Gas cap vent question

[drddan]

On an 88 Festiva, if the gas cap vent is non-functional, can the vacuum created in the tank, stop fuel flow, to the engine mounted fuel pump?

My Festy stopped running Sunday. No gas being squirted into carb when cycling throttle. NEW fuel pump and it has 3/4 tank of gas. I removed gas cap and there was a huge vacuum on tank, which sucked in a lot of air when removing cap.
No fuel being pumped to carb from fuel line at carb. Fuel pump trying to suck my finger in the inlet side when cranking engine, so it is good.
I just checked/cleaned entire fuel system and it is clean tank to carb, including new filter. I was in the tank and it looks new inside. So...
should be getting gas. This car was running great and then this happened out of the blue.
Is the cap stopping it from getting fuel down the fuel line?

[bravekozak]

interesting

I was going to ask that very same question but you answered it for me.
The question was is the Festiva gas cap vented or non-vented. I assumed it was non-vented. The reason why I was asking, is because I threw out all that vapour separator garbage and capped the nipples. I don't want my fuel tank to collapse. I will have to buy a Stant vented gas cap.

Take off the outlet hose at the pump, crank the engine and see if it fills up a big glass bottle. That was you'll know if the gas is getting through or not.

[drddan]

I did remove fuel line from carb, cranked engine---no gas coming out. Thats when I unscrewed gas cap, and it had a huge amount of vacuum and sucked a lot of air into the fill tube. Now I cant seem to get fuel up to the carb and the fuel pump is new and is trying to pull fuel from the tank. Putting a finger over the inlet side, there is suction on the inlet tube.

[DrMaxwellEdison]

can't hurt to try can it?

[MTec007]

actually i dont think that the gas cap is vented. my 88 does the same thing when removing the gas cap, and it runs fine, i would think your problem is else where like a clogged line possible, or a leaking hose

[WmWatt]

While the line is disconnected try blowing down it to see if you can clear it?

[drddan]

WmWatt, I will refer you to post #1. Entire fuel system is clean. Lines, tank, etc...all.
I think I have found my problem, though! Negative back pressure from the cap not venting. It only appears to do it only with the following conditions:

After a complete fillup, when its gets to 1/2 tank, it has built up enough pressure to "suck" the gas back into the tank. Enough that the system looses it's prime and cant pump gas to the carb. It will not do this, say, if i'm really low on gas and stop to put in a quick 1/4 tank, or if I fill up, it is okay until I reach the 1/2 tank mark. It has done this to me 2-3 times now, and I think may be the problem I have had for a while now, just didnt realize what was going on until now.
I am wondering if this can hurt the engine mounted fuel pump, as it is putting quite a bit of reverse pressure on it, when it does this. Could this be messing up the fuel pump, too? Like to much pressure on the diaphram inside the pump? I just bought this one and can return it for another. I am having trouble getting fuel to pump back the the carb now. AND YES, the line IS clear. Disconnected both ends of the fuel line and blew it out with air.
Unscrewing gas cap when it has built up pressure is unbelievable! The amount of suction is really high and goes for 2-3 seconds while unscrewing the cap.
Here are some threads starting from a month ago when this problem started, and this same thing happened then. I thought this problem was acting "weird" from the onset. Was troubleshooting carb, and electrical.

http://www.fordfestiva.com/forums/sh...ad.php?t=30665
http://www.fordfestiva.com/forums/sh...ad.php?t=30959

[MTec007]

just how new is this fuel pump? i had a problem with mine when i replaced mine, there was air in the fuel line and it prevented fuel flow

[drddan]

Just bought it 3 weeks ago, when I had this same kinda thing goin' on. Bought it just so every thing was new on the fuel system. The old one was only a year old. I'm thinking I need to "prime" the lines and pump with gas. Like you said, get the air out of the line.
I had to do that when I installed the new pump, and blew out the fuel lines. I took a small squeeze bottle filled with gas and filled the line from tank to fuel filter with gas. Then it worked. May have to do that again.

See post #7

[Pu241]

So when you let air into the tank by opening the gas cap, does it run normally afterward?
Have you done this with out putting gas in it?

[drddan]

No. It has siphoned the gas out of the fuel line, back into the tank, so there is no gas in line.

[Pu241]

After equalizing the pressure (taking the cap off) and a few cranks of the starter, fuel should, if this is the problem, be getting to the carb without issue.

[drddan]

Thats why I was wondering if the reverse back pressure would hurt the fuel pump. (engine mounted mechanical on carbies)
But, I think it may need primed. When I put the new one on, it needed primed.

[drddan]

By primed, I mean the whole length of the fuel line, from tank to fuel filter, needed to be filled with gas. Filter is in engine compartment.

< < < OH crap. I'm a SENIOR member now.

[Ogrekhan]

Priming should not be needed. The system should self prime after a few cranks of the starter. I don't suppose you have a fuel cell handy the ones for outboard boat motors work very well. In a pinch you can run a line to a gas can hooked to the inlet side of the fuel pump. I have done this to get motors running many times. I would put a fuel filter inline for safety sake. By doing this you could establish that the fuel pump is not the problem. Did you check the vent line from the tank itself?