What is fuel control?

April 1, 2014
Fuel control plays an important part in the engine management system’s ability to deliver, control and keep emission levels at specification. 

If a problem develops, STFT will try to compensate for the fault immediately. LTFT learns the correction over time. If a correction can’t be made, a DTC will set.

Fuel control plays an important part in the engine management system’s ability to deliver, control and keep emission levels at specification. Fuel control is the correct amount of fuel mixture, delivered at the right time and that the Engine Control Module (ECM) constantly monitors and adjusts.

When most technicians think about fuel control, they probably reflect on the days when GM vehicles had carburetors. The objective behind fuel control is to keep the air/fuel mixture at a stoichiometric level (that is, 14.7 pounds of air to every pound of fuel). If you take a trip back to the GM system, you might even remember using the “Integrator” and “Block Learn” scan tool Parameter Identifiers (PIDs) to see what the command was from the ECM. If you were a technician that worked on just European or Asian vehicles, you might not have heard about Integrator and Block Learn, but don’t worry. The modern name for Integrator is Short Term Fuel Trim (STFT) and for Block Learn it’s Long Term Fuel Trim (LTFT). The numbers for Integrator and Block Learn are different than those used for STFT and LTFT. The number used for a perfect mixture adjustment is zero for both STFT and LTFT rather than 128 for Integrator and Block Learn. With the history out of the way, let’s move on to using STFT and LTFT in the real world.

You cannot diagnose vehicles today without using the Generic/Global OBDII PIDs for fuel trim.  In the technician ranks today, there still are many who do not understand what fuel trim/control is all about. This lack of understanding of what is a most fundamental task for the ECM leads to improper repairs and wasted time. In addition, fuel trim data can be used to successfully diagnose a variety of drivability issues, but only if the tech assigned the problem understands how other faults (like misfires and restricted exhausts) will impact the numbers.

On OBDII vehicles, fuel trim is how much fuel the ECM commands the injectors to feed the cylinders. It does so by controlling the time the injectors remain open. Because fuel pressure is a given, the ECM can calculate how long to hold the injector open in order to deliver a specific amount of fuel. But that’s in a perfect world. The upstream oxygen sensors provide feedback to the ECM on how well it’s doing its job. The ECM uses this feedback to alter the injector’s on time. If the oxygen sensors report a LEAN condition, the ECM increases the length of time the injector is open.  To the contrary, if the oxygen sensors report a RICH condition, the ECM decreases the amount of time the injector remains open.  This is the constant switching from negative to positive STFT numbers you see on your scan tool, and is necessary in order to get conventional oxygen sensors to switch voltage. Oxygen sensors, contrary to their name, do not actually measure the amount of oxygen in the exhaust stream, but instead they react to changes in oxygen content. But that’s a whole other topic for another time.

The numbers themselves, both STFT and LTFT are percentage adjustments to the “perfect” amount of fuel as determined by the ECM. Zero is perfect, or stoichiometric, fuel trim.  Fuel trim numbers that are positive means that the ECM is adding more fuel than it originally calculated the cylinders needed and fuel trim that is negative means that the ECM is adding less than it originally calculated.  Fuel trim numbers (STFT and/or LTFT) that is more/less than +10/-10 (I use +6/-6 when I diagnosis a problem vehicle) indicates there is a problem somewhere. Excessively high variances either way (these limits vary by manufacturer) will set a System Lean (ECM is adding more fuel – positive trims) or System Rich (ECM is adding less fuel – negative trims) code.

STFT vs. LTFT

Scan data from OTC scan tool. Notice the high LTFT. This engine has an extremely lean condition. Image courtesy of Pete Meier

STFT is fuel trim in the short term. The ECM constantly moves it up and down, trying to make it as close to zero as possible, equaling the optimal air/fuel mixture. Every time you restart the vehicle, you restart the STFT reading.

LTFT is the average of STFT in the long term. So, if you have STFTs that tend to range between +5 to +15, expect your LTFT to be about +10. While STFT changes rapidly, it takes time for LTFT to move up or down, because it takes a lot of STFT readings to alter the cumulative fuel trim average that is LTFT.

STFT makes constant adjustments in closed loop to keep the air/fuel ratio as close to stoichiometric as possible. Decreasing oxygen sensor voltage causes an increase in STFT. While increasing oxygen sensor voltage causes a decrease in STFT.

But let’s say that the engine starts running lean due to a weak fuel pump. The condition worsens as the fuel pump delivers less and less fuel. To compensate, STFT adds more and more fuel by holding the injectors open longer and longer. As the included chart shows, the trend steadily worsens and the LTFT increases.

Both STFT and LTFT can compensate for a rich or lean condition, but only so far. When they have gone as far as they can, they run out of steam at a point called their Adaptive Limit. When the PCM determines that both are pegged out at their limits, and a severe air/fuel imbalance still exists, it will store a Diagnostic Trouble Code (DTC).

Fuel trim tips and tricks

This failed PCV elbow is all it takes to cause a lean idle condition. Smoke from your EVAP tester, though, makes them easy to find. Image courtesy of Pete Meier

Reset Adaptive Fuel: Do not forget to reset fuel trim / adaptive fuel or fuel trim before sending the vehicle out the door after performing a fuel trim related repair, such as a clogged fuel filter or defective fuel pump.  If not, the vehicle may not idle correctly or suffer new drivability issues until it can correct itself. Clearing the codes is not enough to reset the adaptations. Check your service information on the proper procedure for clearing these learned parameters.

Causes For a Lean Fuel System: vacuum leaks, clogged fuel filter, worn fuel pump, clogged fuel injectors, HO2S problem, wrong PCM command and mechanical problems.

Causes For a Rich Fuel System: high fuel pressure, leaking injectors, EVAP problem, HO2S problem, MAF problem, wrong PCM command and mechanical problems.

Faulty PCV valves can cause vacuum leaks that can force the system to be commanded rich.  Some manufactures keep moisture down and prevent the valve from freezing by heating the PCV valve. Some PCV systems may use engine coolant to heat themselves. Remember if the PCV system is not functioning properly the fuel trim can be affected.

Common DTCs that are associated with fuel control problems are; P0171 (Lean - Bank 1), P0172 (Rich - Bank 1), P0174 (Lean - Bank 2) and P0175 (Rich - Bank 2) along with any O2 Heater, O2 Voltage, or Air Fuel Ratio problems. Always make sure to perform a complete diagnosis of the system to make sure that there is proper input and adjustments from the ECM.

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