Under the Hood: Troubleshooting a misfire indication

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Q: I’ve been thinking about trading in my 2008 Silverado for a new truck. Just the other day my check engine light came on. I used the OBD scanner I bought to see what the code was. It said cylinder #1 misfire. This is creeping me out, as it sounds serious. The truck only has 80,000 miles on it, so I’m thinking the engine must be OK. Now I’m thinking I should really get going on a new truck, but this will make it hard to trade in or sell the truck. Any suggestions on what I can do to see what’s causing this?


A: Let’s start with an easy thing first. Your 5.3L engine utilizes a “coil near plug” ignition system. It’s similar to a coil on plug system except that the coils are mounted about 10 inches from the spark plugs (they’re in a hot area due to the nearby exhaust manifold) and use a short spark plug wire to connect each coil to it’s plug.

Have you felt an intermittent or continuous thumping/roughness from the engine, typically under load? If so, that’s the misfire, and continued driving under this condition should be avoided, to prevent damage to the catalytic converter, and if severe, eventually to that cylinder’s piston rings/bore. The OBD-II system’s misfire monitor is quite sensitive, it’s possible the misfiring was infrequent/slight enough to not be noticed.

Since the coils are so easy to access/remove, how about swapping the #1 coil with it’s #3 neighbor? (Driver’s side cylinders are: 1,3,5,7, from front to rear.) For now, leave the plug wire and spark plugs in their original positions. Take a look also at the terminals of the #1 plug wire. If they’re clean and bright, the wire is likely OK.

Check your scan tool to see if a freeze frame is available; this will indicate the exact driving conditions occurring when the PCM identified the cylinder #1 misfire for the first time. Next, clear the stored diagnostic trouble code and drive the truck as possible to mimic the freeze frame driving conditions. It may take some time, perhaps a few days or more to see another misfire (actually a certain number of crankshaft speed fluctuations per 200 engine revolutions) depending on fault severity.

I’m hoping you’ll eventually see a cylinder #3 misfire code. When the code follows a swapped coil, the coil is the problem! A likely cause is an insulation fault allowing spark to leak to ground rather than jumping the more difficult spark plug gap. Replacement coils are $35-ish apiece.

Should the #1 cylinder misfire reoccur try swapping the #1 plug and wire to the #3 cylinder, clear codes, and try again. If a #3 misfire then occurs, I’d replace all spark plugs and that wire— if need be.

If a #1 misfire code reappears after coil, plug and wire have been swapped, it’s time for professional diagnosis. Possible causes are a dirty fuel injector, or in rare cases, an engine mechanical or active fuel management fault. I’m betting on the coil!



Brad Bergholdt is an automotive technology instructor at Evergreen Valley College in San Jose, Calif. Readers may send him email at bradbergholdt@gmail.com; he cannot make personal replies.


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