Excessive Fuel Tank Pressure

1995 Chevy Lumina 3.1L Sedan w/ ~182K mi

Over the past 6 months, I have noticed fuel odor just about any time I turn off the vehicle, and I am also experiencing progressively higher levels of fuel tank pressure each time I remove the fuel cap. The fuel tank pressure seems to be greater the lower the level of fuel in the tank whenever I remove the cap. 6 months ago (~Sept) it was only a slight "hissing" periodically when removing the cap to fuel up, but now it occurs at every fill up and seems to have enough pressure to push the cap entirely off had I not been careful to slowly loosen the cap to relieve the pressure.

The entire fuel pump and sending unit assembly in addition to the tank and filter were all replaced about 4 years ago in May 2010 or about 28K mi ago. The issue has just come up progressively over the last 6 months.

Today, I inspected the evap. emission components and lines from the tank to the canister to the purge solenoid mounted on the rear of the engine. All lines are connected and seem to be in operable condition. The line from the canister to the purge valve mounted on the engine is completely free with no obstructions. I blew out the line with compressed air and absolutely no exiting particulates were observed. The lines on the bottom of the canister did not show any signs of wet fuel and there were no carbon particles present in the lines from what I could tell from disconnecting and inspecting. The vapor line from the tank to the carbon canister is not kinked or pinched in any manner.

I believe there is a fuel vapor check valve that resides inside the tank connected to the fuel pump assembly/sending unit. This was all replaced as a single unit as mentioned above. I am thinking this check valve may not be functioning properly and not allowing the vapor to escape into the carbon canister. I tried to apply slight pressure and vacuum on the vapor line connected to the check valve on the tank but no air was able to move in or out even with the gas cap off. However, I do have a full tank of gas right now, so I imagine the check valve will be closed.

The check engine light is not on but since this is an OBDI vehicle I am not sure there are any evap. emission codes anyway.

How can I test the gas tank vapor check valve?

How can I test the vapor purge solenoid located on the rear of the engine?

Please let me know your thoughts as to what malfunctioning component could be causing the excessive fuel tank pressure.

Thanks,

Sounds like the purge/vent valve is not functioning or the vent is blocked.

Well, if you had a problem with the EVAP system, you'd think it would have set a code.......however, being a '95, it might not.....

There isn't much you can do to check this system, without a bidirectional scan tool and smoke machine......

Is it a pressure, or is it a vacuum that you are feeling when you open the cap?

You could have a problem with the purge valve, vent valve, or charcoal canister.....the only check valve I can think of in the tank, is a ball check at the fuel inlet, and the check valve that is in the fuel pump....that's it, neither one would have anything to do with excessive pressure......

Well, if you had a problem with the EVAP system, you'd think it would have set a code.......however, being a '95, it might not.....

There isn't much you can do to check this system, without a bidirectional scan tool and smoke machine......

Is it a pressure, or is it a vacuum that you are feeling when you open the cap?

You could have a problem with the purge valve, vent valve, or charcoal canister.....the only check valve I can think of in the tank, is a ball check at the fuel inlet, and the check valve that is in the fuel pump....that's it, neither one would have anything to do with excessive pressure......

I am hearing and feeling pressure when removing the cap, which has become progressively worse over the past 6 months. Maybe there is something going on here with the higher alcohol content in the "winter mix" fuel and the large temp. swings we experience in Texas...it was 75F on Sat. but was in the 20s this Mon. morning. Nonetheless, it should be venting and it is not.

Pic of the actual fuel pump assembly is here...
http://ecat.spectrapremium.com/a/000071874
See part#SP09D1H

The line heading off towards the right rear is the vapor line and is connected to what appears to be a ball/check valve below. This vapor line is connected to the canister.

I have a full tank now, but once I get down to 3/4 I would like to see if I can either blow a little air back into the tank or draw in air from the tank (w/ the cap off of course) through this vapor line where it connects to the canister.

The line from the canister to the purge valve on the rear of the engine is clear as mentioned above. So maybe there is a blockage/malfunction at this valve in the tank connected to the vapor line or clogged carbon canister not letting anything out or malfunctioning purge valve not letting anything into the intake manifold.

Any other ideas?...and how to test each one?

Thanks.

**Update**

I called Spectra and tech support said the "valve looking object" on the vapor line is actually a screen filter. They said no matter if the tank is full, in theory I should be able to suck or blow air through the vapor line connected to the tank with only about 5-10psi. They said the screen filter also acts like a pressure regulator and the valve on the gas cap should also be letting air in and out as pressure or vacuum builds. I'll try removing and blowing out the vapor line from the canister to the tank. If no result, then maybe I will try a new gas cap...it was replaced about 5 years ago.

