After driving around for a while, I can smell fuel vapors. If I unscrew the cap, a fairly large amount of air pressure vents off of the tank. Is there something wrong with the purge system?

Car is basically stock, replaced tank and pump several years ago. All heat shields are installed.

 

This is a common problem and there has been a lot of discussion about here. If you go do a search on the subject, you'll find that a number of things can cause it. Some pressure build up is normal, but if it's enough to start venting out the gas cap where you can smell it's too high. That's the problem I've been chasing for a while, and I've narrowed it down to clogged vapor vent tube in the tank or in the tube leading to the vapor canister. I'm going to replace my fuel pump, sender/tank pipe assembly soon. Hopefully that will fix my problem. In your case, I would read the old posts on the topic and start checking the gas vapor system for problems they talk about in the discussions.

 

I've been dealing with this for over 10 years. Check under your plenum and make sure the small black hard plastic hoses are not burned through. They run off vacuum and open the vapor canister when required. I had a hole in mine and the valve wasn't opening. The test in the book for for checking EVAP will not reveal this, you have to look for it. Also, I took apart my rollover valve in the tank which is connected to the vent line. I removed one ball bearing and enlarged the holes in it considerably, then epoxied it back together. I now get a little puff out of the fuel tank when opening the cap. It used to launch clear across the parking lot. I'm not kidding. The only thing I can attribute to this is a change in fuel chemistry, because this problem is happening on modded as well as bone stock cars.

 

Some pressure is correct- 2psi is designed into the system. More than that, and I would suspect that the vapor lines might be plugged/pinched off. It also sounds like EDGE found another problem that was causing this as well, so you've got a couple of potentials to check out.

 

I have found that if you disconnect the hose going to the charcoal can that is marked "tank" and blow compressed air thru it (remove the gas cap first!) it sometimes works. spiders and bees sometimes get in there and , well stop it up

 

Ive had a pressure problem on mine since repairing another problem ! since ive had the car its had no charcoal canister and the metal pipe has been capped off in the engine bay, this has never been a problem......until i repaired the crack in my filler neck, which has obviously cured the smell of fuel and stopped the leak but now when i take the cap off at the petrol station i get a huge whoooosh like its got loads of pressure.

Im guessing that capped line is the problem now, maybe i should look at putting a charcoal canister back on the car ?

 

there is a white colored pressure relief valve by the axle that is the cause most of the time.

 

If the Vapor Line is capped, pressure can build in the tank. It's designed to run at 2 psi, to help prime the fuel pump. Any more pressure than that, and the valve opens on the vapor cannister line to allow absorption the fumes. If not allowed to vent, it can create enough pressure to split tank seams ( maybe what happened to your filler neck).

My application does not have the white pressure relief valve mentioned, so I can't comment on it's purpose or ability to solve your problem.

 

I did a lot of research on this. As best as I can determine that thing is a vacuum relief valve to let air in as the gas is sucked out and it's not supposed to let raw gas vapor out of the tank. Some people have posted that it is supposed to vent pressure out of the tank, so I looked for months and found a new OEM replacement. The new one made no difference to the pressure build up in my case.

 

Yeah i heard its a one way valve only after i took mine off and cleaned it too, which made no difference. so it only draws air into the system.

Im not sure what to do with mine then, maybe just take the cap off the pipe and let the fuel vapour just vent out naturally, which might make it worse if theres no pressure ?

 

Im just thinking instead of trying to get another vapour canister for my car to get it to vent slightly, if i just change my non vented gas cap for a vented one that will do the same thing in theory ?

 

Do you have the part number to white pressure relief valve?


Maybe its better to block off all vacuumli lines going to evap and just vent tank to air some how?

 

Vented gas caps don't let the pressure release, until you open them. They are meant to allow air in, not out.

 

Ah right, well thats that idea out the window then, lol

 

I've been running without a gas cap for a while after my engine was hydrolocked on a hot summer day due to the tank not venting so it pushed gas through the carb.

Well I started looking into it tonight, pulled my line that goes from the canister to the tank and it was plugged solid with RUST, so I cut off about 4 inches of the metal line to get rid of the rusted part. I can actually smell gas now so I know the rest of the line is clear. I removed the vacuum operated valve since it didn't have a vacuum line going to it anyway, I have a carbed 350 with no emissions devices.

I put a longer rubber line on it and I'm just going to leave it vent into the air, should work fine.