Took my Z28 out today for the first time in around 3 months. It was freak'n hot, but I did an extended drive of about 1 hour as it was good to finally drive the thing again. Got home and smelled fuel vapors. Shut the car off and hear what I thought was the fuel pump continuously running....and not sounding good at all. Pulled the battery positive, but still same noise. Finally figured out the noise was at the fuel cap. Loosened it up...and holy cow, the fuel vapor to come out of the fuel tank was unreal. It vented for at least 30 seconds.

Doing some research on this and I decided to target the fuel tank vent valve and evap pressure control valve.

The fuel tank vent valve is discontinued. I tried to pull it from the hose, but it's in there tight and I don't want to risk breaking it off in the vent tube. Luckly, it comes apart pretty easy. It was fairly dirty inside so I cleaned it all up. Went for a drive and still same problem with excessive fuel pressure vapors.

Tomorrow, I'll look at the EVAP pressure control valve. I did do a little testing tonight with the purge solenoid. Pulled the line from the intake, applied a vacuum, and commanded the valve open with a Tech 1 tool. Vacuum went to zero and I could not pull a vacuum on the line either....so I don't think the charcoal canister is plugged up from the purge side. There is also no SES code for EVAP flow issues....so I think that side of it is fine.

It just baffles me how the fuel tank can get that pressurized with fuel vapor and the fuel tank vent valve not vent the pressure. That thing is supposed to open with 1 psi of pressure....seems like a lot more than that was in there when I cracked the gas cap. Granted, I could not get the fuel tank vent valve out of the tube to test it, but still......looked good after I cleaned it and yet it's still building much higher than normal vapor pressure.

I did shoot the fuel tank with an IR temp gun. It was 112*F. So, not overly hot at all for having run it for 1 hour.

Got this off Firebird Nation. Good picture of what the fuel tank vent valve looks like disassembled.

 

I'll also see if I can post up the FSM description of how the EVAP system works. One thing I learned tonight is that the control tube on the EVAP pressure control valve is to keep it from oscillating or humming....never would have guessed that's what it was for....I thought it was for opening the valve. My thinking was that vacuum on the control valve opened it, but it's just straight up vapor pressure in the fuel tank that opens it. Just shows how I don't really know how the system really works.

 

Is it possible the sound you heard before loosening the fuel cap was vapor venting SLOWLY from the vent valve? Might be indicating excessive heat in the tank is generating more vapor than the vent valve and the EVAP valve can release. A failing fuel pump could generate heat, or is it possible the fuel return line is too close to the headers or some other part of the exhaust?

Could you possibly slit the vent tube longitudinally where It connects to the vent valve and peel it gently away from the valve connector? Then with access to something like a MityVac pump you could test the vent valve for both the pressure and vacuum relief specs. Possibly applying pressure to the tank via the vent tube would tell you if there is flow out through the EVAP pressure control valve when pressure is as low as the 1 PSI spec for the vent valve.

 

The vapor sounded like it was blowing past the seal of the gas cap when I put my ear to the cap. I've never seen anything like it to date. I put a mityvac on the gas tank vent line while I had the valve apart and the line would not hold any kind of vacuum. Using some mityvac adapters, I tried blowing through the vent line, but it didn't seem to flow anything in the opposite direction. That made me raise an eyebrow, but I'm not sure if there is some kind of check valve on the other end. I also started tasting fuel, so I stopped that pretty quick. I'm ordering some inline regulators to drop down the pressure in my air hose for some diagnostic work. I need to get a hold of some low pressure gauges as well.

I may end up having to drop the tank to figure this one out. If I do, there is no way I don't replace that 20+ year old pump while it's down. I kinda hate to disturb the pump since it's working fine.....but I may have to inspect the tank lines for something out of the ordinary going on.

FSM mentions that the EVAP pressure control valve should open at 0.7psi. Which makes sense as it would give the vapors a chance to go into the EVAP canister before getting vented to the atmosphere.

 

Grabbed a couple of pump pics from one of Adsoyo's fuel pump issue threads.

When looking at the pic below trying to identify the lines, going from left to right numbering them 1-4. Just to make sure I'm not goofing the line type....are these the line functions?

1. Fuel return
2. To EVAP canister
3. Fuel feed to injector rail
4. Fuel tank vent valve

For what I think is the fuel tank vent valve line...any idea what crimped to the port? Edit:Looks like fuel tank vent line actually connects into the bucket and is routed behind whatever the crimped thing is.....so is this line submerged into fuel at all times?

Top view....

Side view...

 

Zooming in on this....I may have the EVAP line and fuel tank vent valve line reversed. Even so...I'm not sure why the EVAP line would be connected to the bucket, which is submerged in fuel most of the time.

 

Try this:

https://www.camaroz28.com/g/picture/8176783

The vent line is the one with the rectangular tab right below it.

I think the white plastic cylinder is the rollover valve. If you need better pictures of sending unit, I have a spare unit I can take pics of. Unfortunately it does not have the hose connecting the return to the bucket.

 

10-4 on the rollover valve. I had no idea what that thing was. The pump picture with the labels was very helpful. A good picture of the EVAP line/rollover valve side of the pump would be nice to see. If you can get me a shot of it, I would appreciate it.

