EVAP System Integrity Monitoring

Vehicles Using This Method

^ M96 900 V6 models
^ M96.5- All 900 Turbo

Theory of Diagnostic Operation
This diagnostic will check EVAP system flow and detect any leak, from filler cap to intake manifold, larger than the equivalent of a 1 mm (0.040") hole. We do this by monitoring pressure changes in the fuel tank as we control the purge cycle.

If the diagnostic fails on two consecutive driving cycles the MIL will be illuminated and a DTC set.

To accomplish this, we have added two new parts to the EVAP system. There is an Fuel Tank Pressure Sensor (1) mounted on the tiller neck and a Canister Close Valve (2) which, when closed, blocks the previously unrestricted flow of fresh air into the canister.

The EVAP system leakage test is carried out as follows:

1 Tank pressure is measured during normal purge system operation. This establishes a reference point for evaluating the results of the following steps.
2 The EVAP canister close valve is closed. The EVAP canister purge valve continues normal operation.
3 Tank pressure is monitored for a ten second period. As soon as pressure drops by more than 0.4 kPa we will move to the next step. If, after ten seconds, pressure has not dropped > 0.4 kpa the diagnostic has failed and testing stops.
4 The EVAP canister purge valve is closed, sealing off the fuel tank and EVAP system.
5 Tank pressure is monitored for another ten second period. Pressure should remain constant or increase only slightly. The diagnosis fails if there is a significant increase in pressure.

Enabling and Malfunctioning Criteria
The EVAP system leakage diagnostic runs once per trip. Before it will let the diagnostic run the ECM looks for:

^ The diagnostic must not have been run yet during this trip.
^ Pre-catalyst oxygen sensor transition diagnosis passed.
^ Vehicle speed = 0.
^ The engine must be idling.
^ The EVAP canister purge valve must be active.
^ Less than 10% of the total fuel entering the combustion chamber is coming from the EVAP system.

Hardware Required
The EVAP pressure sensor, canister close valve, filler cap with a high quality two-way check valve, filler tube modified to match the cap and pressure sensor and high quality vent hoses are all new pieces of hardware required for this diagnostic.

Concerns
The most vulnerable part of the EVAP system is the fuel filler cap. It will be necessary to educate owners about the need to fully tighten the cap every time they stop for gas.