to activate under engine suction conditions theoretically frequent but brief periods. Nearly all cars have them & it should be very simple to adapt the plumbing & wiring to just about any other part from any car TBH.
Thanks for the details spend. I'll check the pipe from the fuel tank first, then will look at the carbon canister and purge valve operation.

p5000ko

73 posts

71 months

[news] 
Thursday 10th January 2013 quote quote all
DarkMatter said:
Thanks for the details spend. I'll check the pipe from the fuel tank first, then will look at the carbon canister and purge valve operation.
DM After replacing rear shockers, though they were getting stuck! I still had a bang coming from the back. I have spent weeks trying to find out the cause then i stumbled across this.. Will need to investigate further.
Did you solve the problem?

DarkMatter

Original Poster:

1,167 posts

117 months

[news] 
Thursday 10th January 2013 quote quote all
p5000ko said:
DM After replacing rear shockers, though they were getting stuck! I still had a bang coming from the back. I have spent weeks trying to find out the cause then i stumbled across this.. Will need to investigate further.
Did you solve the problem?
Not totally cured. It appears that the pipes to the roll-over valve were incorrectly fitted. I still have the carbon canister fitted, there is still the occasional banging particularly when the fuel tank is nearly empty but no smell of fuel.

Loubaruch

438 posts

84 months

[news] 
Friday 11th January 2013 quote quote all
Spend is of course correct in saying that there will be negative pressure in the tank. As the volume leaving is bound to be more than the surplus returning. The returning fuel will however be at a higher temperature so that may have a small effect.

Have you tried releasing the filler cap (with the tank nearly empty) and going for a drive to see if the bonking stops.

My son was saying that it is not unknown for a tank to completely implode when racing if the vent is blocked completely.
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