So, what else can I find with fuel trim values?

Problems come up when base pulse width is not correct for assorted reasons. Fuel pressure too high or too low is common. Low fuel pressure is far more prevalent, due to plugged fuel filters or worn pumps. As the pump pressure starts to decay or the fuel filter begins to clog, the computer will respond by adjusting long-term fuel trim numbers in an attempt to get short-term fuel trim back to zero correction. Often we will see this trend more evident under higher load cells.

Fuel trim is also a great help in diagnosing out-of-spec sensors, both in speed density and mass air flow sensing systems. Very often a careful and thoughtful diagnostic approach is necessary. The customer complaint may be that the engine pings or has poor performance. A look at long-term fuel trim will verify that the computer is adding fuel (positive fuel-trim numbers above 10 or block-learn above 145) to compensate for a lean condition across a range of load conditions. If the vehicle has left- and right-bank oxygen sensors, often comparing fuel trim on both banks can tell me if I have a problem peculiar to one bank, such as a badly restricted fuel injector.

But now I need to do pinpoint testing. I always start with the easiest stuff. First, hang that ancient tool that is so often overlooked, and so easy to use -- the vacuum gauge. No point chasing computer controls if my needle indicates a mechanical problem, now is there? Next, are the sensors that affect base pulse width in spec? This is not always very easy to determine. The best way I know of is to use your scan tool and lab scope on known-good vehicles as often as you can. Record the frequency of a good digital MAF or the waveform of a good analog MAF. Remember, too, that BARO readings, whether directly measured or inferred, have a large impact on base pulse width. MAP sensors can be just as flaky sometimes as mass airflow sensors. Honda Map sensors, for example, can cause problems when out of spec by as little as 2/10ths of a volt.

I generally leave fuel pressure and volume testing for later point due to the inconsistency of manufacturers in providing easy access to tapping into the fuel rail. But don't make the mistake of assuming it is okay. The lack of a Schrader port is no guarantee that a pump is not going to fail!

If all my information at this point looks good, it is time to consider restricted fuel injectors. Unfortunately, this may prove difficult as the location of fuel injection connectors for balance testing is sometimes flat impossible. But I have had restricted injectors set system-lean codes and cause massive drivability problems, so don't rule this one out just because it's hard to check.

Fun factors

In OBD II (and even in some OBD I) applications, the computer itself uses the fuel trim values for diagnostic purposes. The computer will look at fuel trim to determine if evaporative purge, secondary air injection, and even EGR (in some cases) are working. It's helpful to know this because sometimes problems with fuel trim will end up setting codes for apparently non-related systems. Also, some manufactures will use the oxygen sensor downstream of the catalyst for fuel trim if problems develop in the upstream O2 sensor. Techs on iATN have proved that even a heater failure in an upstream O2 sensor can cause the PCM to revert to the rear O2 sensor and not switch back to the front unit even when it comes to life. And don't forget, the computer is depending on an accurate reading from the oxygen sensor to make fuel trim adjustments. Use your gas analyzer to verify that a shifted O2 sensor is not "lying" to the computer, causing it to add or subtract fuel when such action is not called for.

In closing...

Fuel trim numbers, unfortunately, are only available with a scan tool. As we independents know too well, many Asian and Euro vehicle manufactures got dragged kicking and screaming into data stream by OBD II, which leaves us with no fuel trim numbers on many, if not most, pre-OBD II vehicles built by said manufacturers. Another unfortunate fact is that Ford, for some reason I cannot fathom, often provided only short-term fuel trim numbers to the scanner on many of its vehicles before the advent of OBD II. This deprives us of the ability to look for trends in vehicle performance and forces us to rely on a trouble code setting, which may not happen until long after a drivability problem surfaces. So, we have to deal with what information we have and make the best of it. Rest assured, if short-term fuel trim is pegged at one end, long-term will also be pegged even though we can't see it on the scanner. Wise use of this data can steer us in the right direction for starting our diagnostic sequence.