Thread: Pressure in the fuel tank

Definitely not normal. A couple of pics in the engine bay showing us the system would help. It sounds like you have the engine breathing going to the fuel tank. This can happen if flame traps block up, the wrong pipes are blocked etc.
Tank breathes through canister so that when stopped fuel vapour is stored in canister and when it starts gets burnt in engine via the breather system.

Thanks for the reply. I will get some pics and put them up and also work out if it's high pressure or low pressure in the tank.

Cheers

Originally Posted by daisy8

Hi everyone

I opened the fuel cap and the car threw fuel out the filler, along with a whole lot of air. (All over my shiny new Converse !) I closed the cap and went for a drive, stopping every now and then to check the cap. Every time there was pressure behind the cap. Don't know if this time it was vacuum or positive pressure. Went for a much longer drive and checked the cap. Air hiss was about the same as a really short drive.

Very interesting. I have had a similar issue with my 91 Range Rover. After filling the tank,I could drive 40+ Km home when the vehicle would have used about 10 Litres, then park the vehicle on a very slight sideways incline (maybe 5 degrees) with the filler on the downhill side while unloading. Within minutes, there was fuel dribbling out of the fuel filler. If I park the vehicle the other way round, meaning with the fuel filler on the high side, the problem does not occur.

The tank was recently removed and refitted with the same problem before and after that. I have also changed the charcoal canister under the bonnet, also changed the fuel pump without effect. Coopers cannot suggest the reason for this. You suggest blocked flame traps? Any suggestions welcome

Mine was also parked on a slight sideways incline when it dumped all the fuel out. I have not seen it throw fuel out again since I changed all the vacuum lines.

But my flame trap was cleaned when I changed the lines and it still sucks or blows air. My flame trap (the little canister on the drivers side rocker cover - right ?) is just filled with what looks like metal fly screen screwed up and packed in. The motor is running much better now with the fly screen cleaned.

Thanks for your advice. I'll try cleaning the flame traps again

Cheers

Alan

Also its not just the flame trap that blocks check the inside of the hose and the orifice the hose connects to.
On the other rocker cover at the back is a crankcase breather with a snap on filter cover which has a piece of foam in it. That all needs to be cleaned as well. Purple Haynes manual p243 in mine.

theres also a couple of valves in the canister or sometimes external to the canister that are regulated by engine vacuum. if these fail, the engine vac is not present they wont open and the tank pressurises.

Have some pics.

Coloured.jpg20200119_121930.jpg20200119_122001.jpg20200119_122025.jpg

On the first pic I have added some coloured arrows indicating where these pipe go.

The far left pipe on the canister goes to the tank and I can blow through it and hear bubbles back there somewhere, but with the cap on it seems to pressurise and then blow back up the pipe back to the canister a little bit.

My motor is a 4.6 and might be a bit different - I only have one flame trap and none on the other rocker. However, when I got the engine a hose was connected to a fitting on the top of the rocker at the back (Blue in the pic with colours.

I only just got home and will drive the car soon to work out if the pressure is positive or negative when I open the cap.

Thanks guys !