S3 Fuel Expansion Tank

[jagwit]

So, I brought my '71 FHC home in the hope of finding an expansion tank related pipe being the cause of the petrol fumes in the car.

Surprise!!!

The '71 FHC fuel tank has one got the "expansion tank drain pipe" labelled "C" in Bob's drawings. No A and dB barbs.

The expansion tank also has only two pipes coming out of it, C, and the vent pipe "D" which exits the body at the exact same spot as the OTS and vents into the IRS cave.

[Bob.]

So unless the filler cap has been replaced by the vented type, whenever the main tank level is above a certain level (vent/return C being the lowest of the 3 standard vents) either by fill level, parking nose down or braking there is no venting of the main tank.

[jagwit]

So unless the filler cap has been replaced by the vented type, whenever the main tank level is above a certain level (vent/return C being the lowest of the 3 standard vents) either by fill level, parking nose down or braking there is no venting of the main tank.

The cap is a vented one. Has a small hole. This setup of my 71 makes no sense to me.

[Bob.]

Looks like a PO tried to simplify the vent system without understanding it first.
As described, as far as I can see the expansion tank is serving no purpose at all. In addition, when the main tank is full the extended filler tube inside the tank will be covered by fuel, negating the effect of the filler cap vent, vapour expansion will force fuel out of line C, passing through the redundant expansion tank and straight out of the vent above the IRS. The level in the filler tube will also rise but would be secondary to the effect on vent C.

This is a description of the system as designed for the S2 emissions cars from 1970 which AFAIK is identical in all S3 cars other than a larger fuel tank and the vent to atmosphere point on non emission cars.

[jagwit]

Thank you Bob! It WOULD make sense for '71 S3s (mine is Oct 71) to have a rather similar setup as for S2.

Its MUCH harder to see what is going on ito breather pipes on the Oct 71 FHC than what it is on the Dec 73 OTS. I'm going to have another look.... It seems the physical locations of the breather pipes are rather different on the S2 than the Dec 73 OTS. And, by the looks of it, much harder to connect (from an assembly point of view) which might have motivated the factory to move them to an easier accessible location.

[jagwit]

I went and had another good look and I found all 4 of them breather pipes. Just extremely hard to see. I applied compressed air (at about 0.2 bar) to the vent pipe and could hear air being blown into the tank (which is near empty) and out of the small hole in the fuel cap.

The two pipes are labelled "A/B" because I can not see which is which.
C is certainly the expansion tank drain pipe.

tn2_20210409_114709 annotated.jpg (72.2 KiB) Viewed 4785 times

Here is the vent pipe D. The red above the yellow arrow is the bottom of the expansion tank.

tn2_20210409_114724 annotated.jpg (68.37 KiB) Viewed 4785 times

My finger is pushing the vent pipe D away to reveal the A and B connections to the tank. They seem to be much further to the rear than the OTS.

tn2_20210409_114854.jpg (77.85 KiB) Viewed 4785 times

[Bob.]

Are you following the "A" and "B" nomenclature from the S3 schematic in post #23? If so then it looks as if your "C" and the "A/B with a horizontal arrow" are the wrong way round and the expansion tank won't drain back to the main tank.

It doesn't matter which of the the two outboard connections on the expansion tank (A & B on the S3 schematic and 1 & 2 in the S2 diagram below), go to which of the two rearmost vent connections on the main tank as functionally they are the same.

BTW the part numbers for the S3 expansion tank, and all 4 hoses are the same as for those listed for S2.

[jagwit]

Are you following the "A" and "B" nomenclature from the S3 schematic in post #23?

Yep.

If so then it looks as if your "C" and the "A/B with a horizontal arrow" are the wrong way round and the expansion tank won't drain back to the main tank.

It doesn't matter which of the the two outboard connections on the expansion tank (A & B on the S3 schematic and 1 & 2 in the S2 diagram below), go to which of the two rearmost vent connections on the main tank as functionally they are the same.

