[I couldn't find a better place to ask about fuel tanks - sorry! - still, fuel tank -> fuel system -> engine... near enough ]

I have one Mr PTM's finest plastic tanks, which has a vent pipe.

I have to make a valve for this, and I have one problem - what does it have to do?! As far as I'm aware, it has 2 main purposes:

1. To stop the petrol coming out when cornering, looping the loop, etc.
2. to let air in as the petrol is consumed.

OK so far - this means a simple one-way valve that I can knock up on my lathe in an hour or less.

My question is, should I make any provision for over-pressure, e.g. for hot days when the fuel and air expands? Or isn't this a worry?

David

David,
I have a PTM tank that didn't have a vent pipe connection. I fitted a pipe into the sender unit that I fitted to the top of the tank near the centre of the tank then into a pipe that loops up under the rear panel then down to the vent valve off the Sierra filler assembly then a pipe down to ground. The valve is mounted onto a bracket welded onto one of the tank straps. The valve is there to prevent spillage if it goes belly up. Seems to work OK.

Unfortunately - no, that's wrong - fortunately, I don't have a Sierra filler!

I'll be using a bought filler cap (when Europa get round to delivering it :mad and a bit of 50mm rubber hose.

I'm just checking to see whether I need to worry about expansion in the tank, as well as low pressure caused by using petrol, inverted flight, etc. My instincts tell me I don't need to worry, but I'm open to the experiences of others.




David

Try again:

(when Europa get round to delivering it )

ChrisW, you must get this editing problem sorted!

DJ

David

The tank MUST vent both ways or you will pressurise the tank as the fuel expands with heat.

I thought on a road car, that most just have a pipe that vents to air.....thats the way it was on my granada and metro - the only cars I looked at.

I guess modern cars have something in the filler cap?

One one way valve kinda idea could be to use an aquarium air valve. These cost about a pound each and is a little plastic chamber with 3 - 4mm or so pipe attatchements each end that take windscreen washer type tubing. If you mounted two of these back to back that might do it.

Also, isnt there something like this in a one man brake bleeding kit?

RS do pressure valves, but they dont look ideal.


atb

steve

by back to back i mean both in opposite parallel(sp?)

David,

I don't have a Sierra filler my Indy. I have a flush fitting one from Merlin which cost twice as much as the donor. I did use the vent valve from the donor though. The valve was mounted near the top of the filler assembly on the Sierra.

stephen_gusterson

posted on 13/3/03 at 06:43 PM

by back to back i mean both in opposite parallel(sp?)


two one way valves in parallel in opposite directions is the same as an open pipe isn't it? Dave

My vent pipe is an inverted U with no valve. It goes up from the top of tank to the top rail of the boot and then down to an open end below the bottom of the tank.

Is that wrong?


Dave

CairB - Sorry - I misunderstood!

Dave A - It depends on the nature of the 2 valves! After some thought I'll probably make something up with 2 valves - both will be sprung-ball valves: the one that stops petrol leaking out and lets air in will operate on a low pressure differential (i.e. a weak spring). The one that lets air out if the weather gets warm (if ever) will have a slightly stronger spring - this will just prevent the tank bursting, but unlikely to operate if the petrol splashes against it.

I think that you will need some form of valve, Dave. Otherwise you risk splashing onto the ground (or under your wheels - this used to happen to Ladas!)

David

Don't want to send this tread into complete disarray, but what about pressurised tanks, i.e. with no valve at all? Anyone have any theory on these, why they don't implode or explode, and how they keep delivering fuel properly under low pressure, i.e. when the tanks nearly empty, do they use a special type of pump?

My main car is a MR2 which has a pressurised tank, when I remove the filler its sucks in a big phsssst of air. I know it uses a submerged pump in the tank.

My guess is that you should keep the same as was originally on the donor of the fuel delivery and engine. I'm assuming the older fords have a non-pressurised system then?

More of a warning for the 4AGE boys really, and for others using a donor with a pressurised tank.

I think my main problem is that in the Escort the filler cap is way above the top of the tank. This means that a tube up to the filler funnel is probably all that it needs.

In my Locost the top of the car is only about 8" above the vent hole - the filler cap will only be 4" or 5" above the tank (and I don't want to run bits of hose around the boot space anyway). Sharp cornering may be enough to push fuel up the vent hose.

Anyway, all this sounds like I'm trying to make things more complicated than they need to be! It'll be a simple enough job to make a vent valve to screw into the tank - I just needed to get a grasp of what's required.

cheers,

David

"I think that you will need some form of valve, Dave. Otherwise you risk splashing onto the ground (or under your wheels - this used to happen to Ladas!) "


So that's why I get through fuel so fast!

I'm sure I've heard of people using a valve for a motorbike fuel tank. They are nice and cheap

I think there are similar things in the demon thiefs catalogue, but they're very expensive

two one way valves in parallel in opposite directions is the same as an open pipe isn't it? Dave


Ermm

NO


they are VALVES

they open at a specific pressure. So, if we say thats a couple PSI, then nowt will flow either way until a couple psi negative or positive pressure occurs.

A two way valve is part of any radiator cap. Useless for this tho.

atb

steve

I have a local company that sell all types of fuel pipes and fittings for drag racing. They told me the simple way to vent the tank is to double coil the pipe before running down to below the bottom of the tank. The double coil will cause enough resistance to stop any petrol sploshing about in the tank getting out, but allow air to travel freely. I'm going to try this first and see if it works, only cost is an extra 12" of pipe.

Cool, might try that.

Now I understand what these valves do too, doh! thanks Steve.

Dave