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Discussion Starter #1
I’ve had this issue since buying my 1987 a few years ago.
Each time I go to fill my tank, there is a big gust of fuel vapour that comes out when I remove the filler cap, followed by a crumple sound of what I’d expect to be the tank relaxing. Its definitely an outward gush and not inward suction. Last night standing in my garage I heard the tank thump, even though the car sat in the garage unused for 24 hours (morning ambient was about 40F and it went to about 70F later in the day).
On an earlier attempt to figure things out, I checked and cleaned the check valve and air inlet at the evaporation canister in the trunk. The hoses are new back there, as are all the hoses in the engine bay (including to the charcoal canister).
I even pulled the clear line exiting the evap canister leading to the front of the car (the clear one) and blew air into it, and felt and listened for air at the hose that enters the air tube over the engine, that comes from the evap canister behind the passenger wheel. I did feel and hear air.

Am I missing something?

The car drives fine. I just fear eventually stressing a seam in the tank.

Thanks
 

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The small black plastic tank like object (canister) above the fuel tank, on the left side of the shelf where the jack is stored, is a large part of the evaporative loss control system which vents the fuel tank. This canister has some one way in-line valves in it as well as a bunch of hoses, one of which goes up into the engine comartment and connect directly or indirectly to the fuel injection feed line on the engine. You obviously have a problem with this loss system and your concerns about stressing the tank are well founded. This condition should not be occuring. I would leave the cap off or at least loose until I could determine what the problem is. Since I have an extra fuel cap I would drill a small hole in the extra fuel cap and use that until I could determine the real problem and get it corrected.

Go to the Technical Digest post, section #4 (fuel system) and thread #3 (pressure in fuel tank); and read up on the evaporative loss control system.
Rbert
 

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that's an odd one Mike, as you seem to have checked the necessary lines on the evap. canister.

What about the little tube actually inside the plastic evap. canister, the topmost connection, marked here in red (these on earlier spiders were metal and do rot and block up)...though i must say my S4 has a hard plastic tube and so can't really rot/block....dunno, maybe worth checking.
And yes, the air inlet valve has to be the right way up (dont ask me which!)...I think there is a little floating plastic diaphragm in this, which when upside down 'might' close? (that is a guess!)

I do also get a whoosh out the tank, but only a slight one and not always, depends on the weather with mine!
Dom
 

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Discussion Starter #5 (Edited)
I definitely have the check valve and air inlet in place, functional, and in the correct orientations. But thinking more about it, if I've confirmed there are no obstructions in the lines running to the engine, and the check valves and inlet work, the vapours are likely not entering the evap canister, making the inlet port to the cannister the possible culprit. I think I checked the flow of air into and out of the trunk mounted canister, but maybe I didn't and will check that tonight. If that checks out ok fine, then leaving the cap a little loose is a good band-aid for now.
 

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i'm curious.... how do these tanks get pressurized ? the return line puts back less fuel than is taken out... even if the vent system is bad, what is making the pressure ?
 

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Mike: as a couple of people have said, if you've checked all that other stuff, it's likely the metal tube in the black separator canister. Take it out of the car and disassemble it and you'll probably find your problem.
 

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Although this suggestion smacks of perpetual motion, I think that the only way for the tank to get that pressurized is if the main fuel pump is over pressurseing the F/I system in the engine compartment and the returning unused pressurized fuel is pressurizing the tank. There is a fuel pressure regulator on the end of the injection fuel rail, which allows proper fuel under proper pressure to the injectors and then returns the un-needed fuel to the tank.

Back in the day when I was wrenching on BMC and Standard Triumph cars, the charcoal canisters would become blocked and the cars would start pulling a VACUUM on the air space above the fuel in the tank. Since the filler caps were air tight (as is your Alfa), when the vacumn pressure reached the same % as the fuel pump pressure exiting the tank, the fuel stopped moving forward and the engine died for lack of fuel. The cars would experience this usually on the interstate where they were moving at high speed and the engine demanding a lot of fuel. Once the engine died it would not restart. When the cars were towed in, I would take the filler cap off and the car would start and run perfectly. The solution was to replace the charcoal canster, or drill a hole in the filler cap (illegal).

It seems like you have a somewhat similiar, but reverse situation. Too much tank pressure as opposed to too little (vacuum).

Robert
 

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But Mad North-Northwest
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Can't be anything to do with the fuel pump since that's just a loop.

Pretty sure it's just the vapor pressure of the gasoline. I'm sure the fuel heats up as you drive from being circulated through the engine compartment, which further increases the pressure.
 

