Evap Can complete removal?
JK Enthusiast
Joined: Mar 2009
Posts: 385
Likes: 0
From: Chugiak, AK
I found some information on why it would be difficult to trick a PCM with the suggestion above:
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Since 1996, Chrysler has been leak-testing the EVAP system with air pressure generated by an on-board pump. This has several advantages, the greatest being that because manifold vacuum plays a different role in the test, the drive cycle is much simpler. Manifold vacuum is used only to power the on-board leak detection pump (LDP), and is controlled with a solenoid valve. Here’s how it works:
The PCM opens a vacuum solenoid valve inside the LDP, admitting vacuum to the top of the LDP diaphragm.
As the diaphragm is pulled up against a spring, filtered air is drawn through a check valve and into the chamber below the diaphragm.
The PCM closes the solenoid valve, venting vacuum above the diaphragm and allowing the diaphragm spring to push the diaphragm back down.
The air under the diaphragm is pushed through an exit check valve and into the EVAP system through the canister.
The PCM continues cycling the vacuum solenoid valve to operate the pump, but the diaphragm stops moving and is held in the full-up position when EVAP system pressure equals diaphragm spring pressure.
The diaphragm spring exerts a pressure of only 7.5 in.H2O, about 0.25 psi, and that’s the pressure used to leak-check the EVAP system.
When this pressure is reached, the PCM stops cycling the vacuum solenoid valve. How does it know when the system is pressurized? The key to the whole EVAP monitor system is the reed switch inside the LDP.
Above the diaphragm is a simple on-off reed switch that the PCM uses to monitor the position of the diaphragm. The switch is open when the diaphragm is up and closed when the diaphragm is down. When the reed switch stays open, the PCM knows the system is pressurized and will stop cycling the vacuum solenoid valve.
At this point, the PCM measures the amount of time it takes for the switch to close again, meaning the system pressure has fallen below diaphragm spring pressure. If the system builds and holds pressure in the correct amount of time, the test is passed.
The PCM calculates the correct amount of bleed-down time before beginning each test, based on conditions similar to those used by Ford and GM. Enable conditions for the leak test are:
Cold start.
Ambient temperature between 40 and 86 degrees F.
Fuel level between 30 and 85 percent for small-leak test; 15 to 85 percent for large-leak test.
Vehicle speed less than 35 mph.
Engine run time greater than 10 seconds.
Battery voltage between 10 and 15 volts.
MAP (BARO) sensor reading 22 in.Hg or more.
As you can see, while the drive cycle is not as critical, it’s still important to avoid generating fuel tank vapor pressure. That’s why more and more vehicles are built with a returnless fuel system.
There are two phases to the leak test. First, the PCM measures the amount of time needed to pressurize the system. If pressure never builds, a gross leak is assumed. If pressure builds within a few pump strokes, this indicates a pinched or plugged hose between the LDP and the canister. Next, it will measure the size of the leak by measuring the amount of time between turning off the vacuum solenoid valve and the reed switch closing as the diaphragm falls. In general, it takes 1.2 seconds to pass the large-leak test and six seconds to pass the small-leak test.
The leak detection pump also includes the canister vent valve. The diaphragm still has a bit of downward travel left after the reed switch closes.
If the PCM doesn’t open the vacuum solenoid valve to pull the diaphragm up again, the diaphragm will continue down and bottom out. At this point, a shaft attached to the bottom of the diaphragm pushes the spring-loaded canister vent valve open. That means the vent valve is closed only when the LDP diaphragm is pulled up with vacuum.
It’s easy to see from the enabling criteria that cars driven in extreme hot or cold weather may not run the EVAP monitor for months, and cars that never leave the high country might never test the system at all.
With a bi-directional scan tool, the purge and vent valves can be commanded closed for testing with smoke, and on any vehicle a scope or graphing meter can be used to monitor the tank pressure sensor or LDP reed switch during a drive cycle.
This can be helpful in states that use the vehicle’s own OBD system for emissions inspection. Just remember that the PCM is looking for stable testing conditions, correct operation of the solenoid valves and/or LDP, and the time needed to build, hold and release system pressure or vacuum.
