Fuel trim for diagnostics

Jan. 1, 2020
Is the cause of a P0171 the same thing on a Ford 5.4 engine as it is on a GM 4.2 engine?

Day after day, we work on cars with drivability problems. Many of us see a lot of GM and Ford vehicles in our shops, which means we see a lot of P0171 and P0174 (System Lean:  Banks 1 and 2) diagnostic trouble codes (DTCs). But is the cause of a P0171 the same thing on a Ford 5.4 engine as it is on a GM 4.2 engine? Hold that thought while we delve into the theory of fuel management.

The Ranger uses a mass airflow fuel management strategy.

The vehicles that come to our shops use one of two types of fuel management strategies: speed-density or mass airflow as measured by the mass airflow (MAF) sensor. In both strategies, the weight of the air entering the engine must be calculated (as opposed to volume) so the Engine Control Module (ECM) can supply the correct amount of fuel (also by weight) to efficiently power the engine. The outcome of these two different fuel control systems is the same: an engine that produces good power, runs efficiently and has a clean exhaust.

Mass airflow engine air calculations are the simplest for me to understand. There is a MAF sensor installed somewhere in the air intake system between the air filter and the throttle plate. This strategy does not need to know anything about throttle position, exhaust gas recirculation (EGR) flow, engine temperature or even barometric pressure. All the ECM is interested in is the total air mass that is flowing through the MAF sensor. In order to be accounted for, all the air entering the engine must travel through this sensor.

The Envoy, on the other hand, relies on the speed-density fuel management system.

If some of the air bypasses the MAF sensor, say through a torn intake tract boot or leaking manifold gasket, the ECM has no way of knowing that and will only add the amount of fuel required by the air mass it does know about. This means a miscalculation that results in too little fuel being added for the actual amount of air that has entered the engine. And that’s not the only way a mistake can be made. If, for some reason, the MAF sensor does not measure the air flowing through it properly or if the oxygen sensor feedback is not reported correctly, diagnostic trouble codes will be stored in the ECM and the Check Engine light will be illuminated.

One the amount of air (mass) is determined by the ECM, it uses this formula to calculate the proper amount of fuel (also by mass) for correct engine operation.

FUEL MASS = AIR MASS * SHRTFT * LONGFT

  EQUIV_RATIO * 14.64

With a man-made exhaust restriction in place, the Ranger exhibited low power and the only PID that changed was the LOAD PID, reading 74 percent when it should be reading closer to 95 percent.

The speed-density method of fuel management achieves the same end result as the mass airflow strategy but the route taken is much different. This method of calculating the weight of the air ingested into an engine depends on several different sensor inputs and several mathematical calculations. Data needed by the ECM to properly calculate the weight of the incoming air (air mass) includes engine rpm, engine volume, throttle plate position, manifold absolute pressure, intake air temperature and barometric pressure.

Each of the two fuel management systems have their strengths and their weaknesses, but all in all, each of them does a pretty good job of keeping an engine running smoothly, efficiently and with a clean exhaust. A few years ago I thought the speed-density system was on the way out, being replaced by the mass airflow fuel management system. Today, though, some of the vehicles with boasting ultra clean exhaust emissions are using the speed-density system, so it would be a good thing to dig a little deeper into this strategy and compare the two fuel management systems. Being aware of the way they operate will give you an edge on the proper diagnostic processes.

Even though the Envoy is struggling to stay at 60 mph, the data PIDs show nothing out of the ordinary. Just goes to show there is more to drivability than just knowing how to read a scan tool.

As we discuss the fuel/air management systems, I would like to present a few things I have found that simplified the understanding of how they work. Both the mass airflow and speed-density systems calculate the weight of the air that enters the engine. The fuel trim Parameter Identifiers (PIDs) on the scan tool are telling you how the ECM is adjusting the fuel injector’s “on time” (pulse width) in order to add the correct amount of fuel based on that measurement. The ECM can only regulate the amount of fuel based on the amount of air it knows about. The ECM cannot do anything about the relationship of air to the amount of fuel, but it has everything to do with the amount of fuel delivered in relationship to the air entering the engine.

Basic Mass Airflow Strategies
Any line tech that has worked with OBDII knows that fuel trim PIDs are displayed in two ways, short term and long term fuel trims. V6 and V8 engines have two sets of both fuel trim PIDs, one for each side (bank) of the engine. Even some in-line four cylinders have two sets, one per cylinder pairing (cylinders 1-4 and cylinders 2-3 t ypically).

Short Term Fuel Trim (STFT) is representative of the current adjustments the ECM is making to injector pulse width (fuel control) and Long Term Fuel Trim (LTFT) is a learned correction over a period of time. Positive numbers indicate the addition of fuel (lean correction) while negative numbers indicate removal of fuel (rich correction.)

At cruise, the Ranger hardly noticed the exhaust was plugged. And neither did the PIDs.

In the mass airflow strategy,the MAF sensor is in control of telling the ECM how much air the engine is inhaling. If there is an air leak in the air intake system any place between the sensor and the intake valves (air boot, leaking vacuum lines leading to the intake manifold or leaking intake manifold gaskets are just a few possibilities), the fuel trims will show a positive number at idle. This is due to air entering the engine that has not been properly measured. The ECM incorrectly thinks the engine has taken in less air than it actually has. The end result is a lean idle condition that the ECM will attempt to correct using STFT and LTFT.

