Fuel trim diagnosis

I have a 2001 jag s type that is misfiring withou a CEL. I hooked up a scantool to check the fuel trims to see of it can point me into the right direction. From what I can tell I know it doesn’t have a vacuum leak but I’m having trouble interpreting the other data.
Does anyone have any experience with fuel trims that can help?

Fuel trims indicate how much fuel the computer is adding to the engine from the signals it receives from the O2 sensor(s). This is displayed as a percentage.

Short term fuel trim is what the computer is doing at that time to control the fuel mixture. Long term fuel trim is what the computer has learned over a certain duration of time to control the fuel mixture.

If the STFT/LTFT is in the + percentages, it means the computer is seeing a lean condition so it adds more fuel to the engine by increasing the pulse widths of the injectors. If they’re in the - percentages, it means the computer is seeing a rich condition so it adds less fuel to the engine by decreasing the pulse widths of the injectors.

STFT"s usually range from -10% to +10%. But can change to -20% to +20% when there’s a sudden change in engine operation such as sudden opening and closing of the throttle.

LTFT’s are more accurate indications of whether the engine is running too rich or too lean because the computer has learned the ideal air/fuel ratio over a longer duration of time.

In an ideal world this would as close to 0% as possible. But factors such as engine condition can cause the LTFT’s to be +/- 5%-8%. But when the LTFT’s starts approaching +/- 10% , there’s an issue that should be diagnosed.

Hope this helps!


Both the short term and long term fuel trim displays look normal from what I can see. The short term is staying within the +/- 5% range, and the long terms are close to zero. I don’t think fuel trim is going to help diagnose this problem, at least not from that experiment. Three of the O2 sensors look to be working correctly also. There’s a 4th O2 sensor channel that is much different from the other three. Is that just an unused data channel? i.e. there’s only 3 O2 sensors on this engine?

You should be able to look at the misfire monitor in global OBD2 by looking at mode6 non continuous monitors if my mind’s working right. Looking at fuel trims wont really give you a clue as to which cylinder it is missing on. The fuel trims I see look fine though.

If I remember right, the fuel trim figures just tell you what the computer is trying to do to compensate for a lean or rich fuel mixture. If the mixture is too lean, it will try to richen the mixture up to its limits. Visa versa if the mixture is running too rich, it will try to lean the mixture out. It just indicates a problem somewhere else if the trim figures are getting to the limits one way or another. So just an indicator of a problem rather than the problem itself.

Give it a thorough inspection for vacuum leaks.

One point to consider about a miss is that unconsumed oxygen will hit the O2 sensors confusing the ECM. If the software is capable it may ignore O2 sensor data closely following a miss. However, you would have to have access to the source code of the ECM or at least a software design outline to know for sure.