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Taking a closer look at fuel injector timing patterns

Thursday, May 31, 2012 - 14:00
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Let’s examine different fuel injector timing patterns on both defective and normal systems. Depending on the system and the mode of injector firing, it can sometimes be difficult to tell what is normal, what is defective or what may be the cause of a defect. We will begin this month with a look at several defective systems.


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The upper capture in Figure 1 is from a V8 Chevrolet with one bank suffering a lean misfire due to a CMP signal defect. The CMP signal on this vehicle should stay high for 4 CKP pulses and then low for the next 4 CKP pulses. As you can see the CMP is staying low for 5 pulses and high for three pulses. The defective Duty Cycle of the CMP signal caused the PCM software processing to regularly drop the same three injector current pulses as well as doubling up on one other. The regularity of the defect CMP signal coupled with the injector pattern as seen on the scope, is a clue telling us that a sensor input, input to output processing or both are the cause of this defect. This PCM did not set a CMP rationality DTC.

The lower capture in Figure 1 is from a V6 Ford. Here we can see that some of the injector current pulses are missing and some are runted. This is an irregular defect pattern caused by the irregular CMP signal input shown in green. This CMP sensor is one of the Fords with the CMP signal mounted on a distributor type shaft. A common failure on these vehicles is that the CMP sensor drive shaft binds in the housing causing the sensor to advance in a stuttering way. As we know from our knowledge of AC sensors the speed of the sensor moving past the reluctor wheel will determine the sensors output amplitude. The speed and thus amplitude of this sensor varying wildly caused the PCM’s input circuitry to track and adapt its signal trigger level incorrectly. This engine did run rough briefly and then stall. Like the previous example, the PCM did not set a CMP DTC.

Be very careful about making assumptions about the presence and integrity of a CMP signal based on vehicle operation. Some vehicles will start without a CMP signal. Some will start and continue to run even if you lose the CMP signal after the engine is running. Some will stall if the CMP signal is lost while running. Some CMP signals if present but defective will cause drivability problems or flat out stalling due to mis-triggering of injection. Neither of these vehicles set a CMP rationality DTC as their PCM software was not sufficiently sophisticated. We cannot afford to make any assumptions about signal integrity based purely on how the engine behaves nor how we believe the PCM software works. We need to see the inputs and outputs for ourselves.

In our next two examples shown in Figure 2 we can see what appear to be two examples of peak and hold style injector current patterns that are being mis-triggered. The defective injector pulses are demarcated in the red circles. However our inputs, the CKP and CMP signals are normal, regular and present. While these defects are from two different vehicles with the same engine they at least look like they might have the same defect.

This is not the case, they are different defects. The capture in the upper portion of Figure 2 is from a vehicle where all injector timing inputs were normal yet the output is defective. Indeed the injector driver section of this PCM was defective, a common failure on the GM 2.2L LN2 engine. This engine simply registered misfire DTCs only. However, the lower capture in figure 2 is indeed an input problem just not any of the inputs shown. The TPS signal on this vehicle had irregular spikey drop outs causing the injectors to go into decel fuel cut only to recover and look like a fast acceleration. This vehicle did not set a TPS DTC because the PCM software has trouble differentiating between an TPS signal variation caused by actual gas pedal movement and TPS signal defect.

Always use all the data at your disposal, scan data, system knowledge and your scope. These cases are yet more examples of not relying on DTCs or the PCM’s rationality software. Computers have huge data capacity and are extremely fast but they are not very smart some times.

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