I have a 2001 Cl Type "S" This car has been flawless for me. Recently I have been encountering an occasional misfire where the car shakes a bit and engine light either flashes or come on. Then car will run normal for awhile. Mechanic scanned engine and said misfire ... probably bad gas. Put fuel treatment in gas. Still happening. Replaced all spark plugs (old plus had 120,000 mi.) and noticed that the ceramic insulator on one plug was quite brown. Could this be the sign of a bad coil? Engine seems to idle smooth/ Help please !

 

After 120,000 miles, the spark plugs will encounter a bit of blow-by through the plug itself; this is normal and should not concern you. It is very typical of vehicles that use high mileage spark plugs. vehicles that only use spark plugs for 30k-50k miles wouldn't encounter this issue because the plugs would not be very old like yours.
Things to consider while doing your spark plugs: top-end fuel injector cleaner, adjust the valve lash, and clean egr ports and you'll be good for another 120,000 miles!!!

 

I concur with Bag's explanation of why the ceramic discolors and that it's completely normal for this to happen. As for the misfire, chances are if you are feeling what appears as a dead single cylinder, you have a bad coil. You didn't say wether or not you had the codes scanned or not but I'm gonna say its safe to say you will see maybe a P030X (X indicates misfiring cylinder) which should help in resolving the matter quickly. If instead you pull a P0300 code than this means either the ECM cannot determine which cylinder is misfiring (due to ECM monitoring logic or even hasn't had adequate time) and so it generically throws a RANDOM/MULTIPLE MISFIRE DETECTED code because it knows by crank/cam sensor monitoring there's at least one misfire present. If you see P0300, the easiest way to identify the misfiring cylinder is by purchasing one coil and installing it in one cylinder at a time. Only switching to another cylinder once the misfire is still felt after driving. This is, of course, if the misfire is not always present thereby not allowing for a diagnosis to be performed on the misfiring cylinder with the engine idling in park and narrowing the bad coil pack down by unplugging each coil at a time until no change is detected in the engines rhythm.

 

Hi yungone501, Thanks for your post, my mechanic scanned it and if the scan would have identified the misfiring cylinder he would have told me and sold me a coil. I suspect that it is P0300 code. Since installing the new plugs and putting a new tank of Union 76 premium fuel, it hasn't misfired or flashed the engine lite. If I can get it to misfire again, i guess I can buy a coil and start as you suggested installing it in one cylinder at a time. My frustration is that it isn't misfiring now. Might it have been bad gas ?? It isn't misfiring while idling!

 

Doubtful on the fuel concern. This is not as common as most people think...though not impossible of course. I'd say its more about the plugs. I often see the gaps on plugs widening from age and therefore requiring the coil to push the spark harder to jump the electrode. This leads to an increase in amperage drawn which can overheat the coil causing intermittent failure. And though sometimes replacing the plug(s) can resolve the issues permanently, there's still a high probability the coil been damaged.

Keep your fingers crossed that the issue doesn't reappear. But if it does, remember the procedure I gave in my first post.

 

I will add to my original post and note that if you do encounter one of your spark plugs with adequately more blow-by or carbon build-up on the insulator like you noticed before, it is possible that the spark plug might have become loose over time and that itself could have caused your misfire. And I've seen it quite a few times. This is just a side note. All of this, of course, would have been fixed when you replaced your spark plugs; which I hope you used factory NGK's. Proven to go at least 100k miles!