Fuel tank pressure after driving.

[ah-kay]

This is something always puzzles me when I open the tank cap. What should be the fuel tank pressure after driving for a while, say 50 miles and consumed 2 gallons of diesel?

1) The pressure should be = atmosphere - because of the vent.
2) The pressure should be < atmosphere - because of fuel consumed.
3) The pressure should be > atmosphere - because fuel is heated and air expanded inside the tank.

What should it be? If it is not what it should be then what is wrong and how to fix it?

Thanks

[mach4]

Originally Posted by ah-kay

This is something always puzzles me when I open the tank cap. What should be the fuel tank pressure after driving for a while, say 50 miles and consumed 2 gallons of fuel

There should be no pressure differential between the tank and ambient because the tank is vented.

If you've got a vacuum, your vent is clogged.

[tangofox007]

Originally Posted by mach4

There should be no pressure differential between the tank and ambient because the tank is vented.

That is factually incorrect. The valve incorporated in the vent will permit both positive and negative pressure differentials in the fuel tank. Same for the fuel cap valve.

[charmalu]

Are you sure it is pressure and not vacuum?
Is the cap hitting you in the Forehead when you remove it? that would be a lot of pressure. LOL.

Im betting you have a vent issue and it is sucking when the cap is loosened.
Even if there is pressure from warm fuel the vent would relieve it.
Check the vent under and behind the L/R wheel.
Not venting can cause a collapsed tank eventually.

Charlie

[Murkybenz]

I run biodiesel, I have a new tank neck seal but also have a square bit of old motorcycle inner tube fitted over the tank neck to protect the seal from the bio which would render it useless in quite short order .
My tank blows out pressure , noticeable as it sprays out onto the rubber as cap removed, it has done this for thousands of miles.

[barry12345]

Most vent valves only relieve vacuum. There may be a chance you are sucking air into the fuel line and this is pressurizing the tank.

Look for a stream of bubbles in the primary filter. There is a pretty constant static bubble there but no visable process of a bubble stream normally. Also try to see if air is also in the fuel exiting the return line from the injection pump. If you do see a bubble stream I would not expect to see a wet spot where the air is entering the system. You might or might not see some evidence of dampness is a possibility though.

[oldiesel]

If you have access to the 124 shop manual look up the tank vent system,it is not a simple vent it is expected to hold some pressure or vacuum.I would send you a link but i am far from home and this laptop does not have the 124 manual

[MBeige]

After driving on a hot day I normally feel the fuel cap apply pressure toward my hand the moment they separate from the neck, so I can tell from there it's pressure and not vacuum. This applies mostly to my 190E, and sometimes to my 300D. Both have the new style caps, not the originals.

There are also times when there is no pressure felt.

One time the fuel cap of the 300TE would not budge. I thought it was a vacuum issue. Turns out the locking mechanism of the cap failed (inside got jammed against fuel filler neck). Took a while and a lot of force to get the cap off.

This link is for the gasser (107 and 126); if they are the same as the diesels, according to the PDF, the vent allows air in and out of the tank, depending on need.

Imploded fuel tank - so sad!

Also states that, "A pressure of up to 50 mbar may occur in the fuel tank in a system which is operating properly. If the passage in the vent line from the fuel tank is restricted...the fuel evaporation gases may escape through the filler cap at a pressure of 100-300 mbar."