During the initial diagnosis of this customer's concern, the original technician could hear compression leaking from the vehicle when the engine was running. At first, he suspected an injector seal to be leaking, creating a density misfire. A smoke machine was used in the cylinder to identify a potential compression leak external to the cylinder. After the smoke machine was hooked up, smoke was present in the intake manifold on TDC compression. A mechanical problem was now suspected but being a GDI engine, this test was not conclusive enough to rule out a carbon build-up issue. The customer authorized the shop to tear the intake manifold off to identify the potential problem. Before the tear down was done, I suggested we prove another way what the other technician was seeing with the smoke machine. These tests and analysis were performed in order to prove what was wrong with this vehicle before expensive tear down was done.
Get a routine, follow your routine and back your diagnosis
As a technician that works on a lot of vehicles that have been to multiple shops, I have to stay centered on my diagnostic routine. Being centered on a routine first means we need to have one. As I have grown stronger in my diagnostic strategies, it has helped me funnel my testing in a way that is logical and produces results. Earlier in my career I simply started with eliminating possibilities until the answer was found.
While deductive logic is the foundation of how we all work it should not be the sole method. Strategy based diagnostics start with the scan tool more often than not. This article isn’t based on scan data strategy but that is where this diagnosis started. Currently I am funneling my testing methods so every test is justified by the previous test’s results. My very first tests are going to start with the easiest and least time consuming to rule out the most possibilities. When it comes to a misfire, the scan tool can offer a big funnel when it comes to weeding out fuel, ignition or mechanical issues.
Once I have made a conclusion that this is likely a mechanical problem, I will stop testing for ignition and fuel and focus on what the data has led me to believe too far.
Starting my routine
I’m constantly learning new techniques to better understand/diagnose the systems that I’m working with. Luckily for misfire concerns I think there is a fundamental first test that we all should think about using to gain a direction as to what our next test should be. This first test for me is secondary or primary ignition analysis. Secondary and primary ignition analysis has not changed much since scopes were first used on the internal combustion gasoline engine. Certainly obtaining these waveforms has gotten a little more difficult over the years with the invention of the COP coil and transistorized coil packs.
|Getting a pattern on a COP coil can be a challenge. A COP paddle probe helps!|
However, the fundamentals of how ignition systems work is the same. Power and ground is provided to a primary coil with few windings. Once the ground side is released, the collapsing magnetic field surrounding the primary coil is induced into the secondary windings. Since there are more windings in the secondary coil, voltage is increased significantly. Now a high voltage in the secondary winding follows the easiest path to ground which we all hope is in the combustion chamber near TDC compression.
Utilizing ignition waveforms allows a technician to look at compression spark and fuel all in the same test. This gives us the quickest and best diagnostic direction for additional tests. i say direction because like I stated earlier in the article I’m not willing to condemn any one part/parts off of one test. I’m going to use my results from the secondary pattern to make a hypothesis on what is going on and what my next test should be. So I grab my newly purchased COP wand and start looking at this misfiring vehicle. As I go down the line of coils the pattern on cylinder 2 has a repeatable event in the waveform at park idle no load (Figure 1).
Making a hypothesis to funnel future tests
Performing secondary analysis is tricky sometimes. You really have to trust your equipment and hope what you’re seeing isn’t noise from a multitude of other contributors. One of the best ways to trust your pattern is to get primary ignition. With primary ignition you are essentially hard wired into the primary circuit. You can trust that what you’re seeing is true. However, we don’t always have the time or ability to get there with transistorized cop designs. If we are going to use secondary for analysis, get to know the good cylinders first before going after the one you suspect to be bad.