Why You Should Probably Change Your Spark Plugs Now

Changes in ignition systems and spark plug design plus cleaner and more efficient engines have led car manufacturers to extend the mileage for recommended spark plug replacement. It isn't unusual to see a recommended mileage of 100,000 for changing plugs. The trouble is that waiting that long can cause problems. The plugs can be "stuck" or very difficult to remove due to carbon buildup. Some plug designs (Ford) are very easily broken when this happens. The job then changes to a more expensive and difficult procedure.
Other then that, the spark plug can very easily be ready for replacement before the scheduled mileage is reached. If you run worn or deposit covered spark plugs the secondary ignition voltage starts to look for new paths to follow. The path of least resistance can be inside the ignition coil or down the side of the plug.
Here is a spark plug from an 05 Chevy Trailblazer 4.2 engine. The recommended replacement mileage is 100,000 and the truck has 105,954 miles on these original plugs. The truck was not here for spark plug replacement though. It was here for a "rough running" engine problem. I could feel an ignition miss just driving it into the shop so I had already started thinking plugs and coil when I brought it inside. Now there were no codes or engine lights on but when I put a scan tool on and watched "current misfires" I could see intermittent counts on #4 cylinder. The hunt began with #4 spark plug removal.
This application uses iridium plugs which give them the longer expected life. The business end of the plug though had quite a bit of wear and deposit buildup.

I could see that the truck needed spark plugs. The smoking gun wasn't at this end of the plug though. The evidence of the misfire was down the side of the porcelain portion of the plug. When the high voltage of the secondary finds that path of less resistance it often leaves evidence. 

The misfire was due to the spark traveling down the outside of the plug instead of as designed. Because of this carbon trail it wouldn't be a fix to just replace the spark plugs. The miss would not only still be there but would cause the new plug in this cylinder to need replacement again. The reason for that is the carbon track isn't just on the plug. It will also be on the inside of the plug boot. The plug boot is between the coil and plug and slides over the porcelain section of the plug. 

I've sliced the boot open so you can see what I'm talking about.

Okay, now if we replace the spark plugs and the plug boot we should be good right? Probably not. The coil is very likely to be damaged internally as well. The coil isn't something that can be easily seen as damaged so the thing to do is replace the coil now rather than put it back and watch for it to fail. 

So....... lets replace those spark plugs now before the complications arise.

Thanks for your time!

Kenneth Hayes

G&G Auto Repair