Gas tank vacuum

I have an '83 308 GTS QV and I get that, too. So I wondered about this as well.

Let's see if other people chime in.

 

No vacuum in my '87. I do not recall ever hearing it hiss (original owner) but have heard it in other vehicles of mine. So I do not know whether a vacuum should be present with this vehicle or not.

 

No vacuum in my 84 GTS QV fuel tank either

 

No vacuum on my 84 308. But it is a slow process filling the car up with fuel. I don't think they vent well while refueling. Wondering if it's related to the vacuum issue?

 

Guys check your emission system. The tanks should vent when you turn the engine off . That's part of the charcoal canister system. The gas cap is non-venting unlike the earlier carb cars.

 

Depends if its a US or Euro car. Euro have a vent pipe open to air.

 

What version/status is your 328? US, CH, Euro, Euro correctly modified for US importation, Euro originally converted for US importation then remodified, etc.?

Euro are always vented to atmosphere as Andy indicated (via an open tube on the filler neck) so they should never have pressure nor vacuum in the fuel tanks. On the US version, the charcoal canister both lets fresh air in if a small vacuum develops or lets air out thru the charcoal bed if a small pressure develops, but the valving doesn't operate with no pressure differences so a little vacuum or a little pressure is normal. Since you say "incredible amount of vacuum" have you looked at the walls of the fuel tanks to see if any have buckled inward?

 

I haven't been over that part of my 328 (either on the car or the parts list), but I had the tank vacuum issue on my '79 Alfa Spider.

On the (SPICA) Alfa, tank pressure would be vented through the charcoal cannister, but vacuum would draw air through a one-way vent in the fuel lines in the trunk area. When the little plastic vent failed, the tank developed vacuum. But the SPICA Alfa has a weird fuel system, as it constantly circulates fuel to the injector pump and back, a bit like an aircraft fuel system.

I'd expect there to be a similar one-way valve somewhere in the US 328's fuel system, to let air in without letting gas vapors out.
I have seen there's a cannister assembly above the right hand tank, on the parts diagram, but I haven't delved into the functions of it.

 

Wonder if that is tough on the fuel pump?

 

I have a vacuum situation on my 1981 NA GTSi.

 

Any updates on this? I also have a vacuum build up

 

FWIW: I can report that I have neither vacuum nor pressure in the fuel tanks on my September 1988 US Spec 328 GTS. It would have shown up on the 1700 mile eclipse watching trip I just drove.

 

Vacuum or pressure? There is a valve that vents the tank. It does not allow vacuum but does allow pressure to build to about 0.05 bar. The valve is present on US 308 and 328 cars, and some others. See item 16 on this diagram.

https://www.ricambiamerica.com/car-diagrams/ferrari/v6-v8/328-group/328-1988/antievaporative-emission-control-system-for-usa-sa-and-ch88-version.html

 

Yeah, I have a US spec 1988 328GTB and I get the vacuum as well, never caused an issue with my car, that I can tell, I've had it since new.

 

If you guys are actually getting vacuum in the tanks you have a problem. The valve I pointed to in post 17 does not allow vacuum. It allows the tank to suck air into the tank through the charcoal canister. And it only allows pressure to build to 0.05 bar. So, if you actually have vacuum you have a problems somewhere in the system.

 

try removing the charcoal canister and whack it a few times. The charcoal settles and pack like sand given time. This will loosen the charcoal to allow better ventilation.

 

Thanks for the info and diagram. My car is 85QV and cannister appears to be the same. I notice on the diagram that there is a line that connects to the intake plenum which I assume would be drawing air from the cannister thru the fuel tanks. Therefore is this causing a buildup in the tanks?

Would it be ok if I disconnect the vacuum line to the plenum and plug the plenum connection or am I creating more problems?

Other option can the cannister be eliminated and just reconnect the vacuum lines?

 

I have a related problem. When I bought my 84 308GTS almost a year ago it wouldn't take fuel-kept chucking it back. I replaced the fuel lines (had to drain the tanks) and the problem went away. Now it's back and the car is running poorly.

I have determined that fuel is not flowing between the tanks. I have a feeling it's must be in the vent lines as referenced in previous responses.

 

The canister is vented to the atmosphere. Plenum sucks air through the canister from atmosphere, thus purging the vapors stored in the canister. Tank also draws air through the canister and should never have a vacuum in it if the vent line, valves and canister are in proper working order. The tank may pressurize slightly and will vent over pressure out through the canister as well, provided the check valve is working properly.

 

Right, so if there is any obstruction in those lines or the check vale has failed it could result in a problem with the fuel delivery system.

Assumptions:
In order to put fuel into the tanks air must escape.
In order to pull fuel out of the tanks air must enter the tanks.

If these assumptions are valid I think my situation involves both those scenarios.
Will start in on inspections tomorrow.

Thanks Johnk

 

The problem with filling is common and not related to those valves. It's simply that with some pumps you can fill the left hand tank faster than fuel can be transferred to the right hand tank, especially as the tanks get fuller. The problem can be mitigated to some extent by not inserting nozzle into the filler neck too far, angling it in a different orientation and not trying to fill at the fastest nozzle setting. I've experienced the same problem, over my 32 years of ownership, since the car was new.

If you have a problem with the way the car is running, I'd look elsewhere. Once there is fuel in the tanks, the fuel pump is going to determine the pressure in the supply line to the injectors unless there is something preventing fuel from getting to the pump.