The EVAP pressure control valve that was on the car was very tough to blow into in order to overcome the spring pressure compared to another one I pulled off the car a few years ago. I put that one on today. Both of them had no trouble with the control side holding vacuum and opening the valve. My test drives today did fine, but it was not as hot and overcast. Got the normal 3-5 second whoosh of vapor when opening the cap after a drive.

I commanded the EVAP purge valve open at idle. The engine stumbled badly and almost died. So, I think that backs up that the purge valve is good as opening it created a massive vacuum leak that the PCM was not prepared to deal with.

The evap canister tested fine. No blockage when testing the ports and no gas coming out of it. I had a hard time blowing into the EVAP line to the fuel pump until I took off the gas cap. I could hear air coming out the filler neck as I would blow into the EVAP tube. I'll know more when I get in some hardware to do some low pressure testing and actually measure the pressure.

I'm still puzzled as to why the EVAP line seems to be routed into the fuel pump bucket.

 

I'll try to take a pic tomorrow. I do have a morning appointment for a cortisone shot in a failing hip. Not sure how agile I'll be for the rest of the day.

The pump bucket has two nipples on it. One for the return is obvious. Not sure about the second, if that’s where the EVAP is supposed to go. My sending unit was a “spare”. When we built the fuel system for the stroker build, the shop pulled the original sending unit and replaced the stock pump/bucket with a new Bosch pump. They threw the pump/bucket in the surplus parts they returned to me when the project was done. May have included the rubber hoses, not sure, but if they did I don’t have them any more.

When the sending unit float level sensor failed, we had to buy a whole new sending unit, less pump and bucket. Removed the broken sensor and pop riveted the one from the new sending unit. In 2001 you couldn’t buy a separate level sensor. I can’t even remember why I put the old pump/bucket in the new sending frame.

 

Just speculating on the EVAP line being routed from the bucket. The fuel in the bucket cools the pump, picking up heat. While it is constantly replaced by the return stream, that stream could be warmed by exhaust or simply by the energy of pumping it. The fuel in the bucket is most likely to be the warmest, and generate more vapor. Send it directly to the EVAP system. Just a guess.

What is unclear to me is how that hose attaches to the rollover valve. Hard to tell, since I don’t have the hoses on my “spare”.

Photos:

 

One more:

 

I appreciate the photos and the theory on the EVAP line. Some of my testing gear came in and I've got to get a few more fittings from Northern Tool.

For a new fuel tank vent valve, the best bet looks to be a third gen one since reproductions are now being made.....provided it has the same specs. Kinda nuts that they discontinued what I would assume to be considered an emissions part. I don't know that mine is bad, but my plan is to pressurize the tank through the EVAP line(where it connects to the EVAP pressure valve) with a gauge attached and see if the vent valve actually vents when the pressure goes above 1 psi. Going to drop my compressor air to around 5 psi to fill the tank as I don't want to throw 90psi at it.

 

Appears there is a discrepancy between my photos and the elevation view diagram of the sending assembly in the 96 factory service manual. There is nothing shown attached to the rollover valve. There is a line identified as "4 - Vent Hose" from the pump bucket, that disappears behind the rollover valve. It is retained in position by "3 - Vent Hose Retainer". The retainer appears to be at a level 3/4" below the bottom of the rollover valve, and the hose slips into an opening. There is nothing like that on my sending unit (which the shop purchased, so I assume they ordered one for a 1994).

 

Finally found a couple pictures that may bring the EVAP picture into focus. Looks like the EVAP bucket hose is open towards the top of the tank. I think that makes the roll-over valve open in the top of the tank until the car gets upside down. On bottom of page 6C-3 the FSM mentions that the roll over valve is pressed into the EVAP tube. I was confused as I thought the bucket hose somehow connected to the EVAP hard line exiting the top of the pump.

 

Just got done with my test of pressurizing the fuel tank for today. I input the air pressure on the input line to the EVAP pressure control valve near the EVAP charcoal tank. The pressure gauge on the mityvac pressure/vacuum pump does not have good low resolution. As in, it considers around 0-2 psi zero. The system would not hold over what I think is 1psi. That 1psi is good for around 3 seconds of venting when opening the gas cap. Which is normal for what I've experienced over the years. I spiked the input of the system to 10psi on the input flow, but the system always quickly settled to that 1psi mark within a second of stopping the flow. So that's a good sign.....

However....there is a problem with my setup. One thing I failed to think about was needing a schrader valve on the input to prevent backflow. All of the air nozzles I've got bleed pressure on the output tube when the flow valve is closed. Given that, I need to get a schrader valve on the input to make sure that the vent valve is doing it's job rather than the pressure bleeding off due to the air input device venting off the pressure.

Another goof up was the pressure regulators I got off Amazon. They are not pressure regulators as advertised, they are flow regulators with a gauge. So be aware if looking at these.....
"Dynastus 2 PCS 1/4"-NPT Air Pressure Regulator Kit, Red & Blue, Air Adjusting Valve Regulator with Gauge for Spray Guns and Pnuematic Tools"