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The importance of asking 'why?' during automotive diagnostics

Monday, December 3, 2018 - 09:00
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Figure 3

To battle the production of the harmful gas, NOx, an EGR valve is utilized in this application. NOx is produced in abundance in temperatures exceeding 2500 deg F. The EGR valve is set to reintroduce exhaust gas back to the cylinder. The idea is to fill the cylinder with the inert gas to make the cylinders’ effective area smaller, reducing the intensity of the combustion event. This in turn cools the combustion chamber and reduces the potential for NOx production. My thought process is, if the EGR valve doesn’t open or fails to deliver EGR, it may be the root cause of the “ping” the engine is suffering from. Let’s have a look. If you refer to FIG 3 It shows the PCM’s intent to introduce EGR at the expected engine load levels. The EGR position feedback is reporting about 90 percent open, indicating that the PCM is hearing what I’m hearing and commanding a spark-retard of 20 degrees. So now the question is why is the engine still pinging, if the EGR is opening as intended? Is there a restriction of some sort, within the EGR system? A stroll through the bi-directional control function of my scan tool can answer that question right from the driver’s seat. I simply commanded the EGR valve open at idle and the engine struggled to maintain idle. Its clear to me the EGR ports were not restricted…Time to roll up my sleeves and dig in deep.

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Brought in for questioning
As I mentioned earlier, sometimes we get lucky and we can get the vehicle to tell us everything we want to know with little effort. Other times, we must push to get the answers we need. In situations like this, maintaining a structured game plan is even more crucial to prevent going down a rabbit hole. Rather than trying to find out what is broken, I chase the symptom. I do this because I know what the symptom is. I’ve felt it and I can easily recreate it. I want to see the ping. I want to see inside the combustion chamber while the engine is running to determine the health of that combustion event. I can’t think of an easier way to do this than to view it through the eyes of an ignition scope.

Figure 4

This vehicle utilizes a waste-spark system, using three coils provide the energy to initiate combustion for six cylinders. This system tethers the coils to the spark plugs with ignition cables. The good news is that I can (unobtrusively) acquire the waveforms capacitively, right from under the hood in seconds. With the help of an assistant in the driver’s seat (to place the vehicle under the fault-conditions), the testing was carried out for all cylinders under heavy brake-torque conditions. Figure 4 displays a Bank #1 ignition event in yellow and a Bank #2 ignition event in red. The waveform displayed (indicated by the red trace) demonstrates an increase in cylinder resistance as the duration of the spark burn-line carries on. We can see this because the waveform slopes upward very sharply. A cylinder that is adequately fueled has less resistance and less energy is expended, trying to maintain the plasma channel (as displayed in yellow). The significance of this event tells me that the bank #2 cylinder is indeed, under-fueled. [all the cylinders were tested, and each shared this similar characteristic].

So, I know understand why the engine was pinging. But must now ask why is the cylinder under-fueled? A quick test of injector current (using an amp probe and lab scope) ruled out any differences between the injectors ability to flow amperage. An injector balance test was also carried out and showed the ability to deliver fuel was equal among all six injectors. Gaining the answers to these tests justified my need to dig even further. I’m not dealing with a flow issue; I’m dealing with a control issue.

Being in control of fuel delivery also means being in control of the fuel injectors on-time. I will have to monitor the suspect bank’s injectors on-time during the fault and compare it to the known good — dynamically! To gather all that data is as simple as sampling current from a single common point in the under-hood fuse box. I positioned my amp probe to acquire the current flow from fuse #12 of the under-hood fuse block. This fuse’s only purpose is to supply current to all six injectors (this was very convenient as there were no other circuits that could skew my results). 

The results of the test exhibit all 6 injectors yielding the same 1-amp current ramp. Hmmm — the ignition waveform clearly exhibited a lean condition on the rear bank of cylinders yet the injector flow test and amperage waveforms yielded no difference between Bank #1 and Bank #2? My mistake was the acquisition was not acquired during the fault conditions. I assumed the fault would be present because I assumed the injectors were restricted or lacked enough current flow. It goes to prove that we learn something new every day and knowledge will continue to beget new knowledge.

Just the facts
After a few moments to gather my thoughts, I had another idea. I would place the vehicle under fault conditions while capturing the injector current. The current is the result. It represents the work performed. I know the bank #2 cylinders are under-fueled so I’m confident I will see the fault reflected in the current waveform. After recreating the fault conditions, the symptom was exhibited, and the injector current trace revealed the cause. If you refer to Figure 5, it’s clear to see the that one injector ramp dropped out and another mis-triggered. This created a lack of injector on-time, which explains the lack of fuel delivery exhibited in the ignition trace, as well as the ping. This only proves that the injector failed to open properly. We have yet to decipher the cause.

Figure 5
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