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maybe shoot some air thru the line #11 in the trunk, in diagram above (that is the one that runs under the carpet, and might get kinked) and check it comes out of both #13 and #6 at the engine (maybe first blocking one off, to check the other and vice-versa?)
this should prove the charcoal canister is not all blocked up.
 

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Tom,
I love and respect you and your knowledge. But mcola lives in Onterio and I live in Memphis TN where it gets brutally hot in the summers. I have never had anyhing close to the problem he is relating, with either of my 88's. There has to be someting more going on here, although I am not sure exactly WHAT it is that is going on.

Robert
 

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maybe shoot some air thru the line #11 in diagram above (that is the one that runs under the carpet, and might get kinked) and check it comes out of both #13 and #6 at the engine (maybe first blocking one off, to check the other?)
That was the problem when I had these symptoms.

My son had a tank pressurization problem with his Milano which was due to a bad exhaust leak which heated the tank. It was bad enough to make it run rich and make it hard to start when it was hot.
 

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But Mad North-Northwest
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Robert, I was explaining why pressure would tend build up at all in the tank. Not saying that the overpressure that Mike is seeing is normal.

As I said, given the other stuff he's already looked at, my money's on a plugged metal tube in the black vapor separator tank.
 

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Discussion Starter #14
maybe shoot some air thru the line #11 in the trunk, in diagram above (that is the one that runs under the carpet, and might get kinked) and check it comes out of both #13 and #6 at the engine (maybe first blocking one off, to check the other and vice-versa?)
this should prove the charcoal canister is not all blocked up.
I did originally confirm the flow of air to #13 using compressed air, but didn't think to check at #6. Seeing as things are fine at #13, is the logic that if the canister is not under vacuum at #6, the vapours are not going to be pulled thru the check valve in the trunk. Lastly, just to make 100% sure... the arrow on the check valve runs towards the engine?
 

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When you get the answer be sure to post it. My money is on Ed Prytherch's answer. He always knows his stuff and since he has been there and done that, I am betting on his answer being the problem.

Robert
 

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Lastly, just to make 100% sure... the arrow on the check valve runs towards the engine?
yep, that is my understanding at least: excess vapour/pressure in the tank (better than negative pressure/vacuum, as that would make the fuel pump struggle) is exhaled at some point (no-one seems to know when!) thru the check valve #3, travels along line #11, enters the charcoal cleaning canister and then (somewhat filtered) out thru line #13 to the inlet plenum nipple, where it gets recycled into the cylinders.

Now (just a thought here) what about that nipple on the intake plenum?.....clean and open?
 

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Generally it's always the red tube marked in the post above . Its got a pinhole opening and does get blocked even though they are made from copper . You can remove tube and fix hose directly . I would even just initially take vent hose a by pass the one way valve and connect to cannistor . The one way valve only works if you're upside down and the way I see it fuel can't flow out if you're upside down as the breather pipe will be higher than the tank . If car is on its side its still won't help as the ball needs to be on top and not on side for it to work . Just my two cents . But I did solve this problem before . Maybe you're cars are different in USA compared to uk
 

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Just an odd afterthought, but have you ever looked at the main pump under the passengers side aft of the seat (on the bottom of the car). Can someone have installed an oddball pump producing too much fuel pressure? As a corollary, have you checked the fuel pressure at the fuel rail? I ran into this kind of a problem on older carberator equipped cars a few times. An improper aftermarket pump producing too much pressure overcame the float and needle and seat in the carb and made the car run ridiculously rich.

I realize that Alfas already use a high boost pump, so this seems a bit far fetched. But you say you have always had this problem since you bought the car, so you can't make any assumptions regarding previous maintanance. I have seen some foolish crap installed on Alfas (and other models too).

Robert
 

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Discussion Starter #19
I came home and tried a few things before logging on...

The evap tank in the trunk is fine, no obstruction in the copper, and both valves are working. Since I knew I could send compressed air from the rear to the air tube, I jumped to the vacuum hose off the plenum. I pulled it off and sucked on it, and no problem. Then I started to prod at the nipple, which has a very small hole and I was either hitting an obstruction or the wire I was using was too big for the hole. So I figured I would start the car with the hose off and listen for vacuum leak. No sound! I felt for suction and nothing. I think I'm on to something. Dom, I just read you going in that same direction!!!

I will have to see what's going on in there. I confirmed vacuum at the ovs nipple, and the car is running, so I have vacuum at the fuel regulator.

So I not only wonder if I have the right hose on the right nipple, but if the right nipples are even installed on the right ports on the plenum?
 
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