On a modern diesel, the exhaust system does far more than carry gas to the tailpipe. It cleans soot, converts NOx and feeds data back to the engine computer, and when part of it fails, the truck can derate or refuse to restart. RDI Power diagnoses and repairs complete diesel exhaust and emissions systems in Brooksville and at customer locations around Central Florida.
The emissions chain, front to back
- Exhaust manifold and up-pipes collect gas from the cylinders and feed the turbo.
- EGR valve and cooler route a measured amount of cooled exhaust back into the intake to lower combustion temperatures and NOx.
- Diesel oxidation catalyst (DOC) burns off hydrocarbons and carbon monoxide and creates the heat used for regeneration.
- Diesel particulate filter (DPF) traps soot until it is burned off.
- SCR catalyst and DEF doser spray diesel exhaust fluid, a mix of urea and deionized water, into the exhaust to convert NOx into nitrogen and water.
- Sensors for temperature, pressure and NOx tell the computer what is happening at every stage.
- Pipes, muffler and tailpipe finish the job.
Most trucks built for 2010 and later emissions rules use EGR and SCR together. Pickups vary by year: for example, Ram added DEF to its 6.7 Cummins pickups for 2013, while the 6.7 Power Stroke and LML Duramax had it from 2011.
How regeneration works
- Passive: steady highway driving keeps the DPF hot enough to burn soot as it goes.
- Active: the computer adds fuel to raise exhaust temperature while you drive.
- Parked or forced: the truck runs a regen while stationary, either on request or started by a technician.
Trucks that idle, run short trips or get shut off mid-regen never clean the filter properly. Our article on DPF and regen problems in diesel trucks explains why in detail.
Emissions warning signs
- DPF lamp, frequent regens or a derate
- DEF warning, speed limit countdown or a no-restart warning
- EGR, NOx sensor or exhaust temperature codes
- Coolant loss with white smoke, a sign of a leaking EGR cooler
- A louder exhaust, ticking at cold start, or exhaust smell inside the cab
Exhaust in the cab is a safety issue, not a nuisance. Carbon monoxide from a leak is dangerous, especially for drivers who sleep in the truck.
Emissions system testing
We read soot and ash load, regen history and DPF differential pressure, then compare temperature sensors at each stage. DEF gets checked for concentration and contamination, and the doser gets a quantity test. NOx sensors are compared upstream and downstream of the SCR. EGR coolers get pressure-tested for internal leaks; our post on a Cummins ISB EGR cooler failure shows what that looks like.
Catalytic converters, mufflers and exhaust pipes
Most people picture the exhaust as a pipe, a muffler and a catalytic converter. On a diesel truck, the catalytic converter is usually a diesel oxidation catalyst (DOC), which turns carbon monoxide and unburned hydrocarbons into less harmful gases and supplies heat for DPF regeneration. Downstream, the muffler quiets combustion noise and the tailpipe carries the cleaned exhaust away from the truck. We repair and replace exhaust pipes, clamps, mufflers and catalytic converters along with DPF, EGR and SCR parts, and we check for leaks upstream of the sensors, because a small exhaust leak can throw off the readings the whole emissions system depends on.
Repairs
Forced regens, DPF removal for cleaning or replacement, EGR valves and coolers, DEF pumps, heaters, dosers and sensors, and the welding and pipe work for leaks, brackets and mufflers. Manifold leaks have their own page: manifold repair. Every repair keeps the emissions system in place and working; we do not delete or disable it.
If a dash warning or a derate is costing you time, call with the codes or the message on the screen, and we will plan the diagnosis.