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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.

Andrew
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2 Answers2

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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.

Pᴀᴜʟsᴛᴇʀ2
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Ok, blue smoke on startup most likely is caused by oil not draining properly in the valve cover and oil being sucked into the intake through the pcv on startup. I had this issue when I first bought my 2008 5.3 silverado. Truck has a brand new jasper engine in it and would blow a tremendous amount of smoke on startup. It would use half a quart every 1000 miles. I took the drivers side valve cover off and noticed after less than 25 hours on this new engine the oil drain ports in the top of the valve cover were plugged. So, I took a drill bit and made about 10 extra holes through the oil drain channel that runs the inside length of the valve cover and have never had another issue with blue smoke on startup. No oil usage either between 5000 mile oil changes. Truck runs perfectly now. Also, a range tuner will keep your afm from turning off 4 cylinders. This will save you thousands down the road when your lifters collapse and won't operate properly...

Kevin
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