Why are semis and heavy vehicles so smokey and loud?


I know nothing about big trucks and their engines, but it seems to me that 1 in 10 or 1 in 20 rigs or dump trucks or vehicles of that scale either spew dark smoke or are extra loud and sound unhealthy.

Are these vehicles subject to the same inspections as our cars?

Have the designers not figured out how to make a truck engine that lasts?

Are the owners too cheap to keep them in top running order?

I only ask because you’ll see plenty of trucks on the road that sound well-tuned and aren’t putting out much more visible pollution than any car on the road. But there’s definitely a higher percentage of clunker trucks on the road than cars.


As large diesel engines age, they smoke more and more. Repairing them is very costly and as long as the vehicle / equipment will start and run, owners usually defer overhauling it. The amount of smoke produced is also affected by driver skill, which in many cases can be less than ideal…

As far as emissions inspections for commercial vehicles goes, and diesels in general, they are a joke. Most are allowed 30% opacity, which means they can smoke heavily and still pass…


[b]The big trucks you see emitting dark plumes of smoke have large diesel engines. Diesel engines by their design are heavy emitters of emissions. And that dark smoke you see coming out of the stacks is diesel soot. But things are changing.

By the year 2010, all on-road diesel engines will have to meet the EPA/CARB requirements for diesel emissions. This includes minimum emissions of soot and oxides of nitrogen.

As a matter of fact, I’m looking in a dyno cell right now that has a Cummins ISX 15 liter diesel engine that is used to run diesel emission abatement component testing.

So wait a few more years, and you’ll see these big rigs running down the road with no emissions out the exhaust.



Not only trucks, but all those little diesel VWs (Ones a few years old) pump out a lot of black choking smoke too.

The worst engines for smoking were the older Macks (Mostly dump trucks).

Constant servicing still didn’t help a friends truck either. He stayed on top of it for 5 years until he found he was just throwing money out the window and said to hell with it.


Tester, we may no longer see the black clouds of carbon, and unburned hydrocarbons and CO may come down significantly, both of which will be welcome, but NOx emissions will not meet gas engine requirements with today’s technology. I truely believe that the only way the technology will develop to enable diesels to meet the Clean Air Act is by mandate. It’s always been a pet peeve of mine to have to have my car/truck pass stringent requirement while trucks continue to pour out huge amounts of pollutants completely unchecked.

However, I’d welcome statutory emissions requirements that required trucks to clean up at least to the extent technically achievable for a mobile diesel engine. It would make a huge difference.

  • mountainbike


my dad has a few old macks and they run fine with no smoke