Long Term Fuel Trim: A Nod in the Right Direction
by ATRA Tech
members.atra.com www.atra.com
transmission or outside, in one of the many computer systems or circuits that control virtually every operation in today’s cars? The problem is, it doesn’t have a
A
code. Faulty connections, slightly out- of-range sensors, and even computer problems can all hide within the range that’s just close enough not to set a code, while being just bad enough to cause transmission issues. Wouldn’t it be great to have some-
thing you could check that would at least indicate whether you were dealing with a control problem or not? There is: Long Term Fuel Trim.
Understanding Fuel Trim Fuel Trim has been available as
a diagnostic tool since OBD-II first showed up on your scan tool. Fuel Trim is the term used to describe how the computer controls the amount of fuel being delivered to the engine. Short Term Fuel Trim is the moment-by- moment changes to fuel delivery, based on the signal from the primary oxygen sensor (or sensors). When everything’s working prop-
erly, Short Term Fuel Trim should vary up and down, generally -5 to +5. That tells you the computer is controlling fuel delivery based on the oxygen sen- sor feedback. Long Term Fuel Trim is a different story. Long Term Fuel Trim is a learned
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customer brings his car in to your shop with a shift- ing problem. Is it inside the
Figure 1: By using the Global OBD II setting your scan tool will display fuel trim the same on every car you check. This’ll give you a chance to become familiar with the readings.
Figure 2a: This vehicle has a v-8 engine and reports bank 1 and bank 2. Notice bank 2 has a large disparity from bank 1, at -6.2. This could be caused by any number of problems: a leak- ing fuel injector, an O2 sensor, a bad spark plug or spark plug wire etc. But since it’s only on one bank it would not be something that would affect both sides, like a MAS or TP sensor.
response, created over a period of time. Its primary purpose is to shift overall fuel delivery up or down, to allow Short Term Fuel Trim to maintain a
full range of control in each direction — from -100 to +100. That is, Short Term Fuel Trim has a range it can oper- ate within. When the computer can no
GEARS January/February 2012
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