Tool Briefing: Lean But Not Mean

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.

At rest, both sides of the injector circuit are high (battery voltage). When the PCM activates a driver, one side of the circuit drops to ground, allowing current to flow. From that point until several milliseconds after the end of the injection event, the scope trace presents quite a lot of information about the injector and its control circuit. Does circuit voltage go all the way to ground or does it 'float' slightly above zero volts? Does voltage stay down when the circuit is active or does it gradually rise? Is there a tell-tale 'hump' in voltage as the injector begins to open? Does injector pulse width increase with load? When the circuit is turned off, how high is the 'fly-back' voltage and how long does it last?

The answers to these and several other questions present a detailed picture of what's happening in the injector circuit. Combined with a current trace, it's possible to determine the condition of the injector, the PCM driver, the circuit between them and many other details about what the PCM and engine are doing. In this case, we're looking to see if injector pulse width (on-time) continues to increase as load increases.

Agreat deal of literature and many training seminars are available that focus on fuel injection and ignition system waveforms on an oscilloscope. With training, knowledge and experience, this can be one of most powerful and versatile diagnostic tools in your shop.

Fuel Delivery

Low fuel pressure will make an engine run lean at all loads, and low fuel quantity will prevent the engine from making power at high loads. On most GM engines, it's easy to check pressure and quantity at the Fuel Pressure Service Connection on the fuel rail.

By activating the fuel pump with a scan tool, or by replacing the fuel pump relay with a manual switch, the pump can be run without running the engine. GM and most other manufacturers specify testing this way. Once the pressure gauge is connected, an accurate reading depends on bleeding any air from the gauge and fuel rail, because air is compressible, so be ready to catch any spilled fuel. Fuel quantity is measured by flowing fuel from the Service Connection on the fuel rail into a calibrated container. A fuel injection test kit includes the container and all the necessary fittings and hoses for all of these tests.

On this particular vehicle, a new MAF sensor fixed the problem. Some techs would also check for high exhaust backpressure with a gauge in the oxygen sensor mounting hole. The point is, no matter what codes or symptoms are presented, it takes more than one test to arrive at an accurate diagnosis.

Loading