Gas tank suction

[timspider]

I've gone through posts going back to 2008 about this issue but didn't see the specific solution. So here goes:
Just installed electric fuel pump when mechanical finally failed. But now I notice suction building in the tank when capped. I had the cap loose (forgot to re-tighten)when I first ran the new pump last night. Car ran well except for rough idle.
then would not start this morning, even with cap loose. Tightened cap and tried to re start. Nothing.
My main question is how do I go about reducing that suction in the tank. I have a return line on my carb hooked up via a fuel filter with a spot for it. I'm guessing that line is not breathing properly?
Should there be any back pressure in the tank at all?
Does loosening the cap achieve the same pressure release that the return line is supposed to provide?
My shop manual has very basic info about the tank with none of the details I need.
Thanks in advance.
ps: I was just bragging to my girlfriend last night about fixing the fuel pump so hoping to get this rectified before she notices.

[bradartigue]

How much pressure is your pump producing (what is the design pressure)? The filters with the return integrated typically take about 3.5 psi to return, so that the float bowl will fill, then the needle seats, then pressure builds up and fuel returns through the filter. Pretty simple. Even if the return isn't working the miniscule pressure of the stock system isn't enough to pressurize the tank. That's why I ask, how much pressure is your pump supposed to produce?

I sucked in a tank from overpressure once, but it was because I accidentally connected the vapor line to the carburetor. Duh. If your vapor line is plugged then unplug it, let it draw in air and you won't have a pressurization problem.

[timspider]

where is vapor line? That's not the return line, correct?
I've got a little cluster of 3 lines at the tank, 2 of which are connected to each other and the middle one is blocked off.
Here's a picture. Is one of these a vapor line?
Do I need to undo something here?
I'm not gonna lie, this is getting exhausting.
But still appreciative of the help. Thanks Brad, Giuseppe, and everybody.

[timspider]

It's rated at 2-4 psi at 30GPH- electronic facet

[4uall]

this might help http://wwwe.fiatspider.com/f08/viewtopi ... +gas+lines

[timspider]

Yes, that helps me to realize this is yet another can of worms to open.
Stupid question. Are the vapor line and charcoal canister needed?
I ask because I've driven this car 10,000 miles in the last year without any of this stuff hooked up.
If it is needed, it looks like I need to re - make the whole system.
This is making me grumpy.
gotta go for a deep breathing walk.

[4uall]

There are different schools of thought on this I am sure others will chime in

Enjoy the walk

[timspider]

Looks like I've got some work to do.
arrggh.

[timspider]

Whenever i see "this might help" from Jay, it's a link to precisely what I need to know about. Thank you Jay.
next dumb question, is it OK to pull that check valve and just bypass it to test for a blockage there?
Or will that create a disaster?
And lastly, aaarrgh.

[NSFIAT]

One of the first things I noticed after driving my '81 for the first time was a crazy amount of suction from within the gas tank, to the point where the rush of air inwards was pretty violent when loosening the filler cap.

Earlier this year I had to replace some components of the fuel system, from the tank to the pump. I decided to tackle the lines, including the fuel vapour lines which have been capped off or cut in your case. The metal lines are still in good shape, and the rubber lines were 'ok' but I decided it would be best to start fresh. When I got to the fuel vapour check valve (not the fuel vapour union that is located in the trunk, but the one located under the car) I noticed an unreal amount of grey grease that had built up on either end of the valve and within the valve itself. It took a while to clean it out, but the value appears to be working the way it should at the moment.

I also decided to pull the carbon canister in the engine bay. The lines in this area were in rough shape and the carbon was pretty much saturated - I've never smelt something that sour.

The loop has been completely replaced and since running the car again I haven't noticed any issues. It also smells a lot better . . .

[carl]

That picture showing the three nipples on the tank show that that one is plugged and the two others are connected to each other. These are part of the tank vent so your tank is essentially non vented with that setup. You can just take the plugged one and run a few feet of fuel line off it to let the tank vent and see if that solves the problem. On my rat spider I did that, including putting an inline fuel filter on it to reduce gas fumes and then ran the hose out the bottom of the trunk.

[timspider]

thanks Carl. that helps. I tried letting the middle vent run through a tube that runs to the hole for the bumper shock.
same result. cranks and cranks with the occasional slight starting rumble, runs roughly for a few seconds, then dies.

I'm really hoping to get this figured out without a tow to the local garage. I can't afford another one of those. Incredible day for a drive today and here I sit, trying to figure out my new fuel pump.

[majicwrench]

If it won't start, this isn't a fuel tank venting issue. Is it not pumping fuel?? Why??

[timspider]

update: pulled the gas line off at the carb and put it in a plastic bottle per something I saw (I think from beek) and had a friend turn the key to the on position while I watched the bottle.
pump engaged when it was supposed to, at on position before cranking, and after a couple seconds a nice flow of fuel came out.
So gas is getting up to the carb.
dumb question. How do I test for spark?
Isn't that the next thing to make sure I have?
also, when I had a friend cranke earlier and looked into the carb barrel, I didn't see gas pumping in.
So something with the floats in the carb maybe?
thanks much!

[azruss]

you have a needle valve that controls fuel to the float bowl. If it is stuck, all the fuel goes to the return line. There is a screen filter at the needle valve as well that could be clogged. to check spark. pull each of the plug wires and one by one plug it into a spare spark plug and ground the plug by putting the tread section on the valve cover bolt. be sure to wear gloves and rubber soled shoe, and dont lean on the body. crank over the motor and you should be able to see the spark.