Investigation
When cold starting this car would misfire, but when warm it quickly cleared. No misfires occurred under normal driving conditions.
The first action was to plug–in with the serial scan tools (scan tools used were Diagbox and SP ACR4/G2) to see what DTCs (diagnostic trouble codes) were present. Sadly in this case there were none stored, and a look at live data didn’t reveal a great deal either, so out with the PicoScope.
The first capture I took was the coil primary current on both sides with misfire present on either cylinder #1 or cylinder #4. See Figure 1.
Figure 1: Coil primary current on both sides with misfire present
A misfire can clearly be seen, and the spark burn time seems to have taken place but dissipated too easily to ground. Is this the insulation in the coil pack? The spark plug? Or an in–cylinder condition? Which cylinder is it?
The next step was to hook up the Coil–On–Plug extension lead kit I recently purchased as secondary analysis was required to pinpoint the offending cylinder (see Figure 2).
Figure 2: Secondary misfire cylinder 4
It can be seen that cylinder #4 is the misfiring cylinder, but still we need to find out why? Again the spark which was mirrored in the primary capture from this secondary is going to ground, it’s time to find out why!
I zoomed in on cylinder #4 to take a closer look, Figure 3.
Figure 3: Cylinder #4 zoomed
I also noticed that being a DIS (Distributorless Ignition System) waste spark that the spark on the waste stroke was intact, so is the coil performing under pressure?
My next step was a coil output test, Figure 4.
Figure 4: Coil output test
This test shows that the coil made around 45 kV with some residual energy left after the burn time, which at this time I am happy with remembering that there is no misfiring under normal driving conditions. So, where to now?
Looking at the downward sloping burn line with no residual energy left (coil ringing) a possibility is high resistance in the secondary, a short to ground, or no fuel (Hydro Carbons) which we all know to be a conductor, but I didn’t really see anything in the coil current ramp that jumped out as a short?
I took out the cylinder #4 spark plug along with cylinder #1 and decided to test the resistance with my DMM (Digital Multimeter), after all I do this with plug leads so why not a spark plug?
Figure 5 is the reading from plug #4 and Figure 6 is the #1 plug!
Figure 5: Cylinder #4 plug Figure 6: Cylinder #1 plug
With the results of that resistance test I can see that cylinder #4 spark plug has resistance to ground and cylinder #1 spark plug does not.
So it is narrowed down to a spark plug issue on cyl #4. Just to confirm my thoughts I swapped cylinder #1 spark plug and cylinder #4 spark plug over, then recaptured the secondary, see Figure 7.
Figure 7: Secondary misfire with swapped plugs
The misfire follows the spark plug. An after fix capture was taken from cold start to confirm the fix, see Figure 8.
Figure 8: Scope trace showing no fault (after fault corrected)
Conclusion
A new spark plug cured this, a full set was not fitted as I service this car myself and the plugs had not been in that long, so I was more than happy to replace the defective one.
My thoughts as to the misfire clearing quickly were that when the plug heated up the internal part of the plug that was shorting to ground was expanding, and not making any further contact to the body of the plug until it cooled again and the same process repeated itself.
I think the hardest part was the condition to catch the misfire as waiting for a cold start was the biggest pain, along with the shift pattern that the owner worked and getting the car was like getting blood from a stone!
This job also came at the right time as I wanted to try the Coil–On–Plug extension leads out that I recently purchased, these helped me no end as without I could not have guessed the offending cylinder by just primary alone.
Once again without the PicoScope and accessories this job would have been guess work and swaptronics…
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