Other Articles - May - 2016

Fuel Trim: Your Crystal Ball

In the last issue of GEARS we looked at oxygen sensor testing, and it brought up some interesting questions about fuel trim. As a result, we’ll spend the next few articles learning to understand fuel trim numbers and their diagnostic value.

In a way, fuel trim is line a “crystal ball” that allows you to see what the PCM is trying to do. This month we’ll cover the basics of fuel trim, to establish a good foundation for the diagnostic techniques to follow.

Defining Fuel Trim

Fuel trim is the correction the PCM makes to try to maintain a stoichiometric air-to-fuel ratio. For gasoline, that’s about 14.7 parts air to one part fuel, or 14.7:1.

When a vehicle rolls off the assembly line, it’s programmed to deliver the ideal mix of fuel for any given condition. But, as conditions change, components degrade or faults develop; the PCM makes corrections to fuel delivery to try to maintain stoichiometry.

These corrections are based on the input from the oxygen sensor, which is why last issue’s article is so important. Before you can trust fuel trim numbers, the oxygen sensors, or air/fuel ratio sensors, have to function properly.

Short Term vs Long Term

Short term fuel trim, or STFT, is the correction that occurs right now. That is, if a lean condition just developed, the PCM would need to add fuel to maintain stoichiometry.

The short term number is always changing; it’s normal for short term fuel trim to bounce around. With a lean mixture, the STFT would be moving positive, or adding fuel, to compensate for the lean condition.

Long term fuel trim, or LTFT, is the correction the PCM learns over time and stores in keep-alive memory. For example, a vehicle that’s running rich will have negative STFT numbers because the PCM will be trying to correct for the rich mixture by reducing fuel delivery below the factory programming.

When the PCM sees that it’s been making negative corrections for a period of time, it “learns” the corrections that it’s been making and adjusts the long term fuel trim.

The LTFT value is stored in the PCM. The PCM will refer to it as if it were one of its initially programmed parameters. These learned values are stored in the keep-alive memory.

Disconnecting the battery can erase these values. This is one of the reasons why a vehicle may run poorly for a while after battery replacement or if the battery goes dead.

Figure 1 shows the fuel trim values from a vehicle that’s operating correctly with no faults. The engine RPM is in red and the vehicle is at idle. The long term fuel trim is in green and has learned a value of +2%. Short term fuel trim is in light blue, and is bouncing between about +3% to –2%; this is perfectly normal.

When dealing with a driveability problem, you’ll need to observe both short term and long term fuel trim values. Faults that only happen for a little while may not give the LTFT time to correct, so they won’t show up in the LTFT numbers.

On the other side of the coin, a problem that’s been occurring for a while may show up in the LTFT numbers while the STFT numbers look normal. We are missing one piece: total fuel trim.

Total Fuel Trim

When diagnosing fuel delivery, you can’t just look at STFT or LTFT, because individually they don’t provide an accurate representation of what fuel corrections are being made.

The actual fuel correction that the PCM is making is the sum of both STFT and LTFT. This total fuel trim number is most likely not displayed on the scan tool. It’s your job to do the math.

For example, if a vehicle has +23% STFT and +2% LTFT, the total correction would be +25%. In this case, looking at STFT alone would indicate a lean condition because the PCM is adding fuel. But the LTFT only indicates the vehicle is running well.

STFT and LTFT help determine whether the issue is recent or not, but total fuel trim is what you should be most concerned with during diagnosis. There are some arguments about what a good total fuel trim number should be, but the general industry standards are total fuel trim of ±10% are acceptable while ±5% would be ideal.

Take these values with a grain of salt: A 1998 Ford Taurus with 247,000 miles and total trim numbers of –12% might be acceptable due to wear and tear, while a 2014 Toyota Camry with 24,000 miles and the same total fuel trim might indicate a problem. The previous example would have been operating in the 0% to +5% range, which is acceptable.

Loop Status

Another important aspect of PCM operation, and fuel trim diagnostics for that matter, is loop status. There are multiple loop modes in which a PCM can operate, but most fit into two main categories: open loop and closed loop. To keep things simple, look at it this way: In open loop the PCM ignores the oxygen sensor’s input; in closed loop the PCM pays attention to the oxygen sensor’s input.

Since fuel trim numbers are based on the input from the oxygen sensors, you can only use fuel trim numbers for diagnosis while the vehicle is in closed loop. In addition, the oxygen sensors have to be operating correctly or your fuel trim numbers will be incorrect.

Conditions

Fuel trim numbers change with engine operating conditions if there’s a fault. When operating normally, the engine would still adhere to the ±5% and ±10% rule no matter how you drive the vehicle.

That’s why it’s important to pay attention to operating conditions when checking fuel trim values. A lean condition will cause positive fuel trim numbers because the PCM has to add fuel to correct the fuel deficiency. But these corrections may only be necessary at idle; not at cruising speed.

Conversely, a problem that’s setting a lean code could present itself in the opposite manner: Fuel trim could be acceptable at idle or low RPM and get more positive as RPM and engine load increase. Observing both the STFT and LTFT while operating the vehicle under different conditions is the key to successful fuel trim diagnostics.

In figure 2, total fuel trim is almost identical at idle and at elevated RPM. Total fuel trim correction at idle is 25% while total fuel trim correction off idle is 24%. This vehicle obviously has a fault, but it’s only one example of how fuel trim behaves when a problem is present.

Additional Thoughts

Another thing to consider while analyzing fuel trim is a question you need to ask yourself: Whole engine or half engine? In the case of a multiple bank engine, which includes most V-type and some 4-cylinder engines, bank 1 and bank 2 have separate fuel trim values. A general bank-to-bank comparison can eliminate a lot of diagnostic tests.

For example, if a vehicle has poor fuel trim values that are similar bank to bank, then the issue would be related to the whole engine. Possible causes might include a MAF sensor or fuel pump. Both of these items affect the entire engine.

Another example would be where the fuel trim is drastically different from bank to bank. This could indicate valve timing issues on one bank, restricted exhaust on one bank, or even a bad oxygen sensor on one bank. Regardless of the actual cause, the fuel trim values provide diagnostic direction.

What direction would you take with the fuel trim values in figure 3? For the sake of full disclosure, this vehicle is a 2004 GMC Yukon with a 5.3 liter, single camshaft engine, and two catalytic converters. Hopefully we’ve laid the fuel trim groundwork necessary for you to start using this data for diagnosis.

Remember, fuel trim is the “crystal ball” that allows you to see what the PCM is trying to do. Next month we’ll apply these principles to a structured method for diagnosing lean codes… and we’ll see what the fault with the Yukon actually is.