Rough Running Chevy or How Can It Be Missing When It Is Sitting Right There?

A misfire. A cylinder that is not contributing to the engine running is said to be misfiring. I remember that when I first started doing car repair that determining a misfire vs poor running took a very long time to learn. Oh I could tell when an engine was running bad, but that isn't necessarily a misfire. It gets even more complicated, there are varying degrees of misfires. There are different types of misfires. There are different conditions and causes of misfires. All these years later though and I can tell a misfire on a car just driving past the shop on the road out front. I could also tell I had an ignition misfire on the 2010 Silverado that I just drove into the shop.
The customer had said it might be a misfire, or just a rough running engine. Just as I said, it can be hard to say one or the other without some experience. When he added that the check engine light was on and sometimes flashing well that clinched it. Since OBDII was born (second generation on-board diagnostics) automobiles are required to have a flashing check engine light to alert that the engine is misfiring badly enough to damage a catalytic convertor.
The slight intermittent miss I felt under light load coming into the shop is pretty typical of a secondary ignition misfire. Secondary ignition is the spark output to the spark plugs. The problem areas there are coils, spark plug wires and spark plugs. I suspected I would need plugs and plug wires. Problems with either goes hand in hand.
Remember, diagnostics is taking a broad range of possibilities and narrowing that down to the actual problem. I had taken "misfire/rough running" to "intermittent secondary ignition misfire", at least in my own mind. Therein lies the next step. I know that I have a pretty good idea that there is a bad plug or bad wire or both but I do need to go further before just throwing parts at it. I have an 8 cylinder engine here but a misfire on only one cylinder. To test further I need to find out which cylinder is misfiring. Testing secondary ignition on cylinders that have no misfire isn't going to be helpful and is a waste of time. Since the misfire is intermittent it isn't as straightforward as looking for a misfire that is always present but it isn't difficult either on this vehicle.
Years ago this would require might putting an assistant inside to create the misfire and me under the hood disabling cylinders until I found the one with the problem. Thankfully it is pretty rare to have to do that these days. Since the misfire was bad enough to flash the light, then I should have a misfire code stored related to the cylinder causing the problem. P0301 is a misfire code for cylinder one, P0302 is cylinder two and so on. Let's take a look at stored codes:

That was a lot less helpful than I had imagined. P0300 is the equivalent of what the customer told me in the first place. It might be a misfire or it might be a rough running engine. No need to surrender and look for an assistant yet though. This truck has the GM datastream items to allow you to see stored misfire histories and even current misfires (live). We'll look there next. 

Looks like we should home in on #7 cylinder. It even racked up a first misfires for me when I started the engine and gave is a slight load. I want to say here that there is NOTHING about a a misfire code that means anything more than the computer on your vehicle has detected a misfire. There is nothing about that code that means you need spark plugs or injectors or valves or anything other than you have a misfire. You may need any, all or none of those things. You have to determine what the problem is for yourself. Now lets look at that #7 spark plug.

Time for spark plug replacement. The engine is burning some oil causing the carbon buildup. 

Thanks again for reading!

Kenneth Hayes

G&G Auto Repair