Blowing and Sucking
Discussion
Just finished investigating a fuel tank breather problem and thought I'd share the findings. (Not what you were expecting is it
)
Symptoms are whooshing when you take the fuel cap off. Awful low speed running and on one occasion when it was particularly hot the engine cutting out completely. That time when I took the fuel cap off the ingress of air was enough to strip trees bare and knock small chilfren off their feet
Having read all the threads I could find I assumed something amiss with the carbon can and its power supply or solenoid.
Not so. The supply is there when the fuel pump is on and after taking the can off and putting 12v across it I proved the solenoid was working fine.
Contrary to everything I have read on here it turns out that the solenoid doesn't open the fuel tank vent at all. The vent from the filler neck of the tank goes via a roll over valve to the lower of two inlets on the can. An outlet is open to the air. The lower inlet is connected (via the filtering in the can presumably) to the outlet.
Its the the upper of the two inlets that is controlled by the solenoid and is the one that goes to the 4 way under the pressure regulator (mines a 4.2) in the engine bay.
So I removed the roll over valve and tubing to see what was what. From carbon can to valve and from valve to filler neck there were no blockages and air was free flowing (when I blew down the hose). With the valve held upside down the ball moves and blocks the inlet. Right way up I could suck and blow air through but its definitely restricted. Presumably theres a small orifice that can be sealed by the ball of the valve.
So with it all put back together I can suck and blow air through the hose that connects to the filler neck. The flow is restricted by the valve but there's definitely air flowing both ways.
So if you think you have a fuel breather problem the simple test is to disconnect the hose that goes to the tank filler neck.
Blow hard - air should flow - I also get a warbling noise from the valve.
Then suck - agin air should flow (maybe a bit petrol flavoured)
Don't suck first. There may be dust in the can that would be unpleasant if inhaled.
(And no I didn't find out the hard way. Fortunately I had already blown the dust through when the can was off the car.)
Isn't hind sight a wonderful thing - all the crawling about with head between the back seats unfastening the lock nuts that hold the fuel filter and carbon can, crawling about under the wheel arch, messing about with wires to test the solenoid.... and all I had to do was blow and suck to prove the fuel tank vent is working.
The million dollar question is of course why was I not getting decent venting to the tank when teh engine cut out?
The straw I am clutching at is that it was a blocked roll over valve that was unblocked by the aforementioned blowing and sucking.
Time will tell. Unfortunately not today cos its now teeming it down and I don't want to drive in the wet until the sikaflex around the bolts and hose has set.
) Symptoms are whooshing when you take the fuel cap off. Awful low speed running and on one occasion when it was particularly hot the engine cutting out completely. That time when I took the fuel cap off the ingress of air was enough to strip trees bare and knock small chilfren off their feet
Having read all the threads I could find I assumed something amiss with the carbon can and its power supply or solenoid.
Not so. The supply is there when the fuel pump is on and after taking the can off and putting 12v across it I proved the solenoid was working fine.
Contrary to everything I have read on here it turns out that the solenoid doesn't open the fuel tank vent at all. The vent from the filler neck of the tank goes via a roll over valve to the lower of two inlets on the can. An outlet is open to the air. The lower inlet is connected (via the filtering in the can presumably) to the outlet.
Its the the upper of the two inlets that is controlled by the solenoid and is the one that goes to the 4 way under the pressure regulator (mines a 4.2) in the engine bay.
So I removed the roll over valve and tubing to see what was what. From carbon can to valve and from valve to filler neck there were no blockages and air was free flowing (when I blew down the hose). With the valve held upside down the ball moves and blocks the inlet. Right way up I could suck and blow air through but its definitely restricted. Presumably theres a small orifice that can be sealed by the ball of the valve.
So with it all put back together I can suck and blow air through the hose that connects to the filler neck. The flow is restricted by the valve but there's definitely air flowing both ways.
So if you think you have a fuel breather problem the simple test is to disconnect the hose that goes to the tank filler neck.
Blow hard - air should flow - I also get a warbling noise from the valve.
Then suck - agin air should flow (maybe a bit petrol flavoured)
Don't suck first. There may be dust in the can that would be unpleasant if inhaled.
(And no I didn't find out the hard way. Fortunately I had already blown the dust through when the can was off the car.)
Isn't hind sight a wonderful thing - all the crawling about with head between the back seats unfastening the lock nuts that hold the fuel filter and carbon can, crawling about under the wheel arch, messing about with wires to test the solenoid.... and all I had to do was blow and suck to prove the fuel tank vent is working.
The million dollar question is of course why was I not getting decent venting to the tank when teh engine cut out?
The straw I am clutching at is that it was a blocked roll over valve that was unblocked by the aforementioned blowing and sucking.
Time will tell. Unfortunately not today cos its now teeming it down and I don't want to drive in the wet until the sikaflex around the bolts and hose has set.
I sometimes get a whooosh of air when I release the fuel cap and on really hot days and the fuel tank getting low, the engine has cut out.
It's only twice it's ever cut out - once in Birmingham on a swealtering hot day last year and once at the Lakes during a very hard afternoon's driving and again, in swealtering heat this year. I'd just put these occurances down to fuel evaporation in the line due to the temperature, but this sounds like it could be worth investigating.
Did you take any photo's or anything, just incase I go blowing down the wrong pipe
It's only twice it's ever cut out - once in Birmingham on a swealtering hot day last year and once at the Lakes during a very hard afternoon's driving and again, in swealtering heat this year. I'd just put these occurances down to fuel evaporation in the line due to the temperature, but this sounds like it could be worth investigating.
Did you take any photo's or anything, just incase I go blowing down the wrong pipe
Here you go.
The hose connected to the fuel tank filler neck is the one to disconnect blow/suck.
The vertical black thing with the hose connected at the top is the roll over valve. The hose is routed out through the body and into the carbon can.
The hose from the filler neck is connected to the bottom of the valve. Its a fair length of hose and I'm wondering if it might have got kinked somewhere behind the fuel tank.
The hose connected to the fuel tank filler neck is the one to disconnect blow/suck.
The vertical black thing with the hose connected at the top is the roll over valve. The hose is routed out through the body and into the carbon can.
The hose from the filler neck is connected to the bottom of the valve. Its a fair length of hose and I'm wondering if it might have got kinked somewhere behind the fuel tank.
Edited by mikesr on Sunday 13th August 18:57
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