My race car had been used for the last few years by a friend. When I got the car back the gas tank was full of rust and when I pulled the tank all the vent return line stubs etc. were plugged. To me this left only the cap to vent the tank. I had bought a new filter/vent piece from GM when I put the car together. the car runs a Q-Jet with a Holley Black pump and HPG-1 filter before it. It has a fuel pressure guage but I didn't get much feedback on numbers it has shown recently. Doesn't this setup need to have more vent capacity?

 

Unless you put on a vented cap, then yes it needs a vent.

 

So the cap alone would keep a vacuum situation from happening? Why is there another vent on the stock setup?

 

The stock cap is unvented so that the charcoal canister can collect the vapours instead of having them vented to the atmosphere.

 

I remember the part about the canister. Ths is is the same crappy gas cap that was on the car when it came from the junkyard-unvented. So stock this car came with an unvented gas cap, vent capability through the canister, and an additional filtered vent coming off the pickup on top of the tank? I thought this part that comes off the pickup was filtered to not let dirt in and vapors out?

 

I think the vent UFO on the bottom is to let air in when you use gas, so that your tank doesn't have a vacuum. The charcoal canister is when your gas tank pressurizes. I think.

 

I called the guy who had the car. He said there was a vacuum on the tank when you opened the cap. I've seen other cars that have that when you open the gas cap. Does that mean they also have a problem with venting? It would seem this could be a problem when the canister goes bad or the cap correct? I remember when I bought that vent filter thingy at the dealership it cost like $25. They don't sell it at parts stores I looked at, but maybe NAPA?

 

A Vacuum? My car 90% of the time is pressurized. Air comes out, not goes in. I've never delt with a carb though. Maybe it's different. But I guess if you did have a vacuum, the little vent on the bottom of the tank would be a place to check.

 

Some pressure is normal (if that's what you mean), and so is the "whoosh" when you remove the gas cap.

JamesC

 

Thanks for the input guys. Final question on this subject: When I rebuild the pickup to eliminate the rusted pickup tubing should I reopen the vent I originally had in it? Never had trouble with it and car ran 12s, but performance has been down lately for many reasons which is why the car is back with me. The guy says he didn't plug all the lines, doesn't know who did. So there is no known timeline for the change. Starting with a new tank also.

 

What was figured out with the vent?? I am having the same trouble. I have an 86 Iroc Z and I have already replaced my fuel pump because it completely burned out. The new Bosch fuel pump is now whining in the tank because the tank is over pressurized. Sounds like I have a dying owl in my tank. Happens more frequently when its hot. So as long as im not driving very far for too long then everything is fine. If its a hot day then after a few miles the pump starts to whine and I have to pull over and open the cap, every few miles. I asked around for the vent and not even the dealer could get me one. Nappa had no Idea what I was talking about untill i had the guy crawl underneath the car to show him. I was also told by some part stores the there is no vented cap for my car, but nappa said that there was. Im confused and I would like to be able to drive over 50 a a day. other than that the car runs great. What has been done to solve this tank vent? should I close the line and get a vented cap, or just take the little plastic piece off and leave it open??

 

The vent is only for vacuum. It lets air into the tank, not out. But me and you have the exact same problem. I had to do the same thing here yesterday on a nice balmy 103* day.

When your fuel runs low, the UFO on the bottom of the car lets air in to make up for the fuel volume lost. The only place I know of that pressure goes to is into the vacuum canister up front. I think my line between the canister and the tank is clogged, pinched, or something, because even removing the fuel line from the canister won't relieve the pressure.

 

On the carburated version there is a check valve between the fuel vapor canister and the metal tank vent line. It is a spring loaded diaphragm that is "fully opened" by vacumm. It allows pressure and excess vapor in the fuel tank to vent into the canister, there is a second pick up line for the vapors to be pulled from the canister to the carb, it is common for people who remove various emmisions equipment to plug the vacumm control on the check valve itself. Bad idea. Not only does this cause fuel pressure to increase in the tank when its running but parked in the sun the tank can only force expanded fuel vapors through the fuel lines, flooding the carb and intake manifold by pushing fuel thru the fuel pump wether the car is running or not. My experience is that occasionally that check valve may fail, or if it was capped off then it needs pulled out and blown thru by mouth to make sure it isn't stuck and suck on the vacuum side to feel the diaphragm move up and down. The check valve is located under the hood between the metal tank vent line and the fuel vapor canister. Your stock fuel cap is a pressurized fuel cap meaning that it will allow air in if the tank is drawn from but will not allow vapor out into the atmosphere, its a one way check valve of its own and is designed to open when vacumm is "above a specific value". A vented fuel cap allows vapors and pressure to relieve in both ways in and out as needed like the cap on a dirt bike or lawn mower. It is designed to be more of a splash guard than anything if its "vented" ala '70s. Also that check valve is spring loaded to a specific load rating before it opens completely, this is to help fuel pick up in the tank, especially when its low by utilizing a low amount of positive pressure around 12-14psi in the tank, to help push fuel thru the pick up line to the fuel pump and prevent it from backflowing when the tank is sloshing around or the pick up is sipping on the last couple of gallons.