Fuel Trim System Monitoring

Regulations
The OBD II requirements for fuel system monitoring state that the fuel delivery system must be continuously monitored for the ability to provide compliance with emissions standards. The fuel system will be considered to be malfunctioning when it causes the emissions levels to exceed 1.5 times the FTP emissions standards.

Fuel Trim
Long-term & short-term fuel trim are J193O terms which will probably be new to many Saab technicians. In fact, they are just another way of describing how the pre-catalyst oxygen sensor and the ECM Work together to fine tune the fuel mixture, familiar concepts in new clothes.

Fuel Trim (previously, Saab used the term "adaptation") is the ability of engine management systems to "reprogram" themselves based on feedback provided by the oxygen sensor. We aren't actually changing the baseline values the ECM pulls from it's memory when calculating how long to trigger the injectors for each possible combination of coolant temperature, rpm, load, etc.

We use information from the oxygen sensor to fine tune the system by applying a correction factor to that calculation. The6 term adaptation came into general use with Saab in 1988 when we began to store this information in a type of memory which stayed alive even after the car was turned off (as long there was +30 power to the ECM). The next time the car was started it remembered what it had learned and modified it's calculations using this information, even when operating in open loop with a cold oxygen sensor. The fuel system was "adapting" itself to it's environment.