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[Andy H]

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.

I wonder if a vacuum in the tank is pulling the base of the tank up and pushing the fuel level sensor up out of the fuel that surrounds it?

[Andy H]

Has anyone got a picture of a carbon cannister they can post up?

[05omegav6]

Has anyone got a picture of a carbon cannister they can post up?

Looks like this...

http://en.bildelsbasen.se/?link=item&post_id=22932104

 

[Kevin Wood]

Firstly, engine ECU has no feedback from the purge valve, so the percentage it reports just relates to the duty cycle of the signal that it sends to the valve, not how far open it actually is.

It detects a wiring fault on the valve circuit and it may also check it is opening based on changes to MAF / fuel trim caused by the additional gas admitted by the valve.

If memory serves, there should be a fresh air vent on the carbon canister, so, unless it's blocked, the valve shouldn't cause the fuel tank to collapse below ambient pressure.

[Andy H]

The picture Taxi Al linked to shows two pipe spigots (presumably one to the tank and one to the purge valve) and a short rubber pipe with a lump on the end (presumably the fresh air vent).

What are the chances of the vent getting blocked? extremely small I imagine unless the vent pipe was kinked when it was first installed

[chrisgixer]

Firstly, engine ECU has no feedback from the purge valve, so the percentage it reports just relates to the duty cycle of the signal that it sends to the valve, not how far open it actually is.

It detects a wiring fault on the valve circuit and it may also check it is opening based on changes to MAF / fuel trim caused by the additional gas admitted by the valve.

If memory serves, there should be a fresh air vent on the carbon canister, so, unless it's blocked, the valve shouldn't cause the fuel tank to collapse below ambient pressure.

It was making repeated attempts to open the valve in that case. In the space of about 20 seconds, the percentage indicator elevated half a dozen times. Failed each time. Until the last attempt where it reached 100% and instantly the tick over stumbled in time with thump from the tank.
It remained at 0% there after.

....that implies to me, the car knows there's a pressure problem in the tank, is trying to open the the valve to release the pressure/vacuum, and failing several times. Then, once eventually successful, is then happy in that state as it no longer attempts to open the valve.

But, given your post, is it just that it automatically does a cycle on start up, and that's that.

...why would it then repeatedly attempt to open the valve until successful? I haven't grasped how maf and fuel trim could help it know fuel tank pressure tbh.

[chrisgixer]

The picture Taxi Al linked to shows two pipe spigots (presumably one to the tank and one to the purge valve) and a short rubber pipe with a lump on the end (presumably the fresh air vent).

What are the chances of the vent getting blocked? extremely small I imagine unless the vent pipe was kinked when it was first installed

The carbon canister has been disturbed in my ownership after replacing the drivers wing. I'm not aware of any damage to it or kinked pipes. I don't remember checking the vent though.
I've also fitted lpg so its possible a bulk of gas pipes may have kinked the purge valve vent pipe. Don't think so though.
I'm also aware there was a branch trapped under the car under the drivers seat, on a country lane during the winter. I guess there vent pipe could be squashed in the run along the bottom of the car.

[Andy H]

Is it possible that mud has found it's way past the wheelarch liner and caked the vent?

or is it possible that the carbon can somehow get blocked and prevent the tank & purge valve from venting?

[Andy H]

or is it possible that the very small hole in the breather bridge has been enlarged during over enthusiastic breather cleaning and now sucks air from the carbon canister faster than it should?
(I think MarksDTM swapped the purge and crankcase breather notes on this drawing - number 3 should be the canister purge)

[05omegav6]

2.5/3.0 centre breather bridge pipes are the same size, so perfectly possible to refit them the wrong way round.

However the 2.6\3.2 pipes are a different size so less likely

[chrisgixer]

Well. Just blasted it back from High Wycombe and its running perfectly, revs out fully at Wot, where as it was slightly stuttering before. Fuel trims where reset though so see how its settles down.

And after opening the petrol flap. No hiss.

[TheBoy]

The petrol light and Range warnings annoy me, so I have to top up (and in slippery winters, I brim the tank for weight), so I wonder if that's why I've never experienced this, or if your car does have a genuine fault  .  Beginning to think former, as you've been unable to repeat it

[chrisgixer]

I guess it depends on atmospheric temp conditions and the vent valve deciding to work, or not. It may be the case that today I go out and the tank is in positive pressure due to the sun being out, and the fault doesn't occur.

Or its a volume thing after lobbing in £20 to put the light off, or to achieve vacuum, the fuel would need to expand and vent, then the vent fail to open after the fuel gets cold at night.

Thing is, when the symptoms occurred with diags connected, I'd already been out in the car that day. Everything was warm, including engine temp. Plus I'd of thought the double sided Irmscher exhaust would be more likely to warm the tank than anything. Even WITH the heat sheild fitted.

Keep an eye on it I guess.

[05omegav6]

I have been digging through what passes for a brain/memory... I have a vague recollection of a previous car, (probably a Granada), which had a vacuum controlled servo on it, aircon related maybe

Any way, if you applied a vacuum to it the lever would operate as expected, but if you tried to manually move the lever to its original position whilst still applying the vacuum, it would simply snap back, a bit like a pneumatic version of the drivers door lock pin when the solenoid is on its way out...

I wonder if this negative pressure in the fuel tank, most likely caused by warm fuel vapour cooling and contracting, is causing the purge valve to remain closed essentially sucking it closed even though the ecu is electrically telling it to open