Fuel tank vent
Discussion
My car occasionally loses power and feels 'strangled'. When this happens there's often a smell of fuel, the exhaust pops once or twice on overrun, and invariably upon removing the fuel filler cap there's a fair bit of pressure in the tank to be released. The engine then starts to run cleanly again (after clearing it with a couple of blips of the throttle).
So my theory is that the tank is pressurising due to the vent pipe being blocked, causing the engine to run rich as the absolute fuel pressure rises (ie absolute fuel pressure is normal fuel pressure + tank pressure, causing the engine to run rich). Sound plausible?
There are no vents in the filler cap as far as I can see - it's a chrome cap with a sprung seal. Original as far as I know. There is a 7 or 8 mm pipe coming off the fuel filler neck, connecting to a length of fuel pipe which runs down the back of the tank in between the tank and the rear bulkhead. This enters a metal valve of some sort which is rivetted to the GRP bulkhead behind the passenger seat. Another length of fuel pipe exits the valve, dissappears down to the bottom of the tank and enters into the bottom inner corner of the rear n/s wheel arch with the fuel feed and return. This then must be the vent pipe? And the valve is there to stop fuel leaking in the event of the car turing over?
My plan is to disconnect the vent pipe at the fuel filler and connect it to an air line to blow it through and clear any blockage. Any idea whether this is a bad idea? Or whether I might damage the valve?
All experience/opinions greatly received, thanks.
So my theory is that the tank is pressurising due to the vent pipe being blocked, causing the engine to run rich as the absolute fuel pressure rises (ie absolute fuel pressure is normal fuel pressure + tank pressure, causing the engine to run rich). Sound plausible?
There are no vents in the filler cap as far as I can see - it's a chrome cap with a sprung seal. Original as far as I know. There is a 7 or 8 mm pipe coming off the fuel filler neck, connecting to a length of fuel pipe which runs down the back of the tank in between the tank and the rear bulkhead. This enters a metal valve of some sort which is rivetted to the GRP bulkhead behind the passenger seat. Another length of fuel pipe exits the valve, dissappears down to the bottom of the tank and enters into the bottom inner corner of the rear n/s wheel arch with the fuel feed and return. This then must be the vent pipe? And the valve is there to stop fuel leaking in the event of the car turing over?
My plan is to disconnect the vent pipe at the fuel filler and connect it to an air line to blow it through and clear any blockage. Any idea whether this is a bad idea? Or whether I might damage the valve?
All experience/opinions greatly received, thanks.
The tank pressure is actually a vacuum and if this is the problem, the engine starts to run lean as the pump cannot maintain the fuel pressure. Some pressure is quite normal BTW.
The chrome cap is a bit suspicious as IIRC they were all black. Yes it is worth making sure that all the venting etc is clear.
The chrome cap is a bit suspicious as IIRC they were all black. Yes it is worth making sure that all the venting etc is clear.
It is all connected to the carbon canister in the front wing, that is where you should check (with the cap off to avoid blowing petrol everywhere). See 'evaporative emissions control' in any LR (even Rover bubble) manual for description, 2 types used early separate purge valve, later clip on integral purge valve.
Dave
Dave
Thanks for replies.
The pressure is definitely positive - the cap almost jumps off into my hand! I'm guessing the recent hot weather may be largely reponsible? Combined with a lack of venting?
Will I risk damaging the roll-over valve by blowing it through with air?
Spend - no carbon cannister as it's a precat!
The pressure is definitely positive - the cap almost jumps off into my hand! I'm guessing the recent hot weather may be largely reponsible? Combined with a lack of venting?
Will I risk damaging the roll-over valve by blowing it through with air?
Spend - no carbon cannister as it's a precat!
Thanks for replies.
The pressure is definitely positive - the cap almost jumps off into my hand! I'm guessing the recent hot weather may be largely reponsible? Combined with a lack of venting?
Will I risk damaging the roll-over valve by blowing it through with air?
Spend - no carbon cannister as it's a precat!
The pressure is definitely positive - the cap almost jumps off into my hand! I'm guessing the recent hot weather may be largely reponsible? Combined with a lack of venting?
Will I risk damaging the roll-over valve by blowing it through with air?
Spend - no carbon cannister as it's a precat!
The fuel pressure is regulated relative to atmospheric pressure, not to the tank pressure. The tank would need to be under a relatively large pressure in order to stop the pressure regulator from doing its job, and I think the tank would blow up like a ballon way before it reached that point. The tank pressure probably isn't anything to do with it.
That's my theory, anyway. To test this theory you could drive with the filler cap cracked open to see if this cures the problem. Having positive pressure in the tank does suggest that you don't have a working breather, and it's well worth fixing this even if it doesn't seem to be causing the current problems.
That's my theory, anyway. To test this theory you could drive with the filler cap cracked open to see if this cures the problem. Having positive pressure in the tank does suggest that you don't have a working breather, and it's well worth fixing this even if it doesn't seem to be causing the current problems.
Umm, evap emissions have nothing to do with cat, the systems were fitted to LR before cats were introduced, timing was close but not exactly correlated IIRC. Dont know when TVR introduced them but guess the quick way to tell is if you have a pipe from the plenum (along with the brake servo pipe) heading to the front wing of the car you will probably have a carbon can/ purge valve.
Dave
Dave
spend said:
Umm, evap emissions have nothing to do with cat, the systems were fitted to LR before cats were introduced, timing was close but not exactly correlated IIRC. Dont know when TVR introduced them but guess the quick way to tell is if you have a pipe from the plenum (along with the brake servo pipe) heading to the front wing of the car you will probably have a carbon can/ purge valve.
Dave
Umm, we've been here before,my 97 factory non cat has no carbon can/ purge valve.
