rogarm

I'm trying to understand the Evap Vent Solenoid operation.
I see with no voltage it is "open" allowing air flow. If I apply 12-14VDC on the bench, it "closes" and thus has no fresh air flow to the canister. When I measure system voltage on the truck plug that goes in the solenoid with engine running it is around 14.3 VDC. If I plug it into the solenoid - nothing happens - no click - no valve movement. Works on the bench,not on the truck. I checked pins and connectors & verified truck and bench voltage within 1 VDC of each other-- cannot see anything.

I understand the functionality of this part, but - when you turn the key, and battery voltage is applied, should it stay closed all the time while drivint, or does PCM drop voltage and allow fresh air to flow (and thus a P0449 large leak code) intermittently?
Gas cap and Evap Vent Solenoid have been replaced
My tighten gas cap message will usually (but not everytime) come on the 1st trip of the day after 4-6 minutes just a temp is getting to normal. Then it goes off. If I shut down and restart the SES lite will be on. It usually will not come on again that day.
When the gas cap message appears, if traffic permits, I can shut down and coast a few feet, restart, and there is usually no SES light.
Also I never get a SES while driving. Only on restart after the gas cap light.
I'm stuck -- those 2014's sure look nice but just not ready yet.

ANY ideas would really be appreciated.
Thank you

fursty

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 vent solenoid valve to the 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 solenoid valve ON, allowing engine vacuum to be applied to the EVAP canister. With the EVAP vent solenoid valve OFF, fresh air is drawn through the vent solenoid valve and the 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 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 restrictions to the purge path in the evaporative emission (EVAP) system. When the enabling criteria has been met, the control module commands the EVAP vent solenoid valve ON and the EVAP purge solenoid valve ON, allowing 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.

Small Leak Test
The engine off natural vacuum (EONV) diagnostic is the small-leak detection diagnostic for the evaporative emission (EVAP) system. While previous leak detection methods were performed with the engine running, the EONV diagnostic monitors the EVAP system pressure or vacuum with the ignition OFF. Because of this, it may be normal for the control module to remain active for up to 40 minutes after the ignition is turned OFF. This is important to remember when performing a parasitic draw test on vehicles equipped with EONV.

The EONV utilizes the temperature changes in the fuel tank immediately following a drive cycle to use the naturally occurring vacuum or pressure in the fuel tank. When the vehicle is driven, the temperature rises in the tank. After the vehicle is parked, the temperature in the tank continues to rise for a period of time, then starts to drop. The EONV diagnostic relies on this temperature change and the corresponding pressure change in a sealed system, to determine if an EVAP system leak is present.

The EONV diagnostic is designed to detect leaks as small as 0.51 mm (0.020 in). The diagnostic can determine if a small leak is present based on vacuum or pressure readings in the EVAP system. When the system is sealed, a finite amount of pressure or vacuum will be observed. When a 0.51 mm (0.020 in) leak is present, often little or no pressure or vacuum is observed. If the test reports a failing value, 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 purge solenoid valve ON, commanding the EVAP vent solenoid valve OFF, and monitoring the fuel tank pressure (FTP) sensor for an increase in vacuum. If the vacuum increases more than a calibrated value, DTC P0446 will set.

Purge Solenoid Valve Leak Test
If the evaporative emission (EVAP) 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 purge solenoid valve OFF and the vent solenoid valve ON, sealing the system, and monitors the fuel tank pressure (FTP) for an increase in vacuum. If the control module detects that the EVAP system vacuum increases above a calibrated value, DTC P0496 will set.

Check Gas Cap Message
The control module sends a class 2 message to the driver information center (DIC) illuminating the Check Gas Cap message when a malfunction in the evaporative emission (EVAP) system and a large leak test fails.

EVAP System Components
The evaporative emission (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 Solenoid Valve

The EVAP purge solenoid valve controls the flow of vapors from the EVAP system to the intake manifold. The purge solenoid valve opens when commanded ON by the control module. 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 Solenoid Valve

The EVAP vent solenoid valve controls fresh airflow into the EVAP canister. The valve is normally open. The control module commands the valve ON, closing the valve during some EVAP tests, allowing the system to be tested for leaks.


Fuel Tank Pressure Sensor

The fuel tank pressure (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. A high FTP sensor voltage indicates a low fuel tank pressure or vacuum. A low FTP sensor voltage indicates a high fuel tank pressure.

Hope this helps some.

rogarm

It is a great help to piecing it together. Thank you.
Especially that the EVAP Vent Solenoid Valve is normally open.
That must be why there are no driveability issues. If the EVAP Vent Solenoid Valve closes for one of the "tests" I have to find out what triggers it and how much voltage it takes to close the valve. In my case, if it will not close then it would cause a "Large Leak" and the GAS CAP/P0449 code. Again it closes with a small Radio Shack DC power supply that puts out right at 15 VDC. It does not close with truck system voltage.

That may also explain why now & again no DIC messages are seen, yet after a shut down, upon restart the SES light is on, and has a P0449 stored.

I will check the operation of the Canister Purge Valve Solenoid next, sure would be nice to have a way to set off the system leak test manually.

Thanks again for the details.

rogarm

It's been fine for a couple of months now. I guess every now & again even a blind squirrel finds an acorn.
My final trouble (although it never gave that specific code) was apparently the EVAP Purge Control Valve. Replaced it an no further issues. BTW there is a nice video on youtube on how to get the front hose connector off. As it was that valve had gotten dirty inside and would not completely seal when closed.

Even with purchasing a gas cap, vent valve and purge control valve as well as a code scanner (family needed one anyway) total cost was under $200.

This is a great source of information. Thank you to all those who input.

Douginoregon

So I have a question. I too have replaced the evap vent solenoid (bench tested OK) but fails to operate when plugged into the truck. I have 12VDC at the pink(ish) wire (KOEO) and .04 on the white wire. I was told that there is supposed to be 12Volts on each wire until you engage the keys, right? then the white is supposed to drop to 0volts?? I am getting PO0499 still.
Like i said., I have tested the new solenoid, I have 12V on the pink wire and nearly nothing on the white. This is while plugged in or not.

Doug