Syncro - fuel tank pressure

[StefDS]

Hi all,

I've searched for the issue, but I only found 4 topics, all for non syncro vans.

When driving for some time, I noticed that the van always takes a "big breath" when opening the gas cap, which is probably caused by the built-up (negative?) pressure.
I'm trying to find a solution without having to remove the engine/fuel tank for now (will be done in future for other jobs, but still want to drive the van this summer season).

The diagram I found from GoWesty at https://gowesty.com/blogs/schematics/fuel-tank-breather-system-syncro

As there are no pressure tanks like in a non syncro, Is there an easy way to diagnose the problem?
Hose #7 on above diagram is only the overflow I would presume? (as this is easily accessible)

Thanks!

[DuncanS]

You have a plugged vent and can collapse the tank. This should help.
https://www.thesamba.com/vw/forum/viewtopic.php?t=606047

Duncan

[MarkWard]

I could be wrong, but the above diagram is for a diesel. The tell tale to me is the loop of fuel hose in the filler box to atmosphere. Gasoline engines would have had the charcoal canister tied to the vent and purged into the intake. Theory was to contain hydro carbons from venting to atmosphere.

The plastic valves in the top of the tank are normally open. They allow the tank to equalize, but if the van ended upside down, they close by gravity. They are sometimes referred to as roll over valves.

I’d start with the hose connected to the charcoal canister. Find the hose from tank and verify it’s not plugged or crimped. There are a lot of hoses on top of the tank and it’s possible if the tank was in and out, one may be caught between the tank top and the chassis.

[DuncanS]

Bentley 20.34 has a full diagram with descriptions of the parts. Should you have an open vent, one that a wasp can fill up, then you will not be able to pull air into the tank to replace the volume of the fuel used. I have had wasps plug lines and holes in my boat many times. It's very firm and has to be drilled out to open; not just some stuff loosely stuffed in. Bits of sand all glued together with serious wasp glue.

Duncan

[DuncanS]

[termuehlen]

If you have a charcoal canister, that hose routing looks like this:

The hose coming out of the bottom of the canister is just tucked into the vehicle frame rail (even though it is labeled "to air cleaner") . A properly vented Syncro fuel system should not have any pressure in the fuel system, the bottom of the canister should be open to the atmosphere. You could work your way back through the system to try and find any "blockage".
_________________
1988 Westfalia automatic Subaru OBD1
1986 syncro tintop wbx

[StefDS]

Hi all, thanks already a lot for the replies.

Yesterday I did a 50 km (31 miles) trip, and again checked the pressure when releasing the fuel cap, but this time no pressure was built up
Previous 4-5 times i've checked it after a trip there was, so the lines "automagically" cleared up I guess?

Anyway I'll have a look at the charcoal canister and the fuel lines around it to inspect the lines.


Thanks again.

[DuncanS]

[joetiger]

I had the same ongoing problem with my Syncro. All my fuel lines and valves were new. In extreme heat, gasoline would even force itself out of the fill port.

I replaced the charcoal canister and that fixed it.

https://vancafe.com/evaporative-emission-canister/
_________________
Joe T.

'86 NAHT Vanagon GL Syncro/ supercharged ABA 2.0 "Pigpen"
'91 Wolfsburg Carat "Barchetta"
'02 Baja-ish Beetle "Bubbles"

"get metaphysical with it. if it's simply a means to get to and from places, it will let you down. if it becomes your zen, it can't fail you." -dabaron

"Still, it's good to be afield."--VWagabond

Check out my book: Vanagon to Louisiana--A Two-Lane Reckoning Through Past and Present

www.josephtrussell.com
www.pigpensyncro.com

[StefDS]

Well, I finally got around on dropping the engine & transmission to get to the fuel tank. After reading all the horror stories it went fairly smooth (the actual dropping of the tank only took 15 minutes)..

But I guess it was taken out by a previous owner because the 2 plastic vents on top are missing and a lot of black silicone is used to plug the holes.. I can only presume the PO broke the vents off when dropping the tank, and they were not available new at the time..

Tomorrow I'll do some dissection of the silicone and hope the 2 holes are not damages, so I can put some proper vents in..

Picture of the top of the fuel tank, with the 2 vents missing (the 2 plastic knobs that hold the fuel lines lines are also broken off):

[4Gears4Tires]

How did your van run right? Eventually there will be a negative pressure and it will reduce fuel flow to the motor?
_________________
'87 Syncro Ferric Oxyhydroxide Superleggera Edition
'85 Westy Sciuridae Domus Edition

[Sodo]

Strange that your tank came out in 15 minutes. Was it “dry”?
Mine was such a fight that I’d never want to do it again.

Maybe some tanks are bigger, some vans are bigger.

Its feasible that a new tank is smaller, then it “grows a little“ the first time you put gas in it. Then after the tank is empty, and aired out for awhile, does it shrink a little?

Anyway, if you have trouble, plug a shopvac in and vacuum physically shrinks the tank size.
https://www.thesamba.com/vw/forum/viewtopic.php?p=10351785
_________________


'90 Westy EJ25, NEW oil rings (!) 2Peloquins, 3knobs, pressure-oiled GT mainshaft, filtered, cooled gearbox
'87 Tintop w 47k 53k, '12 SmallCar EJ25, cooled filtered gearbox
....KTMs, GasGas, SPOT mtb