Auto Repair Q&A



Cooling System










Auto Repair Products

Diesel Engine Black Exhaust Smoke Causes

Duramax 6.6 engine

Diesel engines are a hot topic right now, but most are ignoring that beyond automotive, there are diesel engines in use for a myriad of things. One thing you'll find them in are fire trucks.

User Orien Seaton in our forums had a question about black smoke coming from a Detroit Diesel Series 40-powered fire truck engine. He didn't get back to us to let us know what the problem ended up being, but our forum mechanics had some good ideas.

To begin with, I brought up the obvious:
"Black smoke from a diesel engine is almost always an incorrect fuel:air mix. When diesel runs either too rich or too lean, it creates a lot of soot. Check your air filter, [exhaust gas recirculation] valve, and do a read on the injectors (pull codes). You could have turbo problems as well, but if the engine is running smoothly otherwise, you probably just have a bad mix or clogged injector or EGR."

Big Block 409 had also chimed in, as always getting things back to basics. He pointed out that no matter the engine type, the following are common problems that lead to exhaust smoke:

  • Cylinder compression issues.
  • Engine vacuum leak issues.
  • Engine misfire issues.
  • Fuel injector issues.
  • Air filter issues.
  • Fuel filter issues.
  • Fuel delivery issues.
  • Fuel PSI issues.
  • Fuel volume issues.
  • Fuel regulation issues.

Most of the time, black smoke is caused by too much fuel versus the amount of air coming into the burn. Guys who "roll coal" in their diesel trucks restrict airflow to create the heavy black smoke you see.

As a way to illustrate how the black smoke can happen in an engine that is otherwise normally tuned, picture a diesel rig pulling a steep incline from relatively low altitude to high altitude at the top of a mountain. It emits black smoke as it begins to get higher up because there is less oxygen in the air. Assuming, of course, that the engine's computers are not compensating for the climb by increasing airflow through the intake.

In the case of this fire truck, we can assume that the black smoke is due to the filter, injectors, or the injector pump(s). The EGR valve could also be clogged, so check maintenance records to find out the last time it was cleaned or replaced.