Cracks in the fuel system

Jan. 1, 2020
The Engine Control Module (ECM) constantly monitors fuel trim corrections and will flag a code when it determines that it cannot maintain a lambda mixture. The two codes you might see are system rich or system lean and they might be set for one or bo

Tips for troubleshooting system lean codes

Drivability system lean codes ECM MAF sensors vehicle diagnostics repair shop training technician training A/C training automotive aftermarket The Engine Control Module (ECM) constantly monitors fuel trim corrections and will flag a code when it determines that it cannot maintain a lambda mixture. The two codes you might see are system rich or system lean and they might be set for one or both banks of the engine. Of the two, system lean codes are the more common.

What Causes The Code?

The ECM is in charge of controlling fuel delivery to the engine, and it needs to keep the air/fuel mixture very close to lambda to keep the catalytic converter safe and happy. The only way the ECM knows that it is successful is by the feedback the oxygen sensor(s) provide. The sensors tell the ECM if the mixture it sent was too rich (too much fuel) or too lean (too little fuel). Based on that response, the ECM makes corrections that we can see on our scan tool as fuel trim Parameter Identifiers (PIDs).

There are two types of fuel trim: short-term (STFT) and long-term (LTFT). STFT is an ECM modification to correct for immediate variations and actually is required in order to accurately determine air/fuel mixture via a conventional oxygen sensor. LTFT also is an ECM modification, but is a learned value based on the trends the ECM has seen in STFT. Visit our community at and search our online archives to learn more. Using keyword "P0171" should get you started.

If the ECM has to continue to add fuel corrections short-term, LTFT will learn that correction trend and start to alter the injector on time first. If the long-term correction reaches its predetermined maximum and STFT still is trying to increase on time, the ECM sets the system lean code. It simply cannot correct for whatever condition is requiring so much extra fuel. The question is, is the lean condition caused by too much air, too little fuel or a lie?

First, Information

The first step in troubleshooting a fuel trim related code like this one is to review the conditions the engine was under when the code was recorded. That's found in the freeze frame data associated with the code.

Did the code set under idle conditions: low rpm and next to no vehicle movement? Or did the fault occur under cruise conditions: higher rpm, engine at operating temperature, vehicle at highway speeds? Both provide diagnostic direction.

On a Mass Airflow sensor (MAF) equipped engine (and most of the ones I work on now are), lean codes recorded at idle usually indicate that air is entering the engine somewhere it shouldn't. Codes recorded at higher rpm often are caused by fuel delivery problems. And a sensor that isn't telling the truth can cause both.

The MAF sensor can be used as a diagnostic aid. Start the car and allow it to reach operating temperature. Record the STFT and LTFT numbers and add the two together. This is called Total Fuel Trim (TFT). I know STFT is alternating constantly, and it's supposed to. Use the average PID reading for this exercise. Do this for both banks, if applicable. Now raise rpm to about 2,500 and record the numbers.

If TFT at idle is substantially higher than TFT at 2,500 for all banks, suspect an air leak. Air leaks don't pass that much air, but in comparison to the total amount of air ingested, it is a greater percentage of the total when the engine is idling and the throttle is closed, and has a correspondingly greater influence.

If TFT is about the same, odds are it's not a vacuum leak, so don't look for one. Instead, make sure the MAF reading is accurate and that fuel delivery is correct. And not just pressure — make sure fuel delivery volume is as it should be.

Finding Air Leaks

In the old days, we would use carb cleaner to spray around the intake to find vacuum leaks. If the leak was large enough, you would hear the engine speed increase as it drew in the combustible cleaner. At the very least, you could watch the oxygen sensor signal and see it spike rich when the spray found its mark. These techniques still can be used, but a better alternative involves your EVAP test equipment.

Use the smoke function of your EVAP tester to add smoke to the intake and watch for it to appear. You are looking for any source of air entering the intake downstream of the MAF sensor, and that includes the boot connecting the MAF to the throttle body. Common sources of air leaks include intake manifold seals and Positive Crankcase Ventilation system (PCV) rubber lines.

Not so common are engine seals, oil dipstick tube seals and even oil fill cap seals, especially if there's a problem in the engine's PCV system, like a PCV valve that's stuck open.

Testing MAF Sensors

If the MAF sensor isn't telling the truth, the ECM is operating under false assumptions and that lean code might be of the MAF's making. Most modern MAF sensors are either a hot wire design or hot film design. Both can become contaminated, and when that happens, they generally report less air coming in than there actually is. The ECM, trusting the MAF's report, adds the correct amount of fuel for the air it was told arrived in the combustion chamber. But because more air actually arrived than anticipated, the ECM already is starting out behind the eight ball. The ever-vigilant oxygen sensor is going to let the ECM know the truth by reporting the lean condition, and the ECM will start increasing fuel trims to compensate for the MAF's lie.

One scan tool PID related to the MAF's performance is the Calculated Load PID. This is a general representation of the volumetric efficiency, or VE, of the engine. VE is the ratio of the actual amount of air filling the engine versus the total of what the engine can theoretically take in as a percentage. Most normally aspirated engines operate in the 80 to 90 percent range, while those designs that get outside help breathing (turbo or supercharged) will hit VE numbers of more than 100 percent.

If you suspect the MAF of lying, take a road trip while recording the load PID, engine rpm, fuel trims and engaged gear. On a safe stretch of highway and from a rolling start in first gear, perform a wide-open throttle (WOT) pass until the upshift to second is made. Review your scan tool data, looking for that exact moment just before the shift occurred. You're looking to see what the load value was right at the point the engine was working it's hardest. If the calculated load PID is substantially less than normal, plug the numbers into a VE calculator.

No Delivery

The inability of the fuel delivery system to meet demand is a potential cause of system lean codes. Testing fuel pressure and volume are basic procedures, and we won't waste a lot of time on them here today. There is one technique that might help you narrow the cause down to one of delivery that does bear mentioning.

On your road trip, perform a hard acceleration just short of WOT (to avoid artificial enrichment) and bring it up to 60 to 65 mph while recording the oxygen sensor PIDs. Of course, do this on a stretch of road where it's legal! The sensors should go lean for a moment as you open up the throttle, but should quickly recover and start their normal swing once again. Should they stay lean under load, take a good look at fuel delivery.

When testing the fuel delivery system, always check volume. Too often I see technicians look at only pressure, and pressure can be in specification while volume will be too low to sustain the engine under greater demand.

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