I recently bought a 2008 Silverado 5.3 with 77k on it. After a couple weeks I noticed that at idle the engine would stumble and my rpms would drop slightly. It eventually set off the check engine light with cylinder 1 misfire and random cylinder misfire. A couple days later the light went off but the issues remain.

Since then I've cleaned the MAF with MAF cleaner spray, cleaned the throttle body, and run a couple bottles of techron through to clean up the injectors, but no change.

I dont know if it's related, but I also get a lot of dark blue/black (hard to say) smoke from the tailpipe on cold starts for a few seconds before clearing up. This led me to thinking I was getting oil in the cylinders when the rings cool and contract, which I tracked to known oil burning issues with these AFM engines.

At this point I'm not sure what else to try before spending a fortune at the shop. My only remaining thought is to replace the plugs and coils. But even if that fixes the misfire I'm not sure what to do about the cold start smoke.

EDIT: I had been keeping a casual eye on my oil levels since my last change and did notice a pretty drastic decrease in just a couple thousand miles. I'll just changed it again this past weekend and will keep a close eye on it. Would this affect engine misfires?

EDIT2: I pulled the #1 cylinder spark plug since I had a misfire code for that cylinder and it was fouled up with oil and deposits in the threads and at the firing end.

  • All these signs are symptomatic of oil in the cylinder. I faced the exact same issue some time back. Check your dipstick to see if your engine oil level has dropped. – krthkskmr Apr 11 '16 at 17:45
  • I just changed my oil. I did notice that since my previous change the oil level had dropped significantly. I'll will keep an eye on the levels this time as well, but would this cause the misfires as well? – Andrew Apr 11 '16 at 17:46
  • Pull the plugs and see what kind of shape they are in. – Mobius Apr 11 '16 at 17:47
  • Ah, that does make sense. I'll take a look at them as soon as I get a chance. Would it make sense to change them if they are fouled before fixing the oil issue? – Andrew Apr 11 '16 at 17:52
  • @Andrew, Yes! They will cause misfires, like Mobius pointed out, pull out the plugs. If they are dark black, you have oil getting into the cylinder. I think I already know what is wrong but I am hoping that you can tell me the the spark plugs are not oily or black. – krthkskmr Apr 11 '16 at 18:09

First of all, the 5.3l engine (could be all of the Vortec LS engines, don't know) is thirsty. It drinks a bit of oil. Be aware you could be looking at up to 2 quarts between oil changes. I don't know exactly what the issue is, but it does have a bit of an oil control issue whether it has to do with leakage or burning. My '06 is this way. It has never ran bad due to its use of oil (though obviously too much oil would cause issues for the cylinders as well as the cats).

Your issue with running is most likely a bad coil. The easiest way to test this theory is to change the single coil out with a different one and see if the problem follows the coil change. You may also try to push in both the connector at the top, as well as both ends of the spark plug wire. You may consider changing out the plugs with new ones to see if this helps your issue.

It could also be the spark plug on the No. 1 cylinder as well. If the spark plug is bad, it would give you the misfire, but spark plugs in these are supposed to be good to around 100k miles. You can pull the #1 plug and see what's going on with it. You'd be looking for a tan colored plug if it was running good. Obviously since you've already been getting a misfire code on #1, this may be moot.

The third thing it might be is a stuck/leaking injector. I'm doubting this. You would need to pull the fuel rail, leaving the injector on the rail, then pressurize the system looking at whether it dumps fuel without the engine running to tell if this is an issue. This might account for the bit of black/gray/bluish smoke on startup.

  • I'll test the coil for sure and pull the #1 plug to see if it looks good. The amount of dark exhaust on cold start is pretty high in my opinion. Good luck breathing in the garage even with the door open when starting this thing in the morning. – Andrew Apr 11 '16 at 18:21
  • It is unlikely to be the bad coil. The bad coil does't explain the blue smoke. Also, noticing a significant drop in oil over a matter of few weeks when a 2 quart drop is expected to happen only over (approximately) 5000 miles is a bad sign. The stuck injectors also do not explain the blue smoke. Either way, checking the plug should provide the answer. – krthkskmr Apr 11 '16 at 18:25
  • @krthkskmr - The OP did state "blue/black smoke (hard to say)" ... My experience tells me too much fuel in the exhaust can look black or light blue and be especially hard to tell the difference. They OP's question points towards a bad coil, though if it's intermittent may be hard to diagnose, even with a coil swap. – Pᴀᴜʟsᴛᴇʀ2 Apr 11 '16 at 21:03
  • @Pᴀᴜʟsᴛᴇʀ2 I didn't have a chance to get another coil to test with but I did pull the #1 spark plug to take a look. The threads were darkened and filled with oil, a lot of hard tan buildup on the sparking parts and some dark oily sludge around. I tried to clean it up a bit with a wire brush without much luck. – Andrew Apr 12 '16 at 2:09
  • The insulator around the electrode was also partially missing. – Andrew Apr 12 '16 at 12:36

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