Montreal Subaru Club

Understanding P0440

In order to understand Code P0440, Evaporative Emissions Control System Malfunction (EVAP), we must first understand that it is a performance code rather than an electrical circuit code. What this means is that to pro- duce this code, the computer is not looking for any improper resistances, electrical opens, shorts, or sensor out of range situations. Rather, it uses infor- mation from various sensors to draw conclusions about the proper or improper physical operation of the Evaporative Fuel Emission System. Any electrical failures have different failure codes associated with them.
The job of the Evaporative Fuel Emis- sions System is to prevent evaporating fuel vapors from entering the atmos- phere and contributing to air pollution. In order to keep this from happening, the entire fuel system needs to be able to be sealed from any access to the out- side air. Vapors in the system are trapped in a charcoal canister, and those vapors are later purged into the intake manifold to be burned dur- ing the next driving cycle.
Only vehicles with the Enhanced Evaporative Emissions system can produce Code P0400. To identify which vehicles have the enhanced system and which do not, look for the location of the charcoal canister. If it is small and under the hood, the vehicle does not have the enhanced system. If the canister is not under the hood but is large and under the right rear of the vehicle, the vehicle has the enhanced system.
In the enhanced system, evapo- rative vapors leave the fuel tank and pass through lines to an elec- trically controlled pressure con- trol valve and a mechanical rollover valve (both by the pas- senger’s side front of the fuel tank) heading to the canister. The canister has an electrically con- trolled vent valve (next to the can-
ister) and a line that goes to the front of the vehicle, where it is attached to the electrically con- trolled purge control valve (pas- senger’s side of the engine com- partment). Within the fuel tank, there are fuel level sensors (the same ones that are used for the dash gauge) and a fuel temperature sensor (attached to the passenger’s side fuel sender). Tapped into the fuel filler neck air bypass pipe is a fuel tank pressure sensor (inside the trunk of a sedan or the luggage area of a wagon).
To test the integrity of the sys- tem, the system shuts off all access of the tank to the outside atmosphere. When the engine is running, gasoline is being deliv- ered to the injectors and the excess unused fuel is returned to the tank. Since the injector rails are hot due to their proximity to the engine, the fuel returning to
the tank is warmer than the fuel in the tank. This returning warm fuel mixes with the tank fuel and gradually warms it up. As the fuel in the tank warms, its rate of evaporation increases. Because the tank and system are sealed, the pressure in the fuel tank increases.
The control unit looks for this pressure rise to determine if the system is sealed. If the desired pressure is achieved, the comput- er will continue on to the next part of the test. In this part, the control unit will close the canis- ter vent valve, open the tank pres- sure control valve, and open the canister purge valve. This should draw a low pressure on the tank.
The computer monitors the fuel tank pressure sensor to determine whether the low pressure is achieved. If it is, it then proceeds
to the next part of the test. In this part, it repeats the first part of the test to watch the pressure rise in the tank.
After the pressure rises, the control unit does some math comparing the initial high pressure and the difference between the low pressure and the final high pressure. If the values fall within a specified range, the test passes. If the values are outside the specified range, the test fails. After two consecutive fail- ures, the Check Engine Light is illumi- nated.
The testing of this system is only conducted once per drive cycle and only under very specific conditions:
• The fuel tank must contain less than 9.6 gallons of fuel.
• The fuel temperature must be less than 113°F.
• Engine speed must be over 1500 RPM.
• Vehicle speed over 28 MPH.
• Throttle position must be mid-range.
• Intake manifold vacuum must be equivalent to cruising vacuum.
• 455 seconds must have elapsed since engine start.

