this is what I found in the 97 PCEM
The Evaporative Emission (EVAP) Running Loss System Monitor is an on-board strategy designed to detect a leak from a hole (opening) equal to or greater than 1.016 mm (0.040 inch) in the EVAP Running Loss system. The proper function of the individual components of the EVAP Running Loss system as well as its ability to flow fuel vapor to the engine is also examined. The EVAP Running Loss System Monitor relies on the individual components of the EVAP Running Loss system to apply vacuum to the fuel tank and then seal the entire EVAP Running Loss system from atmosphere. The fuel tank pressure is then monitored to determine the total vacuum lost (bleed-up) for a calibrated period of time. Inputs from the engine coolant temperature (ECT) sensor, intake air temperature (IAT) sensor, mass air flow (MAF) sensor, vehicle speed sensor (VSS), fuel level input (FLI) and fuel tank pressure (FTP) sensor are required to enable the EVAP Running Loss System Monitor.
NOTE:
During the EVAP Running Loss System Monitor Repair Verification Drive Cycle a PCM reset with key on, engine off will bypass the minimum soak time required to complete the monitor. The EVAP Running Loss System Monitor will not run if the key is turned off after a PCM reset. The EVAP Running Loss System Monitor will not run if a MAF sensor failure is indicated. The EVAP Running Loss System Monitor will not initiate until the Heated Oxygen Sensor (HO2S) Monitor has completed.
The EVAP Running Loss System Monitor is executed by the individual components of the EVAP Running Loss system as follows:
1. The function of the EVAP canister purge valve is to create a vacuum on the fuel tank. A minimum duty cycle on the EVAP canister purge valve (75%) must be met before the EVAP Running Loss System Monitor can begin.
2. The canister vent (CV) solenoid will close (100% duty cycle) with the EVAP canister purge valve at its minimum duty cycle to seal the EVAP Running Loss system from atmosphere and obtain a target vacuum on the fuel tank.
3. The fuel tank pressure (FTP) sensor will be used by the EVAP Running Loss System Monitor to determine if the target vacuum on the fuel tank is being reached to perform the leak check. Once the target vacuum on the fuel tank is achieved, the change in fuel tank vacuum for a calibrated period of time will determine if a leak exists.
4. If the initial target vacuum cannot be reached, DTC P0455 (large leak or no purge detected) will be set. The EVAP Running Loss System Monitor will abort and not continue with the leak check portion of the test.
If the initial target vacuum is exceeded, a system flow fault exists and DTC P1450 (unable to bleed-up fuel tank vacuum) is set. The EVAP Running Loss System Monitor will abort and not continue with the leak check portion of the test.
If the target vacuum is obtained on the fuel tank, the change in the fuel tank vacuum (bleed-up) will be calculated for a calibrated period of time. The calculated change in fuel tank vacuum will be compared to a calibrated threshold for a leak from a hole (opening) of 1.016 mm (0.040 inch) in the EVAP Running Loss system. If the calculated bleed-up is less than the calibrated threshold, the EVAP Running Loss system passes. If the calculated bleed-up exceeds the calibrated threshold, the test will abort and rerun the test up to three times.
If the bleed-up threshold is still being exceeded after three tests, a vapor generation check must be performed before DTC P0442 (small leak detected) will be set. This is accomplished by returning the EVAP Running Loss system to atmospheric pressure by closing the EVAP canister purge valve and opening the CV solenoid. Once the FTP sensor observes the fuel tank is at atmospheric pressure, the CV solenoid closes and seals the EVAP Running Loss system.
The fuel tank pressure build-up for a calibrated period of time will be compared to a calibrated threshold for pressure build-up due to vapor generation.
If the fuel tank pressure build-up exceeds the threshold, the leak test results are invalid due to vapor generation. The EVAP Running Loss System Monitor will pass and complete.
If the fuel tank pressure build-up does not exceed the threshold, the leak test results are valid and DTC P0442 will be set.
5. The malfunction indicator lamp (MIL) is activated for DTCs P0442, P0455 and P1450 (or P446) after two occurrences of the same fault. The MIL can also be activated for any EVAP Running Loss system component DTCs in the same manner. The EVAP Running Loss system component DTCs P0443, P0452, P0453 and P1451 are tested as part of the Comprehensive Component Monitor (CCM).
No mention of the Message Center. I didn't see any input from the Fuel Tank Pressure Sensor on the schematic for the Message Center. The input for fuel flow comes from the PCM.
I think Opera House has it right, the PCM uses injector pulse time (and some other info) to estimate fuel use.
. Later Doug904.
