Tool Briefing: Lean But Not Mean

What sapped this engine's passing power?


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.

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