Can I get to the fuel tank vent valve and fuel pressure sensor without having to drop the gas tank?

You might want to post more detailed info if you're having a problem.
There is no Fuel pressure sensor and the 'vent' is your gascap or your purge solenoid on the charcoal canister, at least if I'm understanding you correctly.

There is indeed a FTP fuel tank pressure sensor, located at the gas tank near the sending unit. Its part of the evap system monitoring. And a FVV fuel tank vapor vent valve is listed as availble from GM parts direct. The FVV is either located in a vacuum line near the tank or actually pluged into tank.


I previously posted here about a problem with excessive vacuum pressure being aplied to the gas tank. Tank will actually flex inwards at the bottom.

I replaced the gas cap, and both evap solenoids ( 1 at engine the other on canister)

Made sure all the evap hoses and lines are air tight.

Checked charcoal canister and vacuum lines for blockage.

Reseting computer appears to be a temporary solution, but problem returns in short order.

At this point, I'm down to the FTP sensor, Fuel tank vent valve or a problem with the computer.

Any help is greatly appreciated.............

Yeap, I just got home and logged in because on the ride home I figured it was the tank pressure sensor you were talking about.

And I believe the vent valve is on/part of the charcoal canister.

Got a pressure sensor for ya if you need it. $25.
http://www.bonnevilleclub.com/forum/...ic.php?t=98276

I offer a $50.00 cash reward to anyone that solves my gas tank vacuum pressure problem.

The pressure sensor is mounted on the fuel pump on most GM vehicles. Vacuum is applied to the system, then a bleed down test. The pressure sensor modulates the opening and closing of the vent valve, and reads the tank bleed down pressure.
What are your current codes?
Your on the right track with either of the sensor or vent valve.

Here is more background info on how the evap system works:
Evaporative Emission (EVAP) Control System Description
EVAP System Operation
The evaporative emission (EVAP) control system limits fuel vapors from escaping into the atmosphere. Fuel tank vapors are allowed to move from the fuel tank, due to pressure in the tank, through the vapor pipe, into the EVAP canister. Carbon in the canister absorbs and stores the fuel vapors. Excess pressure is vented through the vent line and EVAP canister vent solenoid valve to atmosphere. The EVAP canister stores the fuel vapors until the engine is able to use them. At an appropriate time, the control module will command the EVAP canister purge solenoid valve ON, open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP canister vent solenoid valve OFF, open, fresh air will be drawn through the solenoid valve and vent line to the EVAP canister. Fresh air is drawn through the canister, pulling fuel vapors from the carbon. The air/fuel vapor mixture continues through the EVAP purge pipe and EVAP canister purge solenoid valve into the intake manifold to be consumed during normal combustion. The control module uses several tests to determine if the EVAP system is leaking.

Large Leak Test
This tests for large leaks and blockages in the evaporative emission (EVAP) system. The control module will command the EVAP canister vent solenoid valve ON, closed, and command the EVAP canister purge solenoid valve ON, open, with the engine running, allowing engine vacuum into the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor voltage to verify that the system is able to reach a predetermined level of vacuum within a set amount of time. The control module then commands the EVAP purge solenoid valve OFF, closed, sealing the system and monitors the vacuum level for decay. If the control module does not detect that the predetermined vacuum level was achieved, or the vacuum decay rate is more than a calibrated level on 2 consecutive tests, a DTC P0455 will set.

Small Leak Test
If the large leak test passes, the control module will test for small leaks by continuing to monitor the fuel tank pressure (FTP) sensor for a change in voltage over a period of time. If the decay rate is more than a calibrated value, the control module will rerun the test. If the test fails again, a DTC P0442 will set.

Canister Vent Restriction Test
If the evaporative emission (EVAP) vent system is restricted, fuel vapors will not be properly purged from the EVAP canister. The control module tests this by commanding the EVAP canister purge solenoid valve ON, open; and commanding the EVAP canister vent solenoid valve OFF, open; and monitoring the fuel tank pressure (FTP) sensor for an increase in vacuum. If vacuum increases more than a calibrated value, DTC P0446 will set.

EVAP Purge Solenoid Valve Leak Test
If the evaporative emission (EVAP) canister purge solenoid valve does not seal properly, fuel vapors could enter the engine at an undesired time, causing driveability concerns. The control module tests for this by commanding the EVAP canister purge solenoid valve OFF, closed; and EVAP canister vent solenoid valve ON, closed; sealing the system, and monitoring the fuel tank pressure (FTP) for an increase in vacuum. If the control module detects that EVAP system vacuum increases above a calibrated value, DTC P0496 will set.

I have read dozens of articles on the evap system.

The issue is that my evap system has decided to stay in closed loop. The vent solenoid is closed to atmosphere and the purge solenoid is open to engine vacuum. Stuck in diagnostic mode. If I reset the computer memory, the system works to spec for a few hundred miles. Then goes right back to pulling full engine vac and the gas tank. The check valve on the gas cap does not allow enough air into the tank to overcome the vacuum.

Now that I've past inspection today by reseting the computer and driving 60 miles. When the problem returns (and it will) This time I'm going to disconnect the evap system entirely and just cover up the darn check engine light.

For those that are curious. I will pull the vac line at the purge solenoid and plug it. Pull and plug the vent line at canister from gas tank. Disconnect electrical connector at purge and vent solenoid. And slightly vent the gas cap to the atmospere.......

reb, you originally asked how to solve your problem. You received some good information and even indicated that you realize what your problem is. The answer to how to properly fix your problem lies in following the test sequence posted on Monday by SSEBONNE4EVA, just above. He didn't just post that article to give you something to read, he actually gave you the information you need to correctly diagnose it yourself.

By choosing to ignore the good information he posted, you are indicating that you are looking for some excuse not to do the right thing, IMO. Just disconnecting the system and then ignoring the DTC light is NOT how to fix this problem. A proper diagnostic sequence, as posted, and then repair or replacement according to what you find is the correct fix.

Remember that we are only able to see what you write and post and are not there to actual do the diagnostics and repairs for you. If you choose not to do the diagnostics that are recommended and report your findings back to us, then there is no possible way short of guessing, and being wildly lucky, that we can help you with a correct fix.

So PLEASE. If you do want to really fix your problem and insure that it stays fixed, do the diagnostics as indicated and get back with us. If you have need of a scanner, there is always your local parts store or perhaps even a nearby member may have a scanner. In any case, realize that there is no magic here and that the information we give you is only good if you actually use it and follow the advice.