Raising the engine speed from idle will cause the airflow into the engine to increase and the trims to trend back closer to normal. Since the air entering through the leak is of a smaller proportionate amount at higher throttle openings, the impact on fuel mixture is reduced, even eliminated. This depends a lot on the size of the air leak and the position of the air leak.

As with the Ranger, the GMC hardly noticed its loss of power at cruise.

Dribbling and leaking fuel injectors, on the other hand, will display a negative fuel trim at idle as more fuel is added than needed. As the engine speed and engine load are increased, the trims will move back closer to normal. Just as with the air leak, fuel leaks at idle represent a greater portion of the total fuel getting to the cylinder at idle than it would at higher loads and engine speeds. The trims will also work this way for a leaking purge solenoid or leaking fuel pressure regulator, since there is an excessive amount of fuel that is not accounted for by the PCM calibrations.

Errors in the actual MAF sensor output (bad sensor ground, contaminated hot wire/film, and the like) will show up with a lower than normal load PID value, the long term and short term fuel trim data will also point to the error in air calculation dependent on the MAF fault. Later on there will be a case study on this issue. Since all MAF sensors do not fail in the same way, it is impossible to say the trims will show specific things, such as “short term trims will follow throttle position”. Over the years I have learned to never say never and or have hard and fast rules for things.  Rather, I try to encourage technicians to learn the theory of operation for their specific system and then apply the theory to the problem they are facing in their bay.

Basic Speed-Density Strategies
Speed-density fuel management systems uses several different sensor inputs to calculate air mass. Any data that is misreported by any of the sensors; MAP, TPS, O² sensor, ECT or IAT can cause a miscalculation in the air mass. Each different vehicle manufacturer will use the inputs of these sensors differently, so don’t think just because the GM product you worked on yesterday calculated air mass one way that Subaru that’s here today will be the same.

The red cursor shows where the vacuum leak was introduced. Notice how the STFT, LTFT and O2 PIDs reacted.

Vacuum leaks on a speed-density engine are not unmetered air and do not cause a shift in fuel trim. Vacuum leaks will also not cause a P0171/P0174 code. And, unlike a mass airflow system, a vacuum leak on a speed-density engine will raise the engine idle speed since the leak is the same thing as having the throttle blade open. Exhaust restrictions can set a P0172/P0175 (System Rich:  Banks 1 and 2)) codes on some vehicles, depending on how the sensor software is written. Low fuel pressure will cause P0171/P0174 codes, just the same as a mass airflow system.

Speed-density And Mass Airflow Compared
Two different vehicles will be used to demonstrate the different fuel management systems.

Vehicle No. 1 is a 2001 Ford Ranger powered by a 4.0 OHC engine. The power runs through a 5-speed automatic transmission and the odometer shows 250,000 miles have been traveled. The Ranger uses a mass airflow fuel management system.

Vehicle No. 2 is a 2002 GMC Envoy. The engine is the 4.2 in-line six-cylinder and is also mated to an automatic transmission. The odometer shows 140,000 miles have slid by under the vehicle. The Envoy uses a speed-density fuel management system.

The vacuum leak is so great the throttle blades are closed to their minimum position. When the throttle blades cannot control the engine speed, the fuel injectors are turned off by the PCM.

Both of these vehicles are in very good shape and have no problems. A scan tool was used to graph data from both vehicles and each vehicle was driven with a restricted exhaust, then each vehicle had a vacuum leak induced so the scan data would show how each fuel management system reacted to the same problem.

With restricted exhausts, the only symptoms were lack of power and the transmissions were reluctant to shift out of second gear while climbing a 4 percent grade at 60 mph. Both fuel management systems were able to keep the air/fuel ratio well in control and the exhaust emissions in check. As the vehicles went into the cruise portion of the test drive, the exhaust flow slowed to the point the restriction in the exhaust had no effect on the engine operation.

When vacuum leaks were induced on each engine a big difference can be seen in the two fuel management systems. The mass airflow system shows a lean condition which will cause the fuel trims to move positive in an attempt to add enough fuel injection pulse width to keep the fuel delivery in check with the amount of air that is actually passing through the engine. Watching the data on the speed-density engine shows the only effect the added air has is an increase in engine rpm. There is no effect on the air/fuel ratio since the air leak amounts to the same thing as opening the throttle blade. The Manifold Absolute Pressure (MAP) sensor is able senses the change in manifold pressure and the ECM opens the fuel injectors to compensate for the added air passing through the engine.

When it comes to diagnosing emission related diagnostic trouble codes, it is very important you are aware of what fuel/air management system you are working on. Without a good understanding of each system operates, you will be wasting a lot of time diagnosing and repairing the problems that come to your bays. 

About the Author

Albin Moore

Albin Moore spent the first 21 years of his working life in the logging industry. In 1992 he made the transition to shop ownership and opened Big Wrench Repair in Dryden Washington. Since opening the shop he has moved the business to specialize in driveability problem analysis, both with gasoline and diesel vehicles. Albin is an ASE CMAT L1 technician, and brings with him 40 years of analyzing and fixing mechanical and electrical problems. Albin enjoys sharing his many years to experience and training with the younger generation as a way of improving the quality of the automotive repair industry.

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