Hi all,

I have a 1985 Camaro IROC-Z with a 383 stroker motor. When I put the car together in 2011, I replaced the fuel pump (with a high-flow model) and gas tank. The tank came with a vented cap, which I'm still using.

I don't drive the car much, I usually only get gas about once a year. I went to fill up yesterday and when I unscrewed the cap, a ton of pressure released. It wasn't a small hiss, but a loud whoosh of air that lasted what seemed like a long time. I filled up the car and drove it a bit longer before coming home and parking it. That was two days ago.

I wasn't certain if I had a vented cap or not, so I just went and unscrewed it. When I did, I could clearly hear fuel running down - almost like it was stuck in the filler neck and when I let air in, it was able to finally run down into the tank.

No idea if this could be related or not, but until the car is fully warmed up, it has a hesitation when stepping on the gas. I've always thought it was fuel-related, but I recently adjusted the timing and it took care of 90% of the issue I was experiencing.

Anyway, I'm trying to figure out what else to check or replace or how to test to see if the pressure I'm getting is excessive or a problem. Thanks.

 

Gas tanks are supposed to develop pressure (since the mid 70s sometime), and they're supposed to hold it. In fact, in California, one of the checks they RIGOROUSLY do as part of inspection is, take your gas cap off, put it on some kind of machine, and check it to make sure it holds pressure. If it doesn't then it means that fuel can escape into the atmosphere. Yes, they physically check your GAS CAP even before the visual inspection (aka colonoscopy without anesthetic) and the dyno test. Which, if you've ever been to CA, you'd know, fuel vapor of all sorts combines with the salt air and turns the air BROWN and SO THICK you can't even see through it, let alone breathe it.

So, sounds like you're all good.

 

Is the evap line still hooked up to the charcoal canister? If it is and the purge solenoid isn't cycling like it should be due to removal of the original ECM this is what will happen. The original system was designed to be sealed, but suck the vapors out and burn them rather than let them free. You could potentially find a gas cap that will fit from a pre emissions vehicle, or undo the evap line at the purge solenoid, route it down and out of the engine compartment and put a filter on it. I did this and used a sintered bronze muffler for pneumatic systems. Haven't had any issues with pressure in the tank and don't even notice any gas smell.

 

With a vented fuel system, and fuel that sits in the vented tank for more than 90 days, in a place like we live, you are basically making some sort of wood preservative or spar varnish in the fuel tank. The more volatile fraction of the fuel will evaporate on days like this, and then resperate water vapor back into the tank when it gets down to 70° overnight. That slush of dead dinosaurs and mosquito water will congeal really nicely in January when it's -25°.

If the plan is to store the vehicle more than drive it, reinstalling the EVAP system would be a worthwhile consideration.

 

^Fill the tank when storing. Problem solved.

 

I never disconnected the EVAP system. At least mostly. I do have a question, though.

See the attached pictures. Specifically the 2nd one. There is a capped line - you can see the little rubber plug I used. It's been so long ago, I don't remember what originally is supposed to be attached there. Can someone help?

 

Looks like you're in luck because that appears to be vacuum control rather than ECM control. There should be a diagram on the hood showing how it's all connected. The gist of it is there should be a tee screwed into the intake that opens when the coolant is up to temp and that allows system vacuum to open the purge solenoid (the round hat looking thing). There should also be a big hose that tees into the brake booster or pcv port on the carb, that's how the can and subsequently the tank gets evacuated.

 

You could also have a bad UFO valve.. .that vent valve at the tank.


No air would get into mine so on long drives, the engine would die until I released the pressure in the tank.

 

The UFO valve opens at some ridiculous pressure, like just shy of tank bursting, Thats its purpose, to prevent the tank from exploding.

 

It opens at around 8psi. It is a one-way valve and its purpose is to allow air IN to the tank.

 

The white and grey one in the back? I could have sworn it was listed as a pressure release valve, not a negative pressure vent.

 

It is both. It vents excess pressure, and allows air in when there's negative pressure, i.e. after some of the gas has been pumped out.

 

it's supposed to do both like Sofa sad.


