Fuel Trim in action

There are plenty of articles and much is spoken of Fuel Trim analysis.

However the message has got confused, after attending seminars by 'experts' I was alarmed at the level of miss information.

Looking at oxygen sensor readings will show trends, however the Trim will continue to adjust fuelling so fuel problems are effectively masked, ensuring correct catalyst operation. So much so that the O2 sensor could show normal response, despite abnormal conditions.

One 'expert' suggested measuring response time of the pre cat sensor after over run as a method of measuring short term fuel trim, utter nonsense. The response time will depend on many variables, the biggest being the distance from the exhaust valve that the sensor is positioned. The quanity and temperature of the exhaust gas content will also effect the response time. How can a 1.1 N/A expect to have the same response time as a 2.0 turbo.

Another misleading piece of advice was the true condition is always reported by the post cat sensor. While this has some foundation, the post cat sensor will report on the effectiveness of the catalyst, so should show less oxygen than the pre cat in a correctly fuelled engine. If the catalyst is faulty the post sensor may indicate a lean condition, that simply does not exist.

The best way to evaluate the trim is to use serial data, comparing long and short term trim at different engine load and speed sites.
Forcing the engine lean (on overrun) and rich (on snap throttle) will show oxygen sensor response but little about trims.