Fuel system leak

[MkIV Lux]

Hi All,
   
   Following happened recently on my car (1993 Cobra Mk IV AKL with EFI engine, under boot flat fuel tank, fuel filler on right hand rear  fender)
   - car parked in a steep parking lot (slope of about 10-15%), nose pointing downwards;
   - tank had been filled up completely including filler neck just about 2 miles before the car was parked;
   - about 10 minutes later, fuel started to run out under the nose  (not by drips but by steady flow);
   - quick check revealed that fuel ran out from the most forward of the 2 "mushrooms" covering the two large ports of the fuel vapor storage canister (sitting on right hand side in front of front crossframe);
   - I assume that the two "mushroom" are ventilation covers;
   
   Turned the car around 180 degrees, now tail pointing downwards and nose pointing upwards, and the leak stopped.
   
   Left the car overnight. Next morning it started normally and worked without any trouble during the first 500 meters. The road then took a very steep hill (gradient about 20%) and the engine started to buck, like getting no fuel for a couple of seconds; no misfires and no afterburning; when reached the top of the slope and we were again running on more or less flat ground with more standard slopes up or down, the engine ran perfectly as always; no further sign during the whole day where I covered some 30 kms with multiple stops. And travelled 200 kms in the evening and 800 kms next day without the faintest indication of any problem.
   
   I assume that the vapor storage had been flooded with petrol when the car was parked  nose downward on steep slope.
   
   Two hoses connect to the 2 smaller ports at the front top of the vapor storage assy:
   - one hose connects the interior port (the larger of the 2) to a copper pipe running alongside the chassis rail (hence obviously starting at the rear end of the car, but so far I have not been able to look under the car and trace the start of that line); this is in fact the return line for the vapours and is connected to the tank at its upper side behind the rear right side wheel (24/7/2018)
   - the second hose connects the exterior port (the smaller one) to the injection system.
   
   Can anybody explain the function of this vapour storage assembly, and the flows of liquid or vapour? If it should be a simple vapor recovery or ventilation system from the tank, and should be principally dry, I would assume that the larger of the two small ports would be connected at some point to the filler neck of the tank (see addition of 24/7/2018 above)
   The image below shows the 2 large ports unvovered; on my car they have a kind of "mushroom" form cover. I'll take another picture tonight.
   
   Grateful for any advice hereabout.
   
   Many thanks in advance
   
   Constant
   

[MkIV Lux]

Digging further on the web, I found this assy to be referred to as charcoal canister, that stores fuel vapour until it is released into the injecton system, the release being controlled by a solenoid valve steered by the ECU.
   So in my "incident", the charcoal was flooded with fuel. Will it need replacement?
   This raises another question: Is the connection from fuel tank to charcoal cannister an open connection or should there be a kind of safety valve that would prevent fuel to access the line?

[Rob.Hendriks]

Constant
   The charcoal canister in question is an EVAP and as you say catches evaporative vapors from the fuel tank.
   The "mushrooms" you refer to are the atmospheric vents i.e. they allow air back into the tank (the filler cap should be sealed on an EVAP system) to prevent a vacuum.
   There are normally no safety valves between the tank and the EVAP filter. The fuel getting into the filter would have been caused by a combination of you overfilling and the car being parked on the incline. Modern cars have a floating ball type arrangement that prevents overfilling and this from happening.
   In all likelihood the majority of the fuel would have evaporated out of the canister overnight and the bucking you experienced was probably fuel starvation, caused by the remaining fuel in the canister sealing off the port and causing a vacuum. Once back on the level it was probably low enough to allow the canister to breathe normally. Any remaining fuel will have evaporated out over the next day or so. If you are concerned, try draining it and blowing air through for a few hours. Of course, the activated charcoal will be totally knackered, however this will be of little consequence unless you are strictly tested for emissions, in this case it would be best to replace the canister
   The small connection goes to the tank the large connection to the engine
   Rob

[westcott]

Hi Constant,
   
   a rollover check valve is placed forward of the filler tube on top of the tank. That valve will close the tank ventilation in case of a roll over to avoid fuel leaking out of the rolled car. If the tank is overfilled until inside the rubber tube connected to the fille neck, fuel will flow trough that open safety valve und trough the evacuation tube into the charcoal canister and fill it up until the fuel level is lover then the above mentioned connectors at the tank. As Rob said the fuel in the canisters will evaporate sooner or later.
   Uwe

[SB7019]

Posts and responses like this are one of the reasons I love this forum.
   
   I had wondereded for many years what these canisters were for and about 6 years ago had their explained to me, but only in general terms.
   