The vast majority of P0400 codes occur due to gas caps that have been left loose or the gas cap retaining tether has become caught under the cap dur- ing tightening. With the cap loose, no pressure rises or falls in the tank, so the test fails. If a loose cap can be positive- ly eliminated as the cause of the code, follow the diagnostics in the service manual. If the diagnostics don’t lead to a definitive failure, there are a few other steps that can be taken.
The Select Monitor has a screen that displays fuel tank pressure. After starting the vehicle, you should see a gradual pressure rise in the tank due to the warming of the fuel. If you open the cap, the pressure should drop immediately.
If you don’t see a pressure rise, there must be a vapor leak to the outside. Closely inspect all lines and hoses. Physically check the operation of the pressure control valve solenoid, the purge control valve solenoid and the canister vent valve. The service manual instructs you to listen to these valves to click, but that does not necessarily mean they are closing completely.
Disconnect the hoses from each
valve, apply a hand vacuum pump to the valve and cycle the valve with the Select Monitor in the compulsory valve operation mode while confirming that the valve actually opens, fully closes and fully seals.
Note: The tank pressure control valve and the canister purge control valve should open when energized,. but the canister vent valve is a normally open valve so it should close when energized.
If the valves work properly, try pres- surizing the fuel tank (engine off) and evaporative system with air to check for leaks. Do not exceed 3 psi when pressurizing the tank or major fuel tank damage can occur. You can pressurize the system through the fuel return line or with a modified gas cap with hose attached. Close or block off the canister vent solenoid to seal the system.
Another situation can exist if you do see a pressure rise on the Select Moni- tor while conducting the first part of the tests when running the engine. In this case, you may have a situation where the pressure won’t go high enough (due to a very small leak or a poorly seating valve) or the pressure won’t go low enough (due to insuffi- cient purging vacuum). A vacuum of - 1.338 kpa in the tank is required to sat- isfy the test. A functional valve test and a check of the supply of purging vacu- um would be required.
Remember, because a precise set of parameters must be met before the test is conducted and two trips are required to turn on the Check Engine Light, a considerable number of days can pass after a gas cap is left loose before a Check Engine Light may come on. This may convince a customer that the light could not possibly be due to their leav- ing the cap loose. Nevertheless, it may well be, and most likely is, the cause.

Enhanced Evaporative Emissions Test

When the car is first started and you begin to drive, the Pressure Control Solenoid comes on a few seconds later. The computer is looking for some pres-
sure change in the fuel tank to indicate that the system has been sealed and can be purged.
If the computer sees a change in the pressure, after about 40 sec- onds the Pressure Control Sole- noid will turn off, and the vehicle will have passed the first part of the test for the Enhanced Evapo- rative Emission System.
If the computer doesn't see any pressure change, the Pressure Con- trol Solenoid will remain on for the remainder of the drive cycle.
If the first part of the test pass- es, the computer will wait until you are driving in a cruising con- dition to perform the next three steps of the test. If the vehicle is cruising over 45 mph but under 80mph and has less than a half of a tank of fuel, the next part of the test will begin.
This second part of the test is the only time when you will ever
see the Pressure Control Solenoid and the Vent Solenoid come on at the same time. When the Pressure Control Solenoid comes on, the computer is looking for a pressure change in the tank. If it sees one, part two of the test has passed.
The Vent Solenoid then comes on to shut off the vent to the out- side air. Since the Canister Purge Valve is also open at this time, the entire evaporative system is drawn down to a low pressure: If the pressure in the tank can be lowered to below -1.338kpa, part three of the test has passed and the Vent Solenoid shuts off, open- ing the vent to the outside air.
The low pressure in the tank should begin to recover toward atmospheric pressure. If the pres- sure rise in the tank is sufficient to satisfy the fault code formula, part four of the test has passed, the Pres- sure Control Solenoid shuts off and the vehicle is judged to be okay.


More P0440 Information

With the introduction of the New Select Monitor (NSM), late model 1997 and newer vehicles with Enhanced Evaporative Sys- tems can be diagnosed more easi- ly for DTC P0440. The NSM can be set up to monitor the Pressure Control Solenoid with one LED and the Vent Solenoid with another LED while simultaneous- ly reading the fuel tank pressure. With this setup, there is no neces- sity for the rapid button juggling that was necessary with the old Select Monitor to catch the chang- ing pressures in the tank during the diagnostic phases of P0440. After the system does its tests, save the data. You can then graph the information on the screen.
LED 6 (on the old Select Moni- tor) corresponds to the Vent
Solenoid (on the new Select Monitor), and LED 8 (on the old Select Monitor) corresponds to the PCV (Pressure Control Valve) solenoid (on the new Select Monitor).