What’s Your Diesel Smoke Telling You

April 17, 2019

We’ve all seen (or have caused); thick, black smoke come from the exhaust of a diesel-powered vehicle. For many of us it is our favorite part of owning a diesel in fact. However, a well running diesel should not produce visible smoke and if yours is, it could be an indication that something is not right with your engine. Let’s talk about the different colors of exhaust smoke and the potential causes.


Blue or Gray. Newer, low mileage truck owners can skip this paragraph, but many of us like to have a worn-out beater truck laying around, or maybe your teenager bought their first car from a not-so-honest guy. Blue usually means one thing: you are burning oil. You might notice a burnt oil smell while stepping on the accelerator, or maybe see blue/gray smoke leave your exhaust on startup or heavy acceleration, these are some of the telltale signs that you are burning oil.

Blue/Gray smoke could be caused by:

  • Stuck rings from stiction
  • Broken piston lands
  • Worn out or broken rings
  • Worn out cylinder bores
  • Worn out valves, valve guides, valve seals, PVC valve, injectors, turbo seals or the turbo itself.

White. This is usually the last color of smoke you want to see. If the smoke is thin, and goes away relatively quickly, then it is merely condensation. However, thicker, longer lasting smoke is a much larger headache. Your engine is more than likely burning coolant. This can be the result of a blown head gasket, damaged cylinder head, or cracked engine block – none of these are easy on the wallet. Diesels can also put out white smoke when fuel passes completely through the engine and reaches the exhaust without having been burned. This might be caused from the engine being too cool to burn the fuel, low compression in cylinder(s), fuel injection timing, defective fuel injector, burnt out glow plugs, clogged air filter or poor fuel quality.

Excessive white smoke is almost always an indication of a water leak either by a:

  • Cracked head
  • Cracked block
  • Blown head gasket

Black. Or as many people call it, “rolling coal”. People throw tuners, modules, or some kind of smoke switch, onto their diesel to make the truck add more fuel than necessary to create black smoke, as well as install larger injectors. But for those who don’t intend to smoke out everyone behind them, there’s quite a few things to check. A little black smoke is normal on a properly functioning diesel but keep an eye on the amount of smoke at different RPMs and loads so that you will be able to tell if something is amiss.

You may be experiencing black smoke because of:

  • Incorrect timing or air/fuel ratio
  • Dirty injectors
  • Worn turbocharger
  • Dirty intake manifold
  • Clogged air cleaner
  • Low cylinder compression
  • Poor quality fuel
  • Excessive carbon buildup in combustion chamber

Clear: the optimal burn in the combustion and exhaust and means everything is dialed in and well-tuned.

With these color indications provided from the tail pipe, you can easily take action for repairs or even better; get ahead of the repairs with common maintenance. “It’s not an option,” says Darwin Hippen, of Riverside, California. “In almost anything we encounter, for anything to endure, maintenance is a priority. Oil changes are critical to the lifespan of your vehicle, for longevity and performance. Friction and heat is not your friend.”
When the EPA required lower sulfur in diesel fuel, it also reduced the lubricity required within the diesel engine fuel systems. “Sulfur is imperative to the function of a diesel engine,” says Hippen. “Sulfur in the fuel itself, lubricates the injectors in the engine. Reduced sulfur jeopardizes the lifespan and the design of this extremely expensive part of the engine.” Fuel additives help counterbalance the lack of lubricity to easily and far less costly, prevent the damages from the ultra-low sulfur diesel fuel.
“For a brand-new truck or a truck like mine with the miles stacked up, and everything in between,” says Hippen, “Your smoke can tell you a lot of information, and thankfully there are products like Hot Shot’s Secret additives to help keep it clear!”
Diesel Extreme: For White Smoke & Black Smoke
Stiction Eliminator: For Blue/Grey Smoke