What Are Diesel Exhaust Tips And Why Are They Vented?

The exhaust tips of a diesel truck have a similar function to the muffler tips of a gasoline vehicle. It routes the exhaust gasses at the vehicle's rear, while serving as a cosmetic purpose on some cars. Meanwhile, aftermarket exhaust tips for diesel engines have different designs, materials, and construction techniques to augment the exhaust sound while adding a dose of style.

However, what about diesel trucks with vents behind the exhaust tips? To understand the purpose behind this quirky feature, we'll need to take a short trip under your vehicle, particularly the exhaust system. All diesel vehicle and engine manufacturers use DPFs — or Diesel Particulate Filters — to meet U.S. EPA 2007 emission standards. Heavy-duty and commercial highway engines and vehicles are required to use "high-efficiency catalytic exhaust emission control devices or comparably effective technologies."

Typically located inside a diesel engine's exhaust system (after the exhaust temperature sensor, and ahead of the NOx catalytic converter), the DPF captures soot and other nasty combustion byproducts and burns it periodically via a regeneration cycle.

The DPF regeneration cycle is a self-cleaning process to rid the DPF of excess soot and ash. Most DPFs are ceramic honeycomb substrates that could get clogged after hundreds of miles. When the computer detects the hampered exhaust flow due to the blockage, it triggers a DPF regeneration cycle to burn off the collected debris and spew it out the exhaust tips as cleaner exhaust emissions.

There are two general types of DPF regen cycles: active, and passive. Active regen uses injected fuel in the exhaust stream to increase the temperature to more than 1,100 degrees Fahrenheit, sufficient to rid the DPF of collected soot. Meanwhile, passive regen happens automatically when driving your diesel truck at sustained highway speeds. It's why manufacturers recommend driving your diesel truck regularly at over 40 mph — at a minimum of 1,800 rpm — for 20 to 30 minutes to facilitate passive DPF regeneration.

The DPF could be clogged sooner than later if the truck is driven constantly at low speeds or shorter trips, excessively idling the engine, using the wrong diesel motor oil, or using low-quality diesel fuel. The symptoms of a clogged DPF include low engine power, excessive smoke, poor fuel economy, and an illuminated engine warning light on the instrument panel.

If you see those weird-looking vented exhaust tips on diesel trucks, it means the exhaust has an active DPF regeneration system. When the system injects fuel into the already-hot exhaust stream, the temperature rises to above 1,100 degrees Fahrenheit. The vents allow cooler air to mix with the scorching hot gasses, lowering the temperature as it passes the tailpipe.

Without those vents, the exhaust tips could get hot enough to damage the trailer you're pulling — or if it gets too close, burn any car's front bumper at the rear of your truck. It also ensures you can use the bumper steps without burning your boots, or do some work near the tailgate without possibly inflicting burns.

Some trucks could use a variation of an active and passive DPF regeneration, and not all diesel cars have vents on the exhaust tips. Nevertheless, paying attention to your DPF's health could save you thousands of repair bills later. In some cases, the DPF may require manual or professional cleaning. In older trucks, replacing the DPF may be necessary to restore efficiency.