CM871 smoke while engine braking

[jaysoto]

So today I was hauling dirt off a job site with a long steep driveway and while I was going down the driveway I had the jakes on to hold me back. Jakes work fine mostly had them on the 1st and 2nd stage occasionally if I was loaded really heavy I would use the 3rd. Anyway I noticed that I could smell raw fuel and looked in the mirror and saw the truck was blowing white/blue smoke. So that's where the smell was coming from but what causes this? Could it be the truck idling for 20 ish minutes from getting loaded or is it still injecting fuel while I have the jakes on? Jakes sound normal and work the way they should. Thanks