This is something we talk about with customers almost every day. Someone buys a diesel because it "uses less fuel," then drives it exclusively around town. Two kilometers to work, three to the store, one to school. And then they wonder when the problems start. In our experience, short city trips are by far the worst driving conditions for a diesel engine.
Diesel Needs Operating Temperature
A diesel engine is designed to run at around 90 degrees Celsius of coolant temperature. On short trips, the engine never reaches that temperature. And that sets off a whole chain of problems.
When the engine runs at low temperature, combustion is incomplete. Unburned fuel washes down the cylinder walls and mixes with the oil in the crankcase. The result is that your oil level rises, and the oil smells like diesel. That diluted oil loses its ability to lubricate, and the engine wears internally, quietly but surely.
The DPF Cannot Regenerate
The DPF (diesel particulate filter) catches soot from the exhaust gases. When enough soot builds up, the engine automatically starts regeneration. It injects extra fuel to raise the exhaust gas temperature and burn off the accumulated soot.
The problem is that regeneration takes 15 to 20 minutes and requires driving at a steady speed, usually above 60 km/h. On a short trip of two or three kilometers, regeneration cannot even start, let alone finish. Soot keeps building up, the DPF clogs, and eventually you get a warning on the dashboard, loss of power, and an expensive trip to the shop.
At our workshop, we see clogged DPF filters mostly on cars that are driven exclusively in the city.
The EGR Clogs Faster
The EGR valve clogs during normal use anyway, but city driving speeds that process up dramatically. At low RPMs and low temperatures, more soot is produced. That soot passes through the EGR and sticks to the intake manifold. On cars that never see the highway, the intake manifold can be completely coated with deposits by 100,000 km.
The Turbo Suffers
The turbo is lubricated by engine oil. When that oil is diluted with fuel because the engine never reaches operating temperature, lubrication is inadequate. Add soot deposits on the variable geometry vanes and you get a turbo that will fail much earlier than it should.
The Oil Gets Contaminated
We already mentioned fuel dilution, but there is another issue. At low temperatures, water vapor from combustion condenses in the crankcase instead of exiting through the exhaust. That moisture mixes with the oil and creates an emulsion, a yellowish paste you can see on the inside of the oil filler cap. That is not good for any engine, and it is especially bad for a diesel.
If You Already Have a Diesel in the City
If you drive a diesel mostly around town on short trips, here is what you can do to reduce the damage:
- Take it on the highway once a week - 30 to 40 minutes of driving at 2,500 to 3,000 RPM does wonders. The DPF regenerates, the EGR partially cleans itself, and the oil reaches proper temperature
- Shorten the oil change interval - instead of every 15,000 km, change it at 10,000 or even 7,500 km
- Use quality synthetic oil - it handles fuel contamination better
- Do not ignore dashboard warning lights - especially the DPF warning. If it comes on, head for the highway, not the parking lot
An Honest Recommendation
If your daily trips do not exceed 10 to 15 kilometers and you rarely drive on the highway, diesel is not the right choice for you. A gasoline car or a hybrid will cost you less in the long run because it will not have DPF, EGR, and diluted oil problems.
But if you already have a diesel and are not planning to change it, stick to the tips above. And if you notice any of the symptoms we have written about, do not wait for them to get worse. Stop by and we will check the engine condition and advise you on next steps.