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[chrisgixer]

Had my code reader on it to check codes after a misfire wobbly on gas. 0300, 0301/3/5 present.

Lpg showed a voltage error. I guess from changing the battery a month or two ago.

Checked fuel trims, all fine at +1 and -2. Happy with that for lpg set up.

Cleared the codes and checked the trims had reset to zero, but in the process of scrolling down to the fuel trims I noticed the fuel tank vent valve data was varying.
  Odd as it was in gas.
Watched the readings for a while and it seemed to start cycling through an increasing scale. Start at zero up to varying levels of 30%. 0 up to 50%. 0 up to 70%. 0 up to 20% and so on.
  On one occasion it went all the way up to 100% and the engine stumbled with a quiet thump from the rear.

How can this affect an engine running on lpg?

[Entwood]

Had my code reader on it to check codes after a misfire wobbly on gas. 0300, 0301/3/5 present.

Lpg showed a voltage error. I guess from changing the battery a month or two ago.

Checked fuel trims, all fine at +1 and -2. Happy with that for lpg set up.

Cleared the codes and checked the trims had reset to zero, but in the process of scrolling down to the fuel trims I noticed the fuel tank vent valve data was varying.
  Odd as it was in gas.
Watched the readings for a while and it seemed to start cycling through an increasing scale. Start at zero up to varying levels of 30%. 0 up to 50%. 0 up to 70%. 0 up to 20% and so on.
  On one occasion it went all the way up to 100% and the engine stumbled with a quiet thump from the rear.

How can this affect an engine running on lpg?

Only thing I can think of... IIRC the fuel tank vent/purge is "powered" by vac .. solenoid opens but vac does the "suck"  ... so could it be that at the 100% you saw when the engine "stumbled",  it, effectively, for that moment had the equivalent of a minor vac leak ??

 

[05omegav6]

Was the thump from the back of the engine or the back of the car

[chrisgixer]

Was the thump from the back of the engine or the back of the car

Back of the car.

As it approached 100% the idle started to die a fraction.

As it flipped over to zero% at the exact same time there was a thump! rpm jumped slightly! then all was well. Steady 0%.

Haven't been able to replicate it since. Where is the purge valve? Rear of engine/carbon canister related?

[05omegav6]

Purge valve lives between the Hbv and breather box, has a pipe either side and an electrical connection

How much petrol in the car? Does the filler cap hiss when you undo it?

Thump was probably the tank changing shape under vacuum when the purge valve opened fully...

Gradual partial opening of the valve allowed the air in the tank to be drawn to the engine, creating low pressure in the tank causing it to 'suck' in. Once the purge valve reached its fully open state, there wasn't enough manifold vacuum at idle to keep the tank deformed. The thump was the tank returning to its normal shape, and the stutter was the result of air being drawn back OUT of the inlet manifold instead of into it.

That the purge valve can be seen to operate, it is functioning correctly. First course of action would be to brim the petrol tank in order to limit the amount of vapour in it. Liquid petrol is much less susceptible to expansion/contraction due to temperature change than a tank of fumes

[chrisgixer]

WHAAAAT.....?

Fill the petrol tank? Have you seen the cost or petrol these days?



...as you can tell, petrol tank runs at minimal level a lot of the time. Usually shows zero range, with the fuel light on, for months.

Oh...
I had also noticed that the fuel light was on prior to the episode. Which wasn't right as I'd put £20 worth in for fear on not starting one morning.

After the episode, fuel level has gone up, and the light is off.

[chrisgixer]

Does the purge valve get gummed up? I've never even touched it before.

Could it have caused the 1,3,5 error codes...? guessing not.

[05omegav6]

Does the purge valve get gummed up? I've never even touched it before.

Could it have caused the 1,3,5 error codes...? guessing not.

The stutter might have caused the codes

I goosed my purge valve first time I cleaned my breathers, as I hadn't removed the rubber hose to the breather bridge before filling it with carb cleaner petrol vapour is pretty clean, so should be fine...

What's the difference between filling the tank in one hit every four months or putting a tenner in every fortnight I suspect it would solve your issue whilst the weather is so changeable  Another option would be to open the filler every day to equalise the pressure in the tank

Incidentally, the purge valve either throws up a code when electrically broken, or stinks of petrol when mechanically broken

[Andy H]

Two thoughts come to mind.
1. the purge valve should be connected to the smallest jet in the plastic strip on top of the inlet manifold. I assume that it shouldn't be capable of passing enough air to cause the engine to stumble.
2. the carbon cannister should have one pipe going to the fuel tank, one pipe going to the purge valve and I assume there must be a breather in the top of the carbon cannister. An open purge valve shouldn't be able to pull a vacuum in the petrol tank because it should just pull fresh air through the breather in the cannister.

[05omegav6]

The purge valve is the breather for the carbon cannister.

Normally the system draws vapour from the cannister, via the purge valve and into the inlet manifold.