>> Edited by nztrev on Monday 18th July 15:11
The roll over valve is designed to let air in and prevent fuel from getting out. As the fuel tank empties, it lets air in to stop a vacuum building up and causing problems. If the tank is positively pressurised, the roll over valve will shut anyway. You can blow air through it but be gentle.
If the breather is blocked, it leads to a vacuum building up in the tank as air cannot get in to replace the fuel that is pumped out. This leads to fuel starvation. If the engine is overfuelling then that is usually due to some other problem. It could be that you have a sticky injector or something similar.
I think what you are seeing is a normal pressure due to hot petrol vapours.
If the breather is blocked, it leads to a vacuum building up in the tank as air cannot get in to replace the fuel that is pumped out. This leads to fuel starvation. If the engine is overfuelling then that is usually due to some other problem. It could be that you have a sticky injector or something similar.
I think what you are seeing is a normal pressure due to hot petrol vapours.
OK, so I blew air through the vent pipe from vent end to tank end (disconnected) and the guage on the air line read 20 psi constant (due mainly to the restriction of the roll-over valve presumably). Blowing the other way the guage reading steadily increased and I switched off at 40 psi (and climbing). There was some leakege through the vent pipe though, presumably due to an imperfect seal. The roll-over valve works as a one-way valve then, and not as a tilt valve?
Does it sound like everything is operating acceptably?
I'm still slightly concerned that the PH consensus seems to be that there should be a slight vacuum in the tank and yet there is (apparently) no provision on non-carbon-cannister cars to relieve positive pressure (assuming the vent valve only allows gas into the tank and not gas out) - the 'symptom' which I am getting (whether or not it's causing the running problem).
Does it sound like everything is operating acceptably?
I'm still slightly concerned that the PH consensus seems to be that there should be a slight vacuum in the tank and yet there is (apparently) no provision on non-carbon-cannister cars to relieve positive pressure (assuming the vent valve only allows gas into the tank and not gas out) - the 'symptom' which I am getting (whether or not it's causing the running problem).
I don't know what the internal design of the roll-over valve is, but I would have expected to find something that closed off when it was upside down, rather than effectively a one-way valve. The shape of the (presumed) valve in your photo is consistent with that, i.e. a vertical cylindrical shape with horizontal pipes in and out. Sure there must be *some* way to let vapour out of a tank when there is no carbon cannister. The pre-cat S series have a vent in the middle of the filler cap. For the cars with a carbon cannister this vent is blocked off and a seperate vent hose is routed out to the cannister and from there to fresh air. Is it possible the cap has been replaced on yours and a new one suitable for a carbon cannister design has been fitted by mistake?
GreenV8S said:
I don't know what the internal design of the roll-over valve is, but I would have expected to find something that closed off when it was upside down, rather than effectively a one-way valve.
Me too - although this would mean the tank was effectively open to air and could slowly fill the garage with fumes. I was hoping for a satisfying pop and the expulsion of a load of gunk into my eagerly awaiting jam-jar, but I chickened out at 40 psi and hence developed the one way valve theory.
GreenV8S said:
The shape of the (presumed) valve in your photo is consistent with that, i.e. a vertical cylindrical shape with horizontal pipes in and out.
Now this is freaky...the only way I could 'observe' the valve was to stick my digital camera behind the tank and blindly press the button, then review the results. But I haven't posted the photos here! Were you the 'fly on the wall'?!!
GreenV8S said:
Sure there must be *some* way to let vapour out of a tank when there is no carbon cannister. The pre-cat S series have a vent in the middle of the filler cap. For the cars with a carbon cannister this vent is blocked off and a seperate vent hose is routed out to the cannister and from there to fresh air. Is it possible the cap has been replaced on yours and a new one suitable for a carbon cannister design has been fitted by mistake?
After 8 previous owners and 13 years, anything's possible! Cap fits well though and looks as old as the car.
Thanks for your input. I'll await shpub to see if he can confirm the valve design and then maybe crank the pressure up to 60 psi. If there's a small explosion in Chepstow this week you know what happened.
resurrecting this thread....
Was there any conclusion to this topic? I'm also pretty certain that I'm getting positive pressure (not a vacuum) in my fuel tank - even after a relatively short run. On one occasion it was enough to ruffle my hair!!!!!
btw: mine is a 99 Chim (serp)
Was there any conclusion to this topic? I'm also pretty certain that I'm getting positive pressure (not a vacuum) in my fuel tank - even after a relatively short run. On one occasion it was enough to ruffle my hair!!!!!
btw: mine is a 99 Chim (serp)
Edited by JonathanT on Monday 4th June 16:42
I am definatly no expert on all this purge tank and pressure business but I will say that I do not believe your petrol cap to original equipment.
My 4.3 which was very original when bought has a manky looking (black as shpub says) what could only be best described as pressed steel cap, with no fancy spring seal, which screws into postition with a quarter turn... nothing fancy about it at all.
I also seem to remember reading somewhere that the precats take a different cap to the 500's as well, but don't quote me on that.
Hoover.
My 4.3 which was very original when bought has a manky looking (black as shpub says) what could only be best described as pressed steel cap, with no fancy spring seal, which screws into postition with a quarter turn... nothing fancy about it at all.
I also seem to remember reading somewhere that the precats take a different cap to the 500's as well, but don't quote me on that.
Hoover.
Edited by Hoover. on Tuesday 5th June 08:04
CaptiV8ed said:
griffter said:
JonathanT said:
Was there any conclusion to this topic?
No! Except that I still get positive pressure on hot days, and it wasn't the cause of my running problem.Gassing Station | Griffith | Top of Page | What's New | My Stuff