vacuum in the fuel tank

[alex 1993 525i auto]

Dear all,

I was reading some old posts for a intake vacuum leak my car have and read an old post about vacuum build-up in the fuel tank (shogun and Russel).

I was curious about the activated carbon canister and fuel tank breather valve, learned they're parts to consider replacing on older cars like mine. I recently had my fuel pump sending assembly, fuel pressure regulator and fuel filter replaced.

I just noticed I definitively have a vacuum forming in the fuel tank, as today I drove 30 miles or so and when I stopped the car, opened fuel cap and heard a clear hissing sound for 3-4 secs before it achieved outside atmospheric pressure.

Should I be concerned with this? I don't want my fuel tank to collapse or bust my new fuel pump!

Is this a sign of a problem with the fuel tank breather valve, or carbon canister, or expansion tank or ???

How could I make sure what is faulty?

Thanks for your advices!
Alex

[Derek A.]

As a safety measure, BMW tech told me to drill a really small hole in the gas cap, to prevent a buildup in the tank. He has come across e34/e32/e38 that have collapsed the tank inwards - to the point that the fuel pump was starved. I saw an e38 on the hoist that had actually sucked the tank in so hard that it split a seam.

[alex 1993 525i auto]

I just found what might be the answer on this url

http://bimmer.roadfly.com/bmw/forums/e38/3879939-1.html

here what it says if it doesn't open:

Author: thesteve on 2003-09-11 at 20:04:01 (posted from: Host: bgp378043bgs.plnfld01.nj.comcast.net IP: 68.36.6.45)


Subject: Not cap, here's what happened: (399 views)

Message: Same thing happened to me and lots of other BMW owners. Fortunately I found my problem before my tank got as bad as yours. Mine only collapsed on the left side about half way. I think we should get new gas tanks for free. Here's the opening text of the service bulletin: I posted a pdf of the entire bulletin at the link below.

GROUP 16 16 04 97 Woodcliff Lake, N.J. February 1998
Fuel Supply Systems Product Engineering
SUBJECT: New Evaporative (Charcoal) Canister Purge Line
MODEL: E38 and E39
Situation: A new style evaporative canister purge line has been introduced into production on all E38 and
E39 models approximately 3/97.
The function of the purge line is to allow outside air to enter the fuel tank via the evaporative
canister. This equalizes pressure in the fuel tank as the fuel is drawn out during engine
operation.
The one end of this line is open to atmosphere and the other end is connected to the center
fitting of the evaporative canister.
If the evaporative canister purge line becomes restricted or clogged the resulting vacuum in the
fuel tank will cause the tank to partially collapse.
The customer complaints may be:
A knocking or a tin canning type of noise from the fuel tank area of the vehicle and / or when
removing the fuel tank filler cap a hissing noise is heard (pressure equalizing in the tank).Cause: Investigation of restricted or clogged evaporative canister lines has indicated the following three
possible causes:
-- line pinched
-- rubber grommet not seated correctly between the center fitting of the evaporative canister
and purge line
-- insect (spider) nests
Note: If this type of line restriction is found blowing through the line with compressed air will
not remove the nests completely due to the sticky nature of this substance. The line must be
replaced in this case.
Correction: If the above customer complaint is received and the evaporative canister purge line is restricted
or clogged on vehicles produced prior to 3/97, it should be replaced with the new style purge
line.
The new style purge line (1) is now shorter than the early style line (2) and incorporates a plastic
screen on the end which is open to atmosphere as shown in the illustration below. The screen
prevents unwanted objects from entering the open end of the line which may cause restrictions
or a complete line blockage.SI New Evaporative (charcoal) Canister Purge Line BMW AG - TIS 11.09.2003 20:00
Issue status (10/2001) Valid only until next CD is issued Copyright Page - 1 -

This technical bulletin is for E38, 7 series, but it look like the culprit!

If anyone knows I should look for other parts to replace or know a more E34 M50 more specific solution please post!

Thanks,
Alex

[whiskychaser]

Mine creates a bit of a vacuum but only takes about half a second to clear. Be interested to hear how you get on with this

[alex 1993 525i auto]

Sorry for the late reply, but I think now the upper url link doesn't apply for e34 M50...

According to the drawings on realoem on fuel tank breather valve and carbon canister, the breather valve is connected to the throttle body, which means the fuel tank should be under vacuum (same vacuum as in the intake manifold), but not more than that.

If I understand right, vacuum in the intake manifold from engine running (breathing air from outside) is the same as in the fuel tank. If the consumption of fuel leads to a vacuum in the tank greater than in the intake manifold, fuel tank breather valve opens and it equilibrates pressure (i.e. vacuum) with the intake manifold vacuum.

So this means no worries about having a few sec hissing sound when you open the fuel cap.

But if it takes clearly more than a few sec, if you see your fuel tank shrinking inwards or if your engine bucks like it is running out of gas (but without problems related to fuel pump, fuel filter, etc.), than the problem could be a faulty fuel tank breather valve.

If I'm wrong please let me know, if not I hope it could answer a few questions.

Alex

[632 Regal]

Get a new gas cap, that fixed it for me. Too simple maybe but 1 less thing to worry about.

[alex 1993 525i auto]

Get a new gas cap, that fixed it for me. Too simple maybe but 1 less thing to worry about.

That's one of the first thing which I changed when I got the car 2.5 years ago, because of a air intake leak (gave check engine light warning, wasn't because of the gas cap...). Problem is solved anyways now.