This article originally appeared in
Big Rig Owner
January 1, 2015
By Bruce Mallinson
“My truck smokes!” Many times during the week, I will answer the phone and hear this statement. Before I can help you, I will first need to know what kind of truck and engine you have, when it smokes, and what color the smoke is. Once I know all that, I can help. There are four kinds of smoke: white, blue, black and black just when you shift. Usually, it is the last one. If you are seeing black smoke when shifting, you are shifting at too low of an RPM or your right foot is too heavy on the throttle when the turbo boost is below 8 psi. Let’s look at this issue first, then go over the other types of smoke later in this column
It takes 8 psi of boost to turn black smoke into clear, and if your fuel system is responsive and you push too hard on the throttle, the engine will produce black smoke until the turbo boost gets to 8 psi. Eight is the magic number, and you should know when your engine has developed 8 psi of boost by the sound of the engine and the turbo. Also, you should have a small spot mirror on the top of your driver’s side mirror bracket looking up at your stack. With one eye on the spot mirror and one eye on the turbo boost gauge, accelerate slowly. Once you get to 8 psi, give her more fuel, just don’t mash on the throttle, roll into it gently. This one change in your driving habits should decrease your amount of black smoke you see when accelerating or shifting.
Back in the days of the mechanical engines, there were aneroid valves that did not work very well so they were removed. Then, in late 1977, Cummins put the aneroid valve into the AFC fuel pump. It had some problems for a couple years, then the engineers at Cummins got them working perfect, but they were a little on the light side for fuel. We figured out how to make the adjustment so you still had response when starting out and when you shifted a gear with only a slight puff of smoke. If you were easy on the pedal, there was no smoke.
Many of the articles compiled here where written by Bruce Mallinson. Attribution to other contributors is given in the specific articles.