Customer brought vehicle in complaining "The car shakes when I've got the cruise control on at 80km/h and go up a slight incline but never any other time. I can't make it do it without cruise control on".
History shows vehicle is properly serviced and spark plugs had been replaced by us 6 months ago. Test drives confirms a misfire, only at 80km/h under very slight load. Fine under higher load.
Coil, leads and plugs removed (Dodge V6, 3 coils on plugs mounted on LH bank with leads to RH bank - waste spark system).
Very obvious carbon track on #1 plug and in the boot of #1 coil (Was much worse than picture before it was scratched off).
It was found the coil boots were a loose fit on the spark plugs. This allows an air gap, which can ionise and create a lower resistance path for the spark current to earth. Once this starts happening, the spark burns a 'carbon track' which creates an even better path for a misfire. The more it happens the worse it gets.
Spark plug replaced and boot cleaned up. New plug was fitted with a smear of dielectric grease inside the boot to fix the insulation problem.
Scan tool OBD2 misfire monitor data showed cylinder 3 and 5 (the other 2 cylinders with coils) also counting occasional misfires (although impossible to hear or notice).
Those coils were removed, boots removed and cleaned, and Di electric grease applied also. Now they all showed 0 misfire counts even after a 10 minute road test.
I've got no doubt they would of eventually developed a carbon track as well.
The whole thing could of been avoided by using dielectric grease when the plugs were originally replaced!
Thanks to Daniel Priestley for submitting this article
