JPTL

88 S4 w/original unmodded fuel system:
After the third session yesterday, 1/4 tank of gas left, after turning the key off, there was a noticeable gurgling or percolating sound coming from the rear of the car.
It was a hot day (85°) and really hard sessions, so everything on the car was hot. The sound continued for a few minutes.
I got under the car and isolated the sound to the gas tank.
Because it was almost the sound of boiling fluid, I felt the tank next to the exhaust to see how hot it was. It wasn't too hot to the touch.
There was no smell of gas whatsoever.
I slowly unscrewed the gas cap and a decent amount of pressure/hissing released.
But the percolating continued. That didn't make sense to me at all. Pressure had been relieved from the tank - at least at the filler neck but the sound continued.
I re-confirmed that the key was off (ruling out fuel pump in my mind).
Not knowing what this sound from the gas tank was, I called it a day and packed it in.
BTW, here's a pic of the rear tire residue as it stuck to something coming from the fuel fill. It's unlikely that this was remnants from a fill-up overflow, because I washed the car right before the track:



Any ideas?

Mrmerlin

looks like fuel leaking from the tank then getting onto the body.

NOTE the bubbling sound was the fuel in the fuel rails percolating,
then returning to the tank via the fuel sender,
its outlet is at the bottom of the sender

JPTL

Percolating at the fuel rails? From heat?
That would explain the boiling sound at the tank.
I'm sure that the fuel rails were plenty hot.
So the "seepage" at the gas cap and the percolating noise may be completely unrelated...
Is the gurgle/percolating noise a regular occurrence at the track or DE's?

mj1pate

Follow up with emeritus such as Stan (Mr Merlin) on technical solutions, but I'll describe my experiences and maybe they'll relate.
During the desert drive portion of my cross country maiden drive; the clogged carbon canister/breathing system prevented proper vapor passage. When I stopped to gas up in Arizona, gas spurted out of the tank in volume once I released the fuel cap. (Fixed long since).
A later opposite pressure instance included vacuum in the tank resulting in a humming/straining fuel pump. More vacuum valve work/replacement helped this situation, but not totally. Bear in mind that that walnut-sized aluminum fuel vapor vacuum valve probably secured on your cross member cost $400 to replace. There's only so much refresh you can achieve.

FredR

JP,

Not sure what you mean by "percolating" unless you have a coffee maker brewing up as you were running!

As with most systems the track finds things you would not normally notice on street duty. The problem with fuel systems such as ours is that they are constantly circulating fuel from the tank to the fuel rail and back again and guess what?- whilst doing this it heats up and the gasoline starts to boil. Refiners try to ensure this does not happen in your car by controlling the boiling point range so that fuel starts to boil at a temperature it should not logically see in service. On carburetted vehicles a starting boiling point of 100F was not going to be a problem but on fuel injected vehicles like ours it became much more "marginal".

Seemingly Porsche reasoned that this could be an issue in more extreme circumstances and duly fitted a fuel cooler into the a/c circuit presumably to take care of this possibility reasoning that on a warm day the a/c system would be operating. However the 928 was not designed for racing and most folks who do race probably do so with the a/c system not even fitted never mind operational. In such cases a separate air cooled fuel cooler might be justified/required

So, you have hot fuel circulating getting ever hotter as you thrash around the circuit, the temperature in the engine bay gets ever hotter but this is no problem for the engine as the fuel is pressurised so it stays in the liquid phase until it leaves the pressure regulator and the pressure drops to atmospheric or close to it. If the fuel is too hot it will flash and thus why the fuel cooler is in that part of the circuit. However, no a/c no fuel cooling!

Now you have the fuel tank which is not a pressure vessel- it can take a little over pressure and a little partial vacuum but not much of either. In days gone by fuel tanks would vent to atmosphere but the good folks in Greenpeace do not like that so [not unreasonably] there is breather system that recycles such excess vapours back to the inlet system and [as I am aware] allows air to enter the tank as fuel is drawn down to prevent a vacuum forming. The exact "modus operandi" of this system I am not fully familiar with but as I understand it responds to engine vacuum.

Now in your reported case, it sounds as though pressure has been building up in the fuel tank- you open the gas cap and you hear a "whoosh" as the pressure is released. As that pressure releases, if the temperature is within the boiling point range of the fuel, a bit more fuel will boil off until the system comes to equilibrium. If the breather system is not working as it should, the tank pressure will be higher than it should be- hopefully there is a secondary [final] protection against over pressure- never really thought about this but hopefully something [fuel cap?] let's go of its seal before the tank ruptures.

As the system cools the vapours will tend to condense and then there is the risk of pulling a partial vacuum if the tank cannot in-breathe. 928 fuel tanks have been known to collapse.

Thus when going racing you need to ensure that these sub systems are working adequately. The 928 fuel system even has a one way valve in it so that if you end up on your roof the fuel cannot flow out of the tank via the return line.

On top of that, if you are racing at a high elevation you may also have to deal with a reduced boiling point due to the elevation.

If you have these factors under control you should have no issues but if you are operating outside the design envelope then it is quite conceivable that you will have issues. This is where other folks with plenty of track experience should be able to advise you as to what issues they found [if any] and how they dealt with them.

I would not mind betting that some simply vent the tank to atmosphere on the basis that such is better than popping or collapsing the fuel tank!