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Discussion Starter · #1 ·
2016 Impala (new body style) with the 3.6L V6, 74k miles.

Every now and then when I start the vehicle after it's been sitting and has cooled down entirely, the car seems to idle rough for a few seconds, then smooths out and idles normally. Most times I start the car it sounds like a slight misfire, but I only hear one or two misfires and it doesn't really affect the idle at all.

I did have two instances where I had a random misfire code, and a cylinder 3 misfire code. I have since replaced the entire rear bank of coil packs, and the spark plugs looked in good condition, so I didn't replace them (per the suggestions of my dealership). I also replaced all of my intake seals at that time since you have to remove the intake to replace the rear coil packs (awful design).

However now the issue is coming back, and although it hasn't produced a code yet, I'm hoping it's not another cylinder 3 misfire. I do have one CEL for EVAP Vent Valve Solenoid Circuit Malfunction, however the dealership assured me that this should not be able to cause any sort of misfire at all. I have attempted replacing the Vent Valve Solenoid as well, and unfortunately it did not remedy the CEL (yes I did reset the entire computer) and the dealership thinks its the brand new part I just replaced at this time, as that's what their diagnostic testing pointed to.

I should be able to get my hands on an HPTuners MPVI2 soon, so I will try to log a few cold starts to see if I can find anything wrong there, but until then, who has had similar issues with this engine? What have you done to fix it? And has that solution been permanent?
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