VEHICLE : Peugeot 206 1.4
engine code : KFW
YEAR : 2006
Symptom : Misfire at cold start that clears quickly.
This car when cold on the initial start would misfire but would quickly clear. No misfires under normal driving.
My first action was to plug in with the scan tool serially to see what DTC’s (diagnostic trouble codes) I had to be getting on with. There was none, scan tools used were Diagbox & SP ACR4/G2.
A look at live data also didn’t reveal a great deal, so out with the Pico.
First capture I took was the coil primary on both sides with coil current with misfire present on either cyl #1 & cyl #4, below (figure 1)
A misfire can clearly be seen, the spark burn time seems to have taken place but dissipated too easily to ground, is this the insulation in the coil pack, spark plug, or an in cylinder condition, which cylinder is it ?
My next step was to hook up the Coil On Plug extension lead kit I recently purchased as secondary analysis was required to pin point the offending cylinder, a First Look Sensor would have done the same too. Figure 2.
It can be seen that the black channel which is cyl #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 easily to ground, so it’s time to find out why?
I zoomed in on cyl #4 to take a closer look, Figure 3
I also noticed that being a DIS 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, Figure4.
This test shows that the coil made around 45KV 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 or 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 cyl #4 spark plug along with cyl #1 & decided to test the resistance with my DMM, after all I do this with plug leads so why not a spark plug ?
Figure 5. is the reading from plug #4 & figure 6. is the #1 plug !
With the results of that resistance test it tells me that no #4 spark plug has resistance to ground & no #1 does not.
So it is narrowed down to a spark plug issue on cyl #4, so just for proofs sake i swapped #1 & #4, then recaptured the secondary. Figure 7.
The misfire follows the spark plug.
An after fix capture was taken from cold start to confirm the fix, Figure 8.
A new spark plug cured this, a full set was not fitted as I service this car myself & 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 & not making any further contact to the body of the plug until it cooled again & 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, 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 & accessories this job would have been guess work & swaptronics..........
Last edited by Robski
on Tue Sep 04, 2012 3:16 pm, edited 1 time in total.