It does look so in my pic but I THINK the second "A/B pipe left arrrow" goes behind the first "A/B pipe down arrow" but its impossible to see where it connects to the tank. This would put both A and B pipes on the left most side of the expansion tank - as per the see through drawing you so kindly posted.

BTW the part numbers for the S3 expansion tank, and all 4 hoses are the same as for those listed for S2.

That helps to add confidence to our conclusion that there is nothing odd happening on my FHC, thank you Bob.

Still have not found why my car has such a strong fuel smell

[lowact]

I think you have? A filler cap that vents to atmosphere and an expansion tank that vents to atmosphere?

[jagwit]

I think you have? A filler cap that vents to atmosphere and an expansion tank that vents to atmosphere?

Correct on both accounts. Should I block the vent in the cap perhaps?

tn2_20210408_211728.jpg (104.82 KiB) Viewed 4770 times

[lowact]

I don’t believe any S3 e-types ever had vented fillers.
Also, I am having trouble believing that any S3’s, even UK cars, did not have their fuel tank vents connecting back to the engine intake (carbs via the engine breather pipes.) Imo this closed ventilation system (with makeup air via the charcoal filter) is essential to minimize petrol fumes and smells. If not so connected the expansion tank does nothing. The expansion tank enables the closed ventilation system.
If there is no expansion tank or if the expansion tank is faulty (not venting the fuel tank), the filler would need to be vented to prevent air lock as the tank emptied. You have an expansion tank and yr blow-through has shown its not blocked, so no reason not the repair your filler cap, but imo unlikely to be any improvement unless you reinstate the entire closed ventilation system, including charcoal canister and vent valve (C37929).

[MLBS3V12]

So whereas on an an emissions car the hose transitions to the steel pipe going to the carbon cannister before going through the grommet at the back of the boot, as shown in post#21, on a non emissions car the hose is simply poked maybe an inch or so through a grommet with a bigger hole and vents into the area above the IRS. It did see the most likely way but it's good to see some evidence as it does not seem to be documented anywhere - thanks Philip.

I m finally conforted on the job I ve done on my S3.
The car I ve is mainly built from a French car ( appologize to say that but the on I ve is one of the very few S3 FHC exported to France) and a US one. I was not sure on the way from the hose coming out from the expention tank. I just let it went through the body around the IRS. It sounds to be finally the right way!
I ve also done a small hole into the tank cap to avoid the fuel smell.
Many thanks

[jagwit]

I ve also done a small hole into the tank cap to avoid the fuel smell.

We both should now close that hole in the fuel cap as there no need for it.

[colin gray]

Hi all, I hope it is okay to jump on this old thread.
A while ago we were getting excessive fuel fumes in our S3 2+2 and on investigation it appeared the expansion/vent tank was porous. The test was compressed air in to the removed main fuel tank which was all okay followed by the same method with the expansion tank, this proved to be leaky around the seam area.
These tanks seem to be unobtainable currently as they were when I originally took to soldering my tank to a leak free condition out of desperation.
I am now getting fumes but to a lesser level and suspect the expansion/vent tank again.
There seems little point in trying to seal it again so I just wondered if anyone had commissioned anyone to make new tanks, failing that can I remove the tank up the pipe stubs on the tank and vent the filler neck or filler cap?
Just to be clear I fitted all new E10 rated fuel pipes throughout the car at the same time as sealing the expansion/vent tank even though I only ever use premium fuel.
I also re sealed the fuel pump with new fibre washers and new pipes that side of the boot.
Thanks in advance
Colin

[MarekH]

...can I remove the tank up the pipe stubs on the tank and vent the filler neck or filler cap?

Yes. Plug the three stubs on the main tank. Seal the vent pipe to the charcoal canister. Drill a hole in the filler cap after checking whether there is already a vent stub above the main fuel tank filler cap hose. (You'd also probably remove the charcoal canister and the pipework to the manifold as they serve no purpose other than to evacuate the fuel tank vapours.)

kind regards
Marek

[Bob.]

If Colin is still around, there is an expansion tank for sale in the US here https://www.ebay.com/itm/185481468289?h ... OSwGlFiuw9~ , although it does say may not ship to UK.