View Full Version : Venting fuel tank

08-17-2004, 05:41 PM
What type of gas filler cap is a 2nd gen (79) supposed to have? I was working on the fuel lines near the tank and when I separated a section of the supply line I got hosed! I don't mean a little gas siphoned off, I mean it was under pressure like there was a pump in the tank! I stuck the line in a 1 gallon gas can figuring it would eventually simmer down - but it filled the can in about a minute. Then it finally dawned on me to have my dad remove the cap. A couple good Ajax cleansing powder scrub downs and I don't smell so bad, but need to find the problem.

It has a metal cap ("clicker" style) with a serrated edge. Is the cap my problem or is there something else going on here?

08-17-2004, 09:16 PM
Do you know which line it was? Remember that the pump pulls the gas from the tank, thereby creating the syphon effect. Taking the cap off should help though.

08-18-2004, 06:16 AM
It was the main supply line running TO the fuel pump, not the return line. I am used to the siphon effect. With it, a little fuel runs or drips out. The line running to the fuel pump did this and stopped with hardly anything lost.

The section still connected to the tank was under pressure. I mean like there was a high pressure fuel-injection pump in the tank (there isn't). I had to hold my thumb over the end of the line until the gas cap was removed. There was a pretty good "whoosh" of air from the cap and the gas flow from the line stopped completely.

When I got everything hooked back up I left the cap loose and drove up the street - everything was fine. I don't wanna tighten this cap down until I know whether it's supposed to be like this.

08-18-2004, 08:12 AM
I don't know about the gas where you are... but I had a similar issue here and there were two factors involved:

1. Huge temperature swings between morning/evening and daytime

2. The formula used for the gasoline.

Basically, my understanding is this:

As gasoline gets warmer, it expands some. Normally this isn't much of a problem, we've all taken a gas cap off and had it 'whoosh' and that's been just fine for what - 100 years? However, a lot of the newer RFGs (ReFormulated Gasolines) expand a lot more than normal gas did, so they can build up quite a bit of pressure.

The issue that I had had was that in the fall and spring, if I filled my tank when it was cool out and didn't burn more than about 3-4 gallons (yes, gallons) before it got warm out, the gas would expand and would actually push it's way past the (non-vented) gas cap. A lot of fuel apparently was coming out this way, the operations group at my work actually called me and asked me to move my car far away from other employee parking because of the danger (?!)

I replaced the cap and the fuel stopped being able to push its way past it - but the gas still expands when it's warm (hot) out and it still builds up quite a bit of pressure in the tank..

I know my tank is a non-vented system (this is my Elky).. I am not sure if yours should be vented or not, or if it should be I am not sure if it'd be through the cap or a separate tube/hose. Hopefully someone else can answer that part.

08-19-2004, 05:24 AM
I think that is my main question - whether or not I am supposed to have a vented cap. I thought this one was but it obviously doesn't release any pressure. It would have to be the cap because there are no vent hoses on the tank or filler tube. One 3/8" supply line, one 5/16" return line, one filler tube out the back - that's it.

08-19-2004, 11:08 AM
Ok, I had to think a little. I've been working a lot and not been able to think straight. There is a line going from the tank to a charcoal cannister in the front of the car. It should be at the front of the engine compartment, at the drivers side. Basically, that is the vent for the tank. Check to see if it's still there, and see if the line isn't plugged.

08-19-2004, 03:15 PM
This puppy is completely desmogged. No cans, no kitties, no extras. Only what is needed to make it move is left. Both lines (supply and return) are brand new from tank to pump.

08-23-2004, 12:51 PM
When you say a return line, you mean like a return from an adjustable presure regulator? If I under stand right, you have the 3/8" going to the fuel pump and a return line (assuming from a regulator) going back to the 5/16" line, correct?

If so, there is your problem. The 5/16" return line is from a cansiter return to allow vapors to be recovered into the tank and not the air. All due to smog stuff. If your returning presurized fuel from a regulator then it would add pressure above the gas and force it out the pickup line. Remember the return line exit at the top of the tank and the pickup tube is at the bottom. You either need to run a vented cap or vent the 5/16 line and add another return line for fuel.

Hope this helps. :fingersx:

08-27-2004, 06:36 AM
Yeah, that helps a ton chevymike - thanks! :icon996: I am going to try a vented cap for now and this winter I can pull it all back down and rework it. I am sure the vented cap will work for now because I have been running it with the cap a little loose for a few days now and it runs pretty good.

The car didn't have any of the smog stuff on it when I got it so I thought the 5/16" line was just a return line.