Dartht33bagger

This is an issue I've been having with my car since I bought it. After driving the car, significant pressure builds up in the fuel tank as it sits, to the point where it is possible to hear the tank 'thud' back to its normal shape when taking the gas cap off. I ignored this for years until a hairline fracture showed up in my fuel tank and dumped gas all over my garage floor last spring. To avoid a repeat incident, I installed the VVR valve suggested in this thread as a temporary fix: https://www.thirdgen.org/forums/tpi/735882-beat-summer-heat-blues.html.

While the VVR valve does fix the issue, I am tired of the car smelling like gas after every drive. Last fall I went through all of the checks of the EVAP system in my service manual. The charcoal canister seems fine, the purge solenoid appears to work correctly (with a big bleed through which my manual mentions as a possibility), the UFO valve on the fuel tank has been cleaned and applying 15" Hg of vacuum to the Fuel Tank Pressure Control Valve opens the valve to allow the pressure through to the canister. On the surface, it appears that my EVAP components are working correctly.

Because the pressure only builds up when the car sits, it has to be the fuel pressure control valve causing the issues. According to my service manual, when vapor pressure in the fuel tank exceeds 1" Hg, the fuel tank pressure control valve is supposed to open. I'm assuming this does not happen on my car as there is still enough pressure in the tank when the car sits to get the 'thud' when I remove the gas cap. This would seem to point to the pressure control valve being bad, but the previous test of 15" Hg vacuum did open the valve. Before I go and drop $30 on a new pressure valve and hope it solves my issue, is there something I am overlooking here?

naf

iTrader: (7)

the UFO valve should release the excess pressure. The EVAP components are designed to recycle the fuel vapors but that system is not adequate to prevent the build up of excessive pressures. That's why the UFO valve is there.

With your return line disconnected, use your compressor to blow air into the tank and you should hear the UFO valve releasing this pressure.

Dartht33bagger

When driving I used to hear the tank 'thud' every once in a while when the UFO valve opened up. While cleaning up the UFO valve I made sure that I could blow through it as well. I haven't tried the compressor test yet but I assume it would pass.

If I remember correctly, I think it takes something like 7 PSI of pressure to open the UFO valve and it shouldn't happen very often as it is more of a "Whoa there is way too much pressure in the tank" fail-safe. It would appear that my tank is building significant pressure to deform the tank but not enough to open the valve.

NoEmissions84TA

iTrader: (1)

You said, the UFO valve on the fuel tank has been cleaned and applying 15" Hg of vacuum to the Fuel Tank Pressure Control Valve opens the valve to allow the pressure through to the canister.
Let's look at this from the other end (of this valve). If 15" of vacuum opened it, wouldn't that also mean that it would take 15 psi on the other end of the valve to open it? If so, then your tank would build enough pressure to most likely rupture before your UFO valve ever opens. Did you stretch the spring a little when you cleaned it?

Dartht33bagger

The UFO valve and the Fuel Pressure Control Valve are two different pieces. The UFO valve runs directly off of the tank and primarily is there to allow air into the tank to prevent a vacuum from being created. It has a secondary function to allow pressure out if it becomes too great as a fail-safe mechanism for the tank.

The fuel pressure control valve is in the engine bay and is the valve inline between the fuel tank and the charcoal canister. To open the valve, either the engine can apply 15" Hg of vacuum to the canister side of the valve or the vapor pressure in the tank can be such that the valve allows vapor to pass into the canister. What isn't clear to me from service manual is how the valve requires 15" Hg of vacuum to open the valve from the canister side but only 1" Hg of pressure from the tank side.

3.4 grape

iTrader: (1)

Did you try removing/bypassing the fuel pressure control valve to see if it or the circuit might be the problem. I removed my fuel pressure control valve in my car many years ago not for any other reason but to clean up the engine bay and just installed a small orifice fitting in it's place. Car runs fine without it.

naf

iTrader: (7)

The 'ufo' valve has two springs in it. One that regulates the negative pressure required to allow air in, and one that regulates the pressure at which it releases excess air.

This valve is the primary means of releasing pressure that would otherwise damage the tank.

Dartht33bagger

That could be a worthwhile experiment. I'll probably end up trying this and see what the post-drive pressure is like.

My UFO valve does open to let out pressure at times. The tank usually stays under that pressure thresh hold though.

EDGE

Check the black nylon control lines beneath the plenum for burn holes .When you test the evap solenoid you're not checking those lines. If they are plugged, burned or otherwise leaking the valve on the evap can will not open. Also your can itself may be plugged. I've been through this more times than I care to remember. Its a royal PIA .