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80 Fuel tank breather

To add my 2c to the mix - diesel does not vapourise much, especially at UK temps. So I think we can rule out "vapour pressure" as a significant source of positive pressure in the tank - unlike a petrol.

With the hissing, I get this occasionally too when opening the filler cap but my impression has been that it's an intake of air, i.e. negative pressure, rather than air coming out.

I agree with Chris that you want the tank under slight positive pressure if possible. Not only would this help stop dusty or moist air being drawn in but negative/low pressure does cause some volatilisation of the lighter fractions of the diesel.
But achieving a positive pressure isn't easy as you've only got the atmosphere to work with (i.e. zero relative pressure, not neg. and not pos.) and the tiny contribution of vapour. Temperature change is probably the biggest contributor to pressure change in a diesel tank, I would think. Fill up with cold diesel at 10-15deg (from an underground storage tank) and your cruiser sits in the sun and warms to 20 or 25 deg. The diesel expands (remember the tank is full) and increased temp also means more volatilisation occurs. So the positive pressure generated probably blows through the breather - Mr T put this there to prevent your tank from rupturing in an extreme case (I'm sure he also made the tank strong enough to avoid this too) You then use up all the diesel and/or the temp drops and hey presto you have a negative pressure - the breather/filler cap should allow some air to be drawn in to equalise pressures and ease the load of the fuel pump. But generating positive pressure is probably not possible here. At best you would have atmos. pressure.

So I think the filler cap valve on many of our diesels is sticking/jammed on not letting air in. As suggested by Rob, I would think that the breather is designed to let air out if excess pos pressure is present but not let anything in the other way to avoid water/dust/mud etc being sucked into the tank, especially if submerged in cold water causing fuel to cool and contract.

Just my ramblings on the subject anyway :ugeek:
 
What about the return fuel line, not entirely sure of its function but could it create positive pressure if the tank was designed to run at positive pressure?
 
Hmm I doubt it - the net flow of fuel will be out of the tank, so the contents of the tanks are declining while the engine is running. The fuel return line AFAIK is there to return any unneeded fuel that was delivered to the IP back to the tank - obviously the fuel demand at the engine end is dependent upon load/speed of the engine but fuel delivery from the tank is fairly constant.
Is there even a pump on the return line? At a guess, this surplus fuel flows back under pressure from the fuel pump supplying fuel to the IP/engine.

For the return line to generate positive pressure, you would need to have a powerful pump returning the fuel. Remember that liquids are essentially incompressible, so high pressure liquid fuel being returned to the tank isn't going to contribute to much of an increase in the tank overall, as the tank is a much larger volume than the small volume returning.
Also if this was the case (returning fuel returned at high enough pressure to pressurise the tank), what would happen when the tank was full and high pressure returning fuel was added? You'd need the breather to be working properly then :twisted:

Cheers,
 
seems like the theory stating that the tank should run at positive pressure and is therefore pressurised is wearing thin, unless someone can convince us that the return fuel line is in fact pressurised and you only need a small positive pressure difference
 
Why not look at it from a different perspective, what if the the lift pump gets tired over the years which blocked breathers only exaggerate, Julian V recommends fitting a walbro fuel pump at the tank to help the flow of fuel.
 
AFAIK the diesel pump uses some of the diesel to cool the pump - this gets returned to the tank.
 
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I think that the theory that the fuel tank is under positive pressure is a myth.

To pressurise the tank, even slightly, you need to add a volume of either air or fuel to it. When the engine is running the net fuel/air in the tank is diminished by the consupmtion by the engine. Even if the return pipe was somehow pressurised by a pump you still have less fuel going back into the tank than it leaving.

When the tank hisses at refuelling it's an ingress of air to make up the slight negative pressure caused by a blocked breather in the cap. The other breather is to vent any fumes from the tank due to any temperature expansion.

Just my 2c.

Cheers
 
The valve is a simple one way valve, air out on expansion of the fuel & then closes if anything heavier i.e water tries to enter tank, the filter from Halfords is not a good idea unless the pipe is extended up to a high point & then turned down again in a u shape. The return line is very low pressure & this should be checked for blockages if you suffer running problems or fuel leaks.
 
Jon

Would you mount at tank end or immediately prior to fuel filter.

Fuel tank end would seem favourite - if you have some small leaks as long as pressure from this pump was higher than suction from IP then I guess you'd still be running pretty well - ie. not drawing in air?

Many thanks.
 
Ian and Toby have more experience of fitting these thatn me, plus Julian V of course, but yes put it at the tank end. I've bought two of these from the place linked to above but haven't fitted either of them :roll: The first one I bought for my old green 80 but broke that before I could fit the pump (sold the pump) and the next one when we had the red 80 back which went back to Ian before the pump was delivered (back ordered so delayed delivery) which is on my desk but belongs to Ian now :D
 
Right guys, I can't solve the negative/positive pressure debate (apart from how could you get positive pressure :?: )

I have extended my tank breather up to beside my filler cap and it is open to atmosphere with no valves or filters.
I may goose neck it in time as there is a possibility of wading to that height. I used the same pipe I used to extend my axle breathers.
The pipe is right next to the filler cap behind the fuel flap.

What I have noticed is, the car feels more responsive and gearchanges are smoother. My only concern now is has my consumption changed :?:, won't know until I fill up next time.
Consequently there is no hiss when the cap comes off so no pressure either way.
 
Manual says that the tank should be under positive pressure, I didn't make it up. Air expands, fuel expands etc with warmth during the day and the return fuel is warmer due to the heat it picks up as it go through the system. I think that most of the time it is very little pressure, but positive it is, I am sure. I believe that the filler cap is one way only and that is to vent out. Breather to allow air in if negative pressure get to much so that flow is restricted. If that's blocked then you get drop off in performance.

Look at the hsst you get when you open the top of a jerry can. Is that negative pressure - err I don't think so.

Chris
 
chris, take the fuel cap apart and you will see that the valve will only let air in. I haven't seen the breather valve underneath so i cant comment on that one
 
It would make sense for negative pressure to be relieved at the filler cap where the air should be (relatively) clean and dry but to vent excess positive pressure underneath where it won't matter if it's dirty / wet.
 
Just to continue the effectiveness of this as a fix - not the in / out debate, I went and 3/4 filled my tank. It went from being alright to really dead. Even revving it on the drive was lumpy. Then I took the cap off. Strewth what a difference. Far more pronounced effect that having it on reserve and loosing the cap. Clearly there is more air in the tank when it's empty and removing fuel to the engine has less effect than when it is full. However it works, it certainly works! It sounds completely different to. Much more like your 80s!

Will get underneath again this week. On the in and out debate, I just find it hard to understand how the breather bit would get blocked if it was venting outwards. Sucking in would eventually block the filter, surely?

Chris
 
Chris said:
Will get underneath again this week. On the in and out debate, I just find it hard to understand how the breather bit would get blocked if it was venting outwards. Sucking in would eventually block the filter, surely?
Click to expand...

EDIT: presssed wrong button! chris check your cap first as the filter in the cap gets blocked and restrictcs air flow. mine was much better after i cleaned it, hissing almost gone
 
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