=====================
Since 1996, Chrysler has been leak-testing the EVAP system with air pressure generated by an on-board pump. This has several advantages, the greatest being that because manifold vacuum plays a different role in the test, the drive cycle is much simpler. Manifold vacuum is used only to power the on-board leak detection pump (LDP), and is controlled with a solenoid valve. Here’s how it works:
The PCM opens a vacuum solenoid valve inside the LDP, admitting vacuum to the top of the LDP diaphragm.
As the diaphragm is pulled up against a spring, filtered air is drawn through a check valve and into the chamber below the diaphragm.
The PCM closes the solenoid valve, venting vacuum above the diaphragm and allowing the diaphragm spring to push the diaphragm back down.
The air under the diaphragm is pushed through an exit check valve and into the EVAP system through the canister.
The PCM continues cycling the vacuum solenoid valve to operate the pump, but the diaphragm stops moving and is held in the full-up position when EVAP system pressure equals diaphragm spring pressure.
The diaphragm spring exerts a pressure of only 7.5 in.H2O, about 0.25 psi, and that’s the pressure used to leak-check the EVAP system.
When this pressure is reached, the PCM stops cycling the vacuum solenoid valve. How does it know when the system is pressurized? The key to the whole EVAP monitor system is the reed switch inside the LDP.
Above the diaphragm is a simple on-off reed switch that the PCM uses to monitor the position of the diaphragm. The switch is open when the diaphragm is up and closed when the diaphragm is down. When the reed switch stays open, the PCM knows the system is pressurized and will stop cycling the vacuum solenoid valve.
At this point, the PCM measures the amount of time it takes for the switch to close again, meaning the system pressure has fallen below diaphragm spring pressure. If the system builds and holds pressure in the correct amount of time, the test is passed.
The PCM calculates the correct amount of bleed-down time before beginning each test, based on conditions similar to those used by Ford and GM. Enable conditions for the leak test are:
Cold start.
Ambient temperature between 40 and 86 degrees F.
Fuel level between 30 and 85 percent for small-leak test; 15 to 85 percent for large-leak test.
Vehicle speed less than 35 mph.
Engine run time greater than 10 seconds.
Battery voltage between 10 and 15 volts.
MAP (BARO) sensor reading 22 in.Hg or more.
As you can see, while the drive cycle is not as critical, it’s still important to avoid generating fuel tank vapor pressure. That’s why more and more vehicles are built with a returnless fuel system.
There are two phases to the leak test. First, the PCM measures the amount of time needed to pressurize the system. If pressure never builds, a gross leak is assumed. If pressure builds within a few pump strokes, this indicates a pinched or plugged hose between the LDP and the canister. Next, it will measure the size of the leak by measuring the amount of time between turning off the vacuum solenoid valve and the reed switch closing as the diaphragm falls. In general, it takes 1.2 seconds to pass the large-leak test and six seconds to pass the small-leak test.
The leak detection pump also includes the canister vent valve. The diaphragm still has a bit of downward travel left after the reed switch closes.
If the PCM doesn’t open the vacuum solenoid valve to pull the diaphragm up again, the diaphragm will continue down and bottom out. At this point, a shaft attached to the bottom of the diaphragm pushes the spring-loaded canister vent valve open. That means the vent valve is closed only when the LDP diaphragm is pulled up with vacuum.
It’s easy to see from the enabling criteria that cars driven in extreme hot or cold weather may not run the EVAP monitor for months, and cars that never leave the high country might never test the system at all.
With a bi-directional scan tool, the purge and vent valves can be commanded closed for testing with smoke, and on any vehicle a scope or graphing meter can be used to monitor the tank pressure sensor or LDP reed switch during a drive cycle.
This can be helpful in states that use the vehicle’s own OBD system for emissions inspection. Just remember that the PCM is looking for stable testing conditions, correct operation of the solenoid valves and/or LDP, and the time needed to build, hold and release system pressure or vacuum.