   As my car now has a heavily revised fuel system  and a Motec ECU I had wondered about removing them as presumed they no longer served any purpose.  Having read the very detailed and informative replies to Constant's post I suspect they may be still be required in order to avoid a vacuum forming in the tank.  Rather than risk this they will now certainly stay in place.
   
   Thank you to Uwe and Rob for explaining one of life's little mysteries!

[MkIV Lux]

quote:Originally posted by Rob.Hendriks
   
....
   The small connection goes to the tank the large connection to the engine
   Rob
   
   

   
   Hi Rob,
   Many thanks for your comprehensive answer.
   Indeed, it is the small connection that links to the tank, the larger one to the injection system, bearing an in-line solenoid valve.
   This kind of charcoal tank has been used on Mustang engine configs from 1979 to 1993, I found out from researching the web.
   
   Tanks for later models look identical but do only have one small port at the front top, connected to the tank, and two main ports: only the most forward is covered by a mushroom while the rearmost bears a two-way solenoid valve linking to the injection system.

[MkIV Lux]

quote:Originally posted by westcott
   
.....
   a rollover check valve is placed forward of the filler tube on top of the tank. That valve will close the tank ventilation in case of a roll over to avoid fuel leaking out of the rolled car. If the tank is overfilled until inside the rubber tube connected to the fille neck, fuel will flow trough that open safety valve und trough the evacuation tube into the charcoal canister and fill it up until the fuel level is lover then the above mentioned connectors at the tank. ....

   
   Hi Uwe,
   Many thanks for these details. So the lesson learned is not to use the rubber tube's capacity and hence keep the whole line from tank ventilation to the charcoal box free of liquid fuel.

[westcott]

quote:Originally posted by MkIV Lux
   

quote:Originally posted by Rob.Hendriks
   
....
   The small connection goes to the tank the large connection to the engine
   Rob
   
   

   
   Hi Rob,
   Many thanks for your comprehensive answer.
   Indeed, it is the small connection that links to the tank, the larger one to the injection system, bearing an in-line solenoid valve.
   This kind of charcoal tank has been used on Mustang engine configs from 1979 to 1993, I found out from researching the web.
   
   Tanks for later models look identical but do only have one small port at the front top, connected to the tank, and two main ports: only the most forward is covered by a mushroom while the rearmost bears a two-way solenoid valve linking to the injection system.
   
   
   

   
   Normally the tube comming from the tank evacuation/ventilation/rollover valve goes down the tank and at main rails height it is connected to a pipe which follows the main rail to the front or is laying inside the rectangular section welded to one of the main rails.
   In the engine compartment that pipe is again connected to a tube and that tube to a charcoal canister. The charcoal canisters will be evacuated by a valve during engine operation.
   
   This is because B.A. had to follow actual 80s US (especially California) emission regulation and therefore used the complete emission equipment of a certified car, the Mustang 5.0.
   
   If you don't need to fullfill US or CA emissions you can delete the canisters and just leafe the tube, comming from top of the tank, open.
   
   I disonnected the tube at its lowest end from the pipe goin to the front and fixed it to the lowest point of the rear inner fender.
   
   Just be aware of the tank fill level and keep it allways below the flap and inside the tank.

[MkIV Lux]

quote:Originally posted by westcott.....
   
   I disonnected the tube at its lowest end from the pipe goin to the front and fixed it to the lowest point of the rear inner fender.
   
   Just be aware of the tank fill level and keep it allways below the flap and inside the tank.
   
   

   
   Would have to be very low to prevent any dripping on a hot exhaust pipe in case of overfill.

[MkIV Lux]

is the charcoal only a physical means of trapping the fuel vapors or does it imply a chemical reaction to neutralize the self-ignition potential of the fuel vapors?

[Rob.Hendriks]

The activated carbon (charcoal) used in EVAP canisters is low density and highly porus i.e. it has a large surface area by volume. This allows absorption of the vapours when the vehicle is static/parked and release of gasoline vapors into the fuel system when the car is running and the solenoid valve opens.
   The activated carbon does not prevent ignition potential of the fuel vapors.

[MkIV Lux]

thanks, Rob, clear !

[westcott]

quote:Originally posted by MkIV Lux
   

quote:Originally posted by westcott.....
   
   I disonnected the tube at its lowest end from the pipe goin to the front and fixed it to the lowest point of the rear inner fender.
   
   Just be aware of the tank fill level and keep it allways below the flap and inside the tank.
   
   

   
   Would have to be very low to prevent any dripping on a hot exhaust pipe in case of overfill.
   
   
   

   
   Yes, for sure it is.[]

[MkIV Lux]

Her is a schematic drawing of the fuel system on my 1993 Mk IV with EFI engine.