I have a 2007 gmc Yukon denali with the 6.2l in the last month it has randomly puffed a bit of smoke on start up what might be wrong only 120k on it
Hi Bryce, For the miles and engine in your vehicle, a bit of visible smoke from the tailpipe on cold start is normal. It indicates a small amount of engine oil is leaking past the "valve-guide seals" in the cylinder head. For now, I wouldn't let it concern you too much because it's actually a VERY small amount of oil making its way into the combustion chamber while you're snoozing.
It only takes a drop or three to produce a huge amount of visible smoke when it burns, and it probably won't even leave deposits on your spark plugs at this point.
This condition isn't harming anything except your nerves, and it won't cause a breakdown or leave you stranded. The oil lost (per mile) is too small to result in the need to check the dipstick more often, but you should get a sense of the loss-rate by periodic checks over a given number of miles.
Top off your oil, and check it every two or three fill-ups, and, my guess is, you won't have to add a quart before driving 1000-1500 miles, which is nothing to worry over, but determine how many miles it takes to use a quart, so you know when to add.
If you want to narrow it down, pop the plugs out and check for any light colored deposits, and note the particular cylinder from which a contaminated plug was removed. You'll probably see little, if any, deposits for now.
Oh, and there are NO magic snake-oil remedies, despite fancy promises on the labels of opportunistic, quick-fix makers. Don't fall for their distractions. The best you can do is to keep up on your oil maintenance. Running dirty oil causes accelerated seal wear.
2007 GMC Yukon...
on Sep 20, 2016