Today I removed the vapor line connecting the tank to the canister and there were not any obstructions in the line. I then isolated the metal vent line/pipe directly connected to the fuel pump/sending assembly and could not get any air in or out of the tank. I also ran a wire through the vent pipe on the fuel pump assembly until the bend in the metal vent line and did not encounter any obstructions. My only guess left is somehow the vent pipe on the fuel pump assembly is clogged inside the tank. I tried spraying B12 and then compressed air into the vapor line on the pump assembly but to no avail. Btw, tank and pump were both replaced 4 years ago as mentioned earlier.

As a benchmark test on another vehicle, I removed the same type of vent line connecting the canister to the tank from my 2000 silverado and was easily able to blow air back into the tank. The tank then slowly relieved the pressure back through the same line when I stopped blowing. This is exactly how I imagine the vent should work to regulate tank pressure.

For now I am just leaving the gas cap lightly snug to hopefully help relieve the pressure. I really want to avoid having to drop the tank.

Does anyone know with absolute certainty whether a gas cap for a '95 Lumina should work as a 2-way valve to regulate tank pressure?...allowing air in when under a vacuum and allowing air out when under pressure.

I have researched gas caps and see conflicting views within online forums. Stant claims their OE caps flow both ways. From their website...
Features and Benefits

Stant caps, made of sturdy and long-life materials, are the only aftermarket caps to include all safety features and valving of the original equipment caps.

OE Pressure relief prevents fuel vapors from exiting the fuel system under normal operating conditions. This allows an escape path for pressure in the event of an impact situation where the fuel tank has collapsed. It also permits pressure to escape if the vapor management system malfunctions.
OE Vacuum relief prevents fuel vapors from exiting the fuel system via the fill tube under normal operations. This allows an alternative path for air to flow into the tank if there is an obstruction in the vapor recovery or vapor management systems. It also permits the fuel system to normalize air pressure during natural vacuum situations.


Just shot an email off the CST to ask if their caps do the same.

fuel vapors [pressure] goes into the canister. the fuel cap allows air in not out. this is how it works. If the fuel tank has too much pressure I would guess that the canister in plugged up from over filling and liquid fuel made it to the canister now a solid block of coal. many have these issues ..I would guess that you have a check engine light on..

So it turns out my theory was right.
Somehow the small vent that resides inside the gas tank and is part of the fuel pump assembly vapor line became restricted and was not allowing fumes to vent from the tank.

The line was NOT clogged with any carbon, dust, debris, etc. and was completely clear all the way through the metal vent line on the fuel pump assembly. The carbon canister also does NOT have any restrictions or carbon in the lines, etc.

I noticed on the new fuel pump, the vent located on the metal vapor line has a small, metal ball bearing located inside the vent to prevent fuel from flowing out if the vehicle flipped over I presume.

On the defective 4yr. old fuel pump, there was not a loose metal ball bearing inside the vent. Maybe they changed the design or maybe somehow the ball bearing became stuck and was restricting the vent on the fuel pump assembly.

Anyways, new pump with a free flowing vent and absolutely no fuel tank pressure build up. I confirmed and tested each pump by blowing directly in and out on the new and old fuel pump vapor lines. I have also been driving the car for a week and checking the pressure by removing the cap after my drive home from work. Temps are also rising here in TX so it seems it is properly venting.

Good update for future reference.

So it turns out my theory was right.
Somehow the small vent that resides inside the gas tank and is part of the fuel pump assembly vapor line became restricted and was not allowing fumes to vent from the tank.

The line was NOT clogged with any carbon, dust, debris, etc. and was completely clear all the way through the metal vent line on the fuel pump assembly. The carbon canister also does NOT have any restrictions or carbon in the lines, etc.

I noticed on the new fuel pump, the vent located on the metal vapor line has a small, metal ball bearing located inside the vent to prevent fuel from flowing out if the vehicle flipped over I presume.

On the defective 4yr. old fuel pump, there was not a loose metal ball bearing inside the vent. Maybe they changed the design or maybe somehow the ball bearing became stuck and was restricting the vent on the fuel pump assembly.

Anyways, new pump with a free flowing vent and absolutely no fuel tank pressure build up. I confirmed and tested each pump by blowing directly in and out on the new and old fuel pump vapor lines. I have also been driving the car for a week and checking the pressure by removing the cap after my drive home from work. Temps are also rising here in TX so it seems it is properly venting.

so the fuel pump caused the pressure build up . never heard that one.

could be the vent line had a blockage but if it did it was a perfect seal.
I would think a kinked vent line or the canister .