406 1.8 fuel tank pressurised ?

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ianb
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406 1.8 fuel tank pressurised ?

Post by ianb »

Advice needed please,
My fuel gauge has slowly crept up to read full during the trip to work this morning even though I know there's only half a tank of petrol in there, and the engine management light came on twice during the trip, at work I opened the petrol cap and there was quite a vacuum had built up in the tank,
Anyone recognise these symptoms? Could there be a blocked air feeder pipe somewhere,
Any advice gratefully received pleasem
Thanks
Linchpin
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Post by Linchpin »

I got that on my 306 the other day... and i ran out of fuel!
Gauge was reading 1/4 tank ... was dry tho :-(
Opened the fuel filler... almost sucked me in!
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Post by jameshughes »

There are breather pipes around the fuel tank /filler area to equalise the pressure in the tank with the outside world. Sounds as if yours are blocked.
Not quite sure the exact location of them on a 406 though.
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Post by Linchpin »

Any idea where they are on a 306?.. Sorry to steal your thread!
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Post by fastandfurryous »

I've had this happen on an old Cavalier once. Most fuel injection pumps are powerfull enough to suck a pretty impressive vacuum, and when combined with blocked fuel tank breathers, you get all manner of odd problems in the tank. Lucky that you're referring to cars with injection moulded plastic tanks, as on pressed steel tanks, it's possible for the vacuum to be strong enough to suck the tank flat! My Cav's fuel capacity went from 61 litres to about 20 in under a week. oops!
Certainly on a 405, and on just about every other car I've worked on, the tank breater pipes go up with the filler neck, double back over it, and then back down, round the tank, and vent at a corner somewhere. This is to prevent the tank from ever draining through the vent. The only thing to do is get under the back of the car, and trace them.
David.
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Post by Linchpin »

Excellent! Ill get on the floor tomorrow and have a good look.
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Post by pug_owner2002 »

I know on my 306 that the pipe that I think is the breather (if it's not the breather it's the over flow pipe?)comes from the filler neck into the rear drivers side wheel arch (behind the plastic liner).
Maybe the same on the 406?
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Post by fastandfurryous »

hmm... the pipe that comes "from" the filler neck is usually a 20mm or so diameter pipe, which allows large-scale venting of the tank while filling with fuel, and hence the fuel cap is off. This venting is inteded to be further up the filler neck than the pump nozzle gets, and so the system works. The tank vent that is in use when the filler cap is on is a much smaller diameter pipe (similar to winscreen washer pipe) which will come from one or two of the high spots on the tank. It can be difficult to trace these pipes sometimes, as they get lost in the mud in the wheelarch.
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Post by Linchpin »

Might this explain why my car takes ages to fill (ie the pump keeps clicking off?)
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Post by fastandfurryous »

It can't help, as it will mean that more air/vapour is being pushed out through the filler neck as you fill, but I doubt that's the main reason. Have you tried other filling stations? some places have ultra-sensitive pumps.
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Post by ianb »

If I understand correctly;
the main vent runs from the tank to the filler pipe just below the cap, this is sealed when the cap's in place, but allows air to be expelled when the cap's off and petrol's being pumped in,
There's another air pipe from the tank to fresh air somewhere that allows air into the tank as petrol is used?
I've run the engine with no filler cap and everything seems ok, as soon as the cap is refitted and the engine left running for a couple of minutes - a vacuum bulds up. This seems to point to this second pipe being the culprit, where would I look on the tank to find this pipe, and where would it's most likely open end be?
Thanks for advice so far
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Post by fastandfurryous »

Yes... you are correct.
Although primary vent isn't actually sealed when the cap is on, it's just another bit of pipe that goes from the top of the tank up to the filler neck. It doesn't go out to fresh air though, so yes, when the cap is on, it is irrelevant.
The second vent pipe will come from the top of the tank, track the filler neck up into the wheelarch, loop around over the filler, back down the filler pipe, around the tank, and vent somewhere around the tank. On the 405 it's on the front left corner. A 306 may be in a different location.
Your first task is to find the vent connection on the tank, take it off, and jam some compressed air down the pipe. This often blows out a large lump of yuck from the other end of the pipe. If this doesn't clear it then it's probably easier to get some new hose and replace it, although the compressed air trick has always worked for me. There may also be some connections along this pipe that you can disconnect and try blowing through a shorter length at a time.
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Post by ianb »

I've found and disconnected the smaller vent pipe, I think on the 406 it comes down at the back of the rear arch, certainly the car ran ok at this point with no vacuum building up in the tank, once the pipe was refitted I had the same sucking problem( actually I called it something different after 2 hours rolling around under the car).
Tomorrow I'll try the compressed air trick, if I have to replace the pipe does it need any one way valves or it just a piece of pipe from the tank to fresh air?
thanks for all the advice so far,it's helped me pinpoint the problem.
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Post by fastandfurryous »

As long as the car isn't equipped with a charcoal cannister for hydrocarbon emission, then yes, it's just a pipe. Do make sure that you route any new pipe in the same place as the old one came from.
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Post by ianb »

Another question-
I think the ventpipe is clear right through now but I'm still getting suction with the cap on, I've disconnected the pipe at an elbow in the boot, air is being sucked through the pipe going up the channel over the back door - presumably back to the tank, the remaininpipe from the elbow goes under the rear wing to the filler pipe, I can blow air throuh that & feel it coming out just inside the filler pipe below the cap, there's a small button in the inside of the filler which I think is held in with the cap in place - does anyone know what this is for ?
Sorry for the lengthy message but I think I must be close to sorting this,
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