Why one spark plug may be black
Dear Car Talk:
I have a 2013 Camaro with the V-6 engine. When I changed the spark plugs, the old plugs from cylinders two to six were light tan in color, but plug No. 1 was dark and sooty. The engine seems to be running fine. Is this a problem to worry about? – Michelle
RAY: I’d worry a little, Michelle. When a spark plug is black, that’s usually a sign that there’s incomplete combustion in that cylinder. And when it gets bad enough, it’ll turn on your Check Engine light, and might even create a misfire. You’re not there yet, but you’ll likely get there eventually.
The cheapest and easiest thing it could be is a bad spark plug.
If a spark plug isn’t firing hot enough or is badly misgapped, it won’t combust all the fuel and you’ll be left with a coating of black soot.
So now that you’ve replaced the plugs, check them in 30 days and see if No. 1 is getting black again. If it’s clean the next time you check, then all you had was a bad spark plug, and you’re all set. If it’s black again, then the next thing I’d test would be the coil.
This car has what’s called “coil on spark,” where there’s a coil on top of each spark plug. If that coil isn’t sending enough voltage to the plug, you’d get incomplete combustion.
Since you changed the plugs yourself, you sound like a do-ityourselfer, Michelle. It’s pretty easy to test a coil by simply swapping two of your coil packs.
Switch the coils from cylinders No. 1 and No. 2, and check back in 30 days. If your No. 2 plug is now getting black, then you’ve identified your bad coil, and you can just buy a new one for about $40.
If the blackened plug still shows up in cylinder No. 1, next on my list would be a bad fuel injector. Each cylinder has its own fuel injector to spray in the precise amount of fuel at exactly the right moment. If your No. 1 injector is leaking, or its spray pattern is messed up, too much fuel can be sent into that No. 1 cylinder, and not all of it will be combusted.
You’d test the fuel injector the same way you test a coil, by swapping two of them. But, unfortunately, getting to the fuel injectors is a lot more involved.
So if you’ve got a free weekend – or better yet, a nice holiday three-dayer – you can dig in and try swapping around two of the injectors. Then check back in 30 days and see which plug is black.
On the other hand, if you’re going to go through the trouble of removing the intake plenum, the fuel rail and all the wiring, you might want to just go ahead and buy a replacement injector for $150 or so, and replace the No. 1 injector while you’re in there. Once you’ve ruled out the plug and the coil, there’s a pretty decent chance that the injector is what’s causing your problem.
Or if that all sounds like too much trouble, you can wait until your Check Engine light comes, like most red-blooded Americans would, and then let your mechanic figure it out while you spend the holiday weekend watching Netflix. Good luck, Michelle.
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