A lot of times it won't let air IN and creates a strong vacuum as the pump sucks fuel.....Its like a capri sun juice pack....... eventually as you suck the bag implodes on itself...

The vacumm can get so strong that the tank "pops in". and the pump cant overcome the vacuum and cant suck fuel.

 

That's actually a great point. I too have had this problem for a very long time. The tank is always pressurized. So if I had enough pressure that you can hear and feel it when you open the gas cap, how then is that UFO valve letting air into the tank? This may be the entire problem with this system. That UFO valve is basically closed to letting air in by the internal pressure. You basically have a gas tank that's like a closed bottle of pop. Air isn't going in or out. Nor is the fuel.

 

No, that is not how this works. The tank is SUPPOSED to hold pressure. This is to keep fuel vapor from escaping the tank to the atmosphere constantly. The UFO valve is designed to allow air to escape somewhere in the vicinity of 8 psi as a general safety measure, but pressure below that is considered acceptable.
The UFO valve only allows air IN when tank pressure drops to a point that it would otherwise become a vacuum. The valve keeps the tank at or above atmospheric pressure. Think back to grade school and warm air/cold air and expansion/contraction. If your tank has pressure in it there is no vacuum condition, which means your UFO valve doesn't need to let any air in and is just holding pressure as a one way valve.

Edit for further clarification:
The UFO is designed to function as a one way valve IN, but has the ability to vent if tank pressure gets a little too high. I have found from experience that the other components of the EVAP system will actually vent excess tank pressure before that point, basically rendering the vent ability of the UFO valve to be a secondary/emergency function only. I have a hard time even imagining the UFO valve ever being pressured to that point (~8 psi) if all other EVAP components are in place.

 

My understanding is that its supposed to allow air in at X amount of PSI to equalize the tank pressure. If i recall it was a fairly low amount. Mine didn't allow any air in at all. It would get so bad that the tank would suck itself in and cause the fuel starvation. I used to hear bubbling which I believe was the vacuum in the tank sucking whatever air / fuel from the pump / line back into the tank.....When I released the gas cap a HUGE amount of vacuum pressure would be released, and you could hear and feel the massive woosh by the cap as it sucked air in....and you can actually hear the tank pop back out a little. I could drive for maybe another half hour before this happened again....


I remember there was a thread about this... people had the same issues with stalling and "bubbling" and theorized the fuel got so hot it was bubbling / boiling in the tank or something like that... I believe the issues were all just bad vent valves cause gas getting so hot it boils seems ridiculous.... it was probably just the fuel getting sucked back that they heard


back when I had this issue the valve was long discontinued, so I had to use the roll over vent valve mod from summit.... but now Hawks makes a replacement vent valve.

 

It could have been both. A bad UFO valve that doesn't let air IN could create enough vacuum to cause the fuel to boil at a much lower temperature. In general, liquids boil at a lower temperature in a vacuum and at a higher temperature under increased pressure.

This link allows you to play with the temperature difference for water. For example, 8 psia vacuum difference translates to water boiling at 183.5°F.

https://www.engineeringtoolbox.com/w...re-d_1686.html

 

I did that mod too and recently went back to the UFO valve because my fuel was 100 percent boiling. There was no pressure in the tank so I'd here it bubbling and could smell it. I understand the concept of pressure raising the boiling point. That's what the Rad does with water. I'm just nervous driving around because for years I had stalls that left me stranded in intersections. I'm pretty sure it was a bad evap system because I found burned nylon vacuum lines which is what controlled the evap can. Haven't had the issue since and I'm using a racetronix pump with the additional power relay. It's just unnerving to hear that pressure release when you open the cap. Can also feel the temperature of that fuel when it releases. I have 3 heat shields above the muffler as well.

 

what part of the evap susem gave you issues / heated the fuel?

 

Heated fuel yes. But the nylon vacuum tube which is under the plenum was burnt and basically welded closed. This was the line that went to the canister for the purge. So the tank could not vent to the canister.

 

I'll need to check that!

 

Check it all the way back to the distributor. It goes to a connection down the side of the distributor on the passenger side of the intake manifold