How to diagnose stalling engine problems

Different load conditions can cause challenges when diagnosing the stallng engines.


Once the microprocessor has calculated the crank angle space, the ignition discharge and fuel injector on time will be commanded. However additional data will be needed for these commands to take place. The air weight that each cylinder contains will need to be calculated along with the throttle position, air temperature and engine coolant temperature. This data is used by both the fuel control map and ignition control map. If this data is incorrect for the engine operating condition and the microprocessor cannot correct for this data, or the microprocessor commands inappropriate functions based on this data, the engine can stall. 

Now let us look at a stalling engine using the oscilloscope. As can be seen in Figures 1, 2 and 3, the stalling engine sensor data is displayed; the yellow trace on channel 1 is the crankshaft position sensor, the red trace on channel 2 is the RPM signal, the green trace on channel 3 is the electronic spark timing signal, the blue trace on channel 4 is the camshaft position sensor, the white trace on channel 5 is the injector, the purple trace on channel 6 is the ignition coil, the orange trace on channel 7 is the throttle position sensor, and the brown trace on channel 8 is the manifold absolute pressure sensor.

In figure 1, in order to analyze the data from the stalling engine, you will need to compare the input and output data. When looking at the data you can see that the yellow trace, which is the crankshaft sensor, failed first. The CKP sensor goes to the ignition module where this signal is converted to a digital signal that is the RPM signal. The microprocessor uses this RPM signal to produce the spark command which is the EST signal. When the CKP signal fails, the RPM and EST signal stop. By evaluating the data, it is easy to see the CKP sensor is bad. It will be important to find which signal failed first.

In figure 2, in order to analyze the data from the stalling engine, you will need to compare the input and output data. When looking at the data, you can see that the purple trace, which is the ignition coil, failed first. When evaluating this data, you can see that the EST command is present but the module did not complete the coil circuit, thus indicating the module, module power, or module ground is bad. The scope leads will need to be moved to the module power, module ground, and module command signal (EST) to determine which is bad.

In figure 3, in order to analyze the data from the stalling engine, you will need to compare the input and output data. When looking at the data, you can see that the blue trace, which is the camshaft position sensor, failed first. When evaluating this data, you can see that the CMP is pulled to ground. This indicates that the CMP sensor is bad, the PCM output voltage to the CMP is failing, the signal wire is bad or the power or ground to the CMP sensor is bad. The scope leads will need to be moved and the CMP sensor will need to be disconnected to determine which is bad.

(Editor's Note: Bernie Thompson, president of Automotive Test Solutions (ATS), offers a comprehensive approach to diagnosing stalling engines. To contact ATS, visit www.automotivetestsolutions.com.)

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