1. Home
  2. Car Maintenance
  3. Pressurised Fuel Tank - How/why

Pressurised Fuel Tank - How/why

Nothing wrong with it I'm just interested in why. Its a Talbot Express (motorhome) 2.0 Petrol engine (carb) with a small mechanical fuel pump and a non-vented cap fuel cap All standard and working fine. The tank is fitted with a pressure valve leading to the atmosphere that lets air in easily but restricts it to a higher pressure if air needs to escape. The fuel pump has a return feed to the tank and after a run the tank is pressurised noticably. I wondered: a) what the thinking behind this was? b) how it manages it unless it is simply heat building up in the fuel as it passes through the pump and pipework then back to the tank.

Regards, Steve

Reply to
SandS
Loading thread data ...

Fumes are kept in for environment protection, shaking fuel makes it try to expand and the fumes get pressurised, all this is extremely normal.

mrcheerful

Reply to
mrcheerful

Hi. Thanks for the info. I'm not worried about it, just interested in how things work. Cheers, Steve

Reply to
SandS

The message from "SandS" contains these words:

Are you sure it's pressurised? They're often slightly evacuated, which makes a whoosing noise as you open the cap.

Reply to
Guy King

There is a vacuum only when there is a fault.

Some cars have a sensor and if the tank does NOT pressurise, a fault light comes on. First question to customer with fault light: Have you got a fuel cap?, second question: Did you do it up till it clicked three times?

mrcheerful

Reply to
mrcheerful

The message from "mrcheerful ." contains these words:

Really? All the Corsas I had did it, as does my Montego and the wife's Clio.

I haven't yet tried the Audi, mostly 'cos I'm putting off the awful day when I have to fill it up. Depending where I get the diesel, a full tank will be around £70!

Reply to
Guy King

yes, the caps and breather systems are designed to let air in to replace the fuel as it is used. in the absence of proper breathing the tank can collapse or the fuel flow can stop.

mrcheerful

Reply to
mrcheerful

The message from "mrcheerful ." contains these words:

True, but the air admission valve always seems need a psi or two to open. When the tank's low, 1psi deficit is quite a bit of volume - hence the whooshing noise.

Reply to
Guy King

All the cars I've seen try to keep a slight vacuum. The newer ones that try to detect leaks do that with vacuum too. Vacuum is free, you just get it from the inlet manifold. Pressure is a bit more difficult to come by. Also, if there is a small leak, air will leak in with a vacuum rather than fuel/vapour leaking out.

IIRC the regs my Saab was meant to comply with allow anything up to a

2mm hole in the system to go undetected. After that, it has to flag up a problem.

Cheers,

Colin.

Reply to
Colin Stamp

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.