Tool Briefing: Lean But Not Mean

What sapped this engine's passing power?


Vehicle Application:

• 2005 Chevrolet Express 2500 4.8

Avg. Reported Mileage:

• 11013

Customer Concern:

Trouble codes P0171 and P0174. The vehicle does not go over 60 MPH under heavy throttle but will accelerate with less throttle.

Steps

Tests/Procedures

1. Scan test the Powertrain Control Module (PCM) engine load to be above 80 percent under heavy engine loads. If low, clean the Mass Air Flow (MAF) sensor and retest.

2. If the engine load is high, check the injector pulse to be high under heavy engine loads.

3. If OK, check for a fuel delivery problem, install a fuel pressure gauge, start the engine and bleed any air/take fuel sample by opening the valve (depress the button) on the fuel pressure gauge and the engine should maintain idle and fuel volume should exceed 1 pint in 30 seconds.

4. If the engine stalls during the test, replace the fuel filter and/or fuel pump.

Data and DTCs

The Diagnostic Trouble Codes (DTCs) indicate the air/fuel ratio is lean on both cylinder banks. It's important to remember that air/fuel ratio is a calculation, not a direct measurement. It's based on data from several sensors, so if the data from any one sensor changes, it will change the air/fuel ratio calculation. In Step 1, a scan tool is used to check the data reported by the Mass Air Flow (MAF) sensor.

Scan tools communicate with the Powertrain Control Module (PCM), not with the sensors themselves. When a sensor malfunctions, the PCM makes the decision to turn on the Malfunction Indicator Light (MIL) and set a DTC. But a MAF sensor can 'fail within spec,' meaning it still works well enough that the PCM won't detect anything wrong, but it's no longer accurate, and the PCM cannot detect the inaccuracy. However, if you know what to look for, you can see it on a scan tool.

This engine's MAF sensor generates a frequency signal, alternating between zero and 5 volts. As air flow increases, so does frequency. As long as the signal goes all the way down to 0 volts and all the way up to 5 volts, the sensor is working and there will be no trouble code. But if the sensor's heated element becomes encrusted with dirt, more air must flow past it to make the signal's frequency increase. The sensor still works, but it's no longer accurately measuring air flow. One way to check its accuracy is to check Engine Load on a scan tool.

Looking at the datastream or Parameter IDs (PIDs) in scan tool Mode $01. Load should increase dramatically under full-throttle acceleration (this test requires two people or a scan tool that can record data in 'movie' mode). Load is also a calculated number, using data from the MAF sensor and the Manifold Absolute Pressure (MAP) sensor. That calculation is used to adjust or 'trim' the air/fuel ratio. If the MAP sensor is working (no fault code) and Load doesn't reach at least 80 percent under a short burst of full throttle, the MAF sensor definitely needs attention.

There are some spray products available for cleaning MAF sensors, but most manufacturers recommend against it because there's no way to get it clean enough to regain its calibration. But before buying a new one, many techs will clean the MAF sensor anyway just to see if that makes a difference. Replaceable sensor cartridges are now available, saving the cost of replacing the entire assembly.

Injector Pulse

Step 2 calls for checking the fuel injectors' pulse width. Since the engine is running lean, we're looking for 'injector pulse' or 'pulse width,' the amount of time the injectors are turned on. The amount of fuel being injected depends on pulse width and fuel system pressure. In this particular case, Steps 2 and 3 could be interchanged, but both are needed for a complete diagnosis.

The best way to check injector pulse width is with an oscilloscope, and there are two ways to view that signal: current and voltage. Acquiring the current signal requires a low-amp current probe, while a voltage signal can be acquired by back-probing the injector connector. Each signal by itself provides useful information, but both are needed for a complete picture. Current provides more information about the injector itself, while the voltage waveform shows what's happening in the control circuit. In this case we're looking for pulse width, a PCM command, so a voltage signal will be enough by itself.

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