Jadz928

Hi guys,
This is on the 87 S4.

The first noticeable symptoms were a intermittent fuel pump BUZZ, then an erratic fuel sender (guage now pegged indicating broken sender).

After the BUZZ came a gas fill where I noticed a lot of back pressure when fill cap was removed. Looked under the car and the fuel tank has a big "dent" in it. Looks like the pressure sucked it in. Left the fuel cap loose and drove home.

RL search indicated fuel tank breather hose is crimped or plugged. This requires to pull down the tank.

At the same time I want to rehab the system and fix my imploded tank.
Any WYAIT advice and ideas on how to "pop" the tank back out will be much appreciated. Thanks in advance.

PS. apparently it's pretty easy to pop the tank back out with compressed air.

Mrmerlin

I would drop the tank and then flush it out if you have access to hot water fill some in tank and then add the air, let the water be on the dented section , this will help it return to normal without damage, OR you sit the tank in the sun for a hour . on the dented portion

Leon Speed

If you can't fix it now, get a full tank of gas, the dent will pop out. Put the cap back on and turn it back so that it leaves a little bit of room for air to pass into the tank but doesn't come loose. Loosen the cap regularly.

SharkSkin

That's not pressure, it's vacuum that you heard. Most likely suspect is the fuel cap. Replace that and fill the tank -- see if the weight of the fuel pops it out. If it does, then all you have left is replacing the sender.

Leon Speed

Dave, is the cap supposed to vent?

SharkSkin

Yes, IIRC it is supposed to let air in but not out.

Jadz928

Interesting! Yes, I mispoke, vacuum not pressure. My understanding was there is a vent line on top of the tank that equalized vacuum/pressure.

I'll swap out gas caps this evening and fill up... if that doesn't work, I'll just have more gallons of fuel to drain.

Thanks everone for your input. Will keep you posted.

Rehan

My tank collapsed, removed it from the car took in to the shower and filled it with some hot water and then took a heat gun and heated it some more around the collapsed area Then I took a shaft made of wood and easily pressed it out.

I did try to press it out when the tank was still in the car and had some fuel in it, I failed, the plastic was to stiff.

Hilton

There certainly is a vent line on S4's - not sure on earlier cars due to lack of experience.

The vent goes from a T behind the filler neck up to the expansion tank above the right-rear wheel. From the other end of the expansion tank is a line that goes to a check valve. On an S4, if you remove the plastic cover on the front side of that rear wheel well, you can see the line and check valve.

SharkSkin

On the 78 at least, the "check valve" forward of the accumulator is a tipover valve which does not block flow unless the car is upside down(barely visible behind the bracket in the pic below), and the line goes forward to the charcoal canister. IIRC there is a check valve up front somewhere too, but I can't find a reference at the moment. So unless the tipover valve is installed upside down or the lines to the charcoal can are plugged, it should vent pressure but AFAIK the only place it is supposed to ingest air to displace consumed fuel is through the gas cap.

ZEUS+

The evap system is a closed system, cap does not vent either way. When the engine is off the charcoal cannister absorbs fuel vapor. When engine is running engine vacuum burns the fuel absorbed in the charcoal. A restricted breather/vent hose is usually the cause of your problem.

SharkSkin

Thanks Zeus, every other car I've owned had a one-way valve in the cap. I looked at my cap on the 928, and it is a solid block of plastic that, as you say, does not vent either way.

Jadz928

I'd driven Perl a few times since I figured out the fuel tank breather issue. I just kept the gas cap loose until I had time to deal with the issue.
So I decided to do a fuel tank R&R to include new fuel level sender, fuel pump, strainer, and all new rubber.
Just got around to it and found the initial damage to the fuel tank itself, was pretty bad. It must've really sucked itself in... enough to destroy the sender and put a nasty bend in it.
Also, it broke the internal baffle loose, so that was just floating around in there.

Needless to say, the tank is toast. Mason at 928Worldwide is sending me a new/used one right now, god bless him.

Moral of the story... know your fuel tank system. I didn't take long for the damage to be done, prior to identifying the issue.

Mrmerlin

would you please add as much detail to,
what caused this problem,
how you realised it,
and what your doing to fix it,
pictures would be very helpful, Thanks Stan

Leon Speed

To add to this discussion, in my case it was:

what caused this problem: I replaced the charcoal cannister and needed to reroute the hoses attached to it because the new charcoal cannisters available are three way instead of two way with an y-splitter. I used soft hoses instead of the original hard hoses and the hoses got piched by the wheel well liner - there is less room for the new cannister. The problem was the vent hose that leads from under the rear bumper cover forward to the cannister.

how you realised it: less power at high speeds, difficulty starting and a tremendous hiss of air flowing in when removing the gas cap. Look under the tank and see a huge dent. Fill it up and dent pops out (over time).

and what your doing to fix it: moved the cannister aft as much as possible and carefully routing cannister hoses.