Evaporative Emissions Canister Flow Monitor

Vehicles Using the Flow Check Diagnosis
Evap system operation can be monitored using several different approaches, Saab will 'use two in 1996. The models using the flow check method are:

^ All 9000s
^ 4-cyl Motronic 900s
^ M96-M96.5, Turbo 900s

Theory of Diagnostic Operation
If the Evap Canister purge valve, wiring, hoses, etc. are ok we should see a change in the fuel trim and idle control circuits when we open and close the purge valve.

1 Once normal purging starts, the system is allowed to run for a while to establish stable short term fuel trim and IAC operation.

2 The duty cycle for the purge valve tapers down until the valve is completely closed (0% duty cycle). The short term fuel trim and IAC valve duty cycle are sampled. Their average values over a period of time are calculated to provide a reference point for the next step.

3 The purge valve is opened and, as the volume of flow from the Evap canister increases, the system looks for changes in short term fuel trim and IAC duty cycle. The purge valve will continue to open until either the total fuel trim change has reached 6% or the IAC duty cycle changes by more than 1%. At this point the test will have been passed and the diagnostic will be terminated.

If the purge valve duty cycle reaches 90% without passing the test, we will store a new idle air reference' value and then taper back the purge valve duty cycle to 0%. As this is happening we will monitor just the IAC duty cycle. If we see a change of more than 1%, the test will pass. Anything less than 1% and the system will fail.

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