Testing

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 vent 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 purge valve ON, open, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent valve OFF, open, fresh air will be drawn through the 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 purge 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.
Canister Vent Restriction Test

If the 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 purge valve ON, open, and commanding the EVAP vent valve OFF, open, and monitoring the FTP sensor for an increase in vacuum. If vacuum increases more than a calibrated value, DTC P0446 will set.
EVAP System Components

The EVAP system consists of the following components:


EVAP Canister - The canister is filled with carbon pellets used to absorb and store fuel vapors. Fuel vapor is stored in the canister until the control module determines that the vapor can be consumed in the normal combustion process.

EVAP Purge Valve - The EVAP purge valve controls the flow of vapors from the EVAP system to the intake manifold. This normally closed valve is pulse width modulated (PWM) by the control module to precisely control the flow of fuel vapor to the engine. The valve will also be opened during some portions of the EVAP testing, allowing engine vacuum to enter the EVAP system.

EVAP Vent Valve - The EVAP vent valve controls fresh airflow into the EVAP canister. The valve is normally open. The control module will command the valve closed during some EVAP tests, allowing the system to be tested for leaks.

Fuel Tank Pressure Sensor - The FTP sensor measures the difference between the pressure or vacuum in the fuel tank and outside air pressure. The control module provides a 5-volt reference and a ground to the FTP sensor. The FTP sensor provides a signal voltage back to the control module that can vary between 0.1-4.9 volts. As FTP increases, FTP sensor voltage decreases, high pressure = low voltage. As FTP decreases, FTP voltage increases, low pressure or vacuum = high voltage.

EVAP Service Port - The EVAP service port is located in the EVAP purge pipe between the EVAP purge valve and the EVAP canister. The service port is identified by a green colored cap.


Large Leak Test

This tests for large leaks and blockages in the EVAP system. The control module will command the EVAP vent valve ON, closed, and command the EVAP purge valve ON, open, with the engine running, allowing engine vacuum into the EVAP system. The control module monitors the fuel tank pressure (FTP) sensor 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 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 is more than a calibrated level on 2 consecutive tests, a DTC P0440 will set.
Purge Valve Leak Test

If the EVAP purge 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 purge valve OFF, closed, and vent valve OFF, open, sealing the system, and monitoring the FTP for an increase in vacuum. If the control module detects that EVAP system vacuum increases above a calibrated value, DTC P1441 will set.
Small Leak Test

If the large leak test passes, the control module will test for small leaks by continuing to monitor the 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.

Thank you for the info on how the computer tests the system. That'* basicly what I found on alot of the videos i watched on the EVAP system. The system is testing itself and then setting the po440 code Looks like it is probably a large leak or blockage somewhere.

I need to know what the system is doing just under normal driving conditions. That'* what I'm dealing with now to try and test the system myself to find the problem.

This is what I have found out so far. I can remove the green cap from the service port and take out the schrader valve and put on the modified green cap i can attach a vacuum gauge too. With the throttle open the evap test port shows it has 1 inch of vacuum which means the purge valve is opening allowing vacuum into the evap line. My autotap program can read the fuel tank pressure sensor reading and it shows zero with the throttle open and 1 inch of vacuum at the service port. This is were I'm stuck. Is there a blockage in the line or canister? Is the vent valve open at this time? If so would I actually be getting a reading on the service port vacuum gauge? Or is the fuel tank pressure sensor bad and just not throwing a code that it is bad?

I may try driving it again and monitor the evap system and fuel tank pressure sensor to see what they do.

Steve

I think I figured out a way to test the fuel tank pressure sensor. I will take the hose off of the canister that goes to the vent valve. Plug the opening on the canister. This will eliminate the vent valve and I can make sure the system should be sealed. Start the car and open the throttle were it shows vacuum at the evap test port and the fuel tank pressure sensor should show some kind of reading on the autotap program. If there is no reading then there must be something wrong with the fuel tank pressure sensor.

it seems to me you are kinda doing things the hard way trying to see what the car is doing while running, i would do some static tests before trying to get into that. the purge valve is normally closed so with the connector unplugged you should not be able to suck through it, if you can its stuck open and bad, if not you can cycle it with some jumper wires attached to the battery. then you can test the vent valve in a similar way with jumpers then you can test the lines, the easiest ive found is capping off and putting a vac with a hand vac tool and see if it holds,. most problems ive seen are one of these things, one of the valves or the lines especially the metal ones were they transition to rubber, by far its mostly the lines.

I've already tested the purge and vent valves and replaced both. I'm trying to verify the fuel tank pressure sensor is working properly. I've also started to cap off lines to see if it will hold a vacuum but haven't had a chance to do all of them yet.

If the fuel tank pressure sensor is okay the it has to be the lines of seals.

I had a chance to look at my mom'* car this past weekend. Vacuum checked the canister and it held vacuum for 10 minutes without losing any. Capped off the line going to the canister that comes from the throttle body and it won't hold vacuum.

The lines are very hard to get to the connections so i'm going to use a very long piece of hose I can attach from the purge valve to the canister and see if I get it to hold vacuum and get a reading on the fuel pressure sensor. Then the task of trying to change the the vacuum lines in the engine compartment.

We did try something else. My brother has a 2000 pontiac that we attached the modified vent valve to the evap test port. His vacuum readings stay erratic even at idle and most of the time while driving. My mom'* car will start out erratic and then smooth out to a steady 1" of vacuum while driving and go to nothing when at a idle. Should they be the same or is the computers programmed to do it differently?

i dont if they would be the same, you would have to know the exact parameters and the way that the computer says to purge the fumes, they are likely different from car to car.

if your line wont hold vacuum you've found your problem, replace it with some new rubber line all the way and see what happens

That'* what I have in mind this weekend to do. I'm have a long piece of tubing to run from the purge valve all the way to the canister and see what that does. Hopefully it will show fuel tank pressure.

Went to my mom'* today and tried using a long piece of tubing to bypass the vacuum lines to the canister. Didn't change the fuel tank pressure sensor reading. My brother and I managed to get the tank down far enough to replace the fuel tank pressure sensor. After that we got readings on the autotap program.

Put everything back together and cleared the codes. Double checked to make sure it would work with the original vacuum lines. Still has fuel tank pressure readings. Told my mom she could drive the wheels off of it now and the light shouldn't come back on. Atleast not for that.

Thanks for everyone for all the suggestions. Hopefully it is fixed.

Did you change the gas cap?