The recent cold snap might have caused a partial vacuum in the fuel tank by reducing the volume of vapour in the tank.

The gradual opening of the purge valve might have been enough to cause the tank to partially collapse.
Eventually the tank would be unable to deform any more, and the full opening of the purge valve might have dropped the cannister/inlet manifold pressure differential to reverse as the tank returned to its inteded shape.

Say the tank had 10 litres of fuel and 65 litres of vapour at 20 degrees C.

Reduce the temp down to 5 degrees,C and the vapour volume might drop to 50 litres, resulting in a pressure drop.

Purge valve opens halfway, and the inlet manifold pressure at idle gradually causes that volume to drop to 40 litres. The tank starts to collapse in on itself. The valve opens fully and the manifold pressure cannot maintain the plastic tanks' collapsed state.

As the tank returns to its original 75 litre volume, that draws the air back from the inlet, causing the splutter.

40 litres of space expanding to 65 litres (don't forget there's still 10 litres of petrol), means the tank a vapour defecit of 25 litres. This has to come from somewhere, and because the system is closed,  it can only come from the filler if opened or the inlet manifold.

Hopefully that explains it a bit clearer

The numbers are imaginary, but shoukd illustrate the point...

[Andy H]

The purge valve is the breather for the carbon cannister.

Normally the system draws vapour from the cannister, via the purge valve and into the inlet manifold.

The recent cold snap might have caused a partial vacuum in the fuel tank by reducing the volume of vapour in the tank.

The gradual opening of the purge valve might have been enough to cause the tank to partially collapse.
Eventually the tank would be unable to deform any more, and the full opening of the purge valve might have dropped the cannister/inlet manifold pressure differential to reverse as the tank returned to its inteded shape.

Say the tank had 10 litres of fuel and 65 litres of vapour at 20 degrees C.

Reduce the temp down to 5 degrees,C and the vapour volume might drop to 50 litres, resulting in a pressure drop.

Purge valve opens halfway, and the inlet manifold pressure at idle gradually causes that volume to drop to 40 litres. The tank starts to collapse in on itself. The valve opens fully and the manifold pressure cannot maintain the plastic tanks' collapsed state.

As the tank returns to its original 75 litre volume, that draws the air back from the inlet, causing the splutter.

40 litres of space expanding to 65 litres (don't forget there's still 10 litres of petrol), means the tank a vapour defecit of 25 litres. This has to come from somewhere, and because the system is closed,  it can only come from the filler if opened or the inlet manifold.

Hopefully that explains it a bit clearer

The numbers are imaginary, but shoukd illustrate the point...

I can't see how that could work.

1. When the car is standing unused the fuel tank vents through the carbon canister. The purge valve remains closed when the engine isn't running hence there needs to be an open vent in the carbon cannister.
2. When the engine is running the purge valve opens to draw fresh air through the carbon filter. That fresh air cannot come from the tank hence there needs to be an open vent in the carbon cannister.
3. There is no way that an Omegas plastic fuel tank can withstand manifold vacuum hence there needs to be an open vent in the carbon cannister.
4. Drawing fuel from the tank requires that air is allowed to enter somehow otherwise the tank would collapse. It can't come from the purge valve when the engine is running because there would need to be a vacuum in the tank greater than manifold vacuum

[chrisgixer]

Good explanation. Thanks Al. And Andy

My old zx6 would fail to start to on cold mornings, until the filler cap was popped.

What dictates the valve opening? The readings implied several attempts to open to 100% but failed! until it was fully open and the eventual thump and hick up, and eventual steady zero reading. It then seamed happy.

Makes me think its stuck/sticking/needs servicing. Somehow

If the odd/excessively low fuel level reading is indicative of these symptoms and the purge valve, its been happening for a while, as the fuel level does similar on a regular basis. Although I've never noticed the hic up and thump before.

Presume vacuum in the tank capable of sucking in the plastic, would also be significant enough to effect fuelling, or at least strain the fuel pump excessively ?

[chrisgixer]

Just to add, fuel level indicator seems to vary on start ups.

Some days its fine. One morning it could be low and the light come. Re start same day, level back to normal, light off.

Wonder if its cycle sometimes works on start up, and sometimes not, giving the level issues.

[chrisgixer]

Running on lpg remember, so the fuel level rarely changes much. The variances day to day are as much as 10 to 15 litres when the fault occurs.( if it is a fault )

[05omegav6]

Running on lpg remember, so the fuel level rarely changes much. The variances day to day are as much as 10 to 15 litres when the fault occurs.( if it is a fault )

Next shift on, open the petrol filler every time you get in the car to go to work/come home to see if that makes a difference...

I suspect that with a vacuum behind it, the valve would be unable to open fully this may eventually lead the failure of the purge valve as the motor tries to overcome the vacuum holding it shut.

Of course it might be that it has failed and the symtoms are infact the cause