Symptoms
- Rough, uneven idle
- Visible smoke (black, white, or blue depending on cause)
- Possible knocking or rattling sound
- Reduced power or hesitation
Likely causes
- CommonFailing or dirty fuel injector(s)Diesel injectors rely on very precise spray patterns; wear causes rough combustion and smoke.
- CommonWeak or failing glow plugs (cold conditions)Glow plugs help ignition in cold engines; failing ones cause rough cold starts and idle.
- PossibleAir in the fuel systemAir pockets from a low tank or a loose fitting disrupt the precise fuel delivery diesels need.
- PossibleClogged fuel filterDiesel fuel filters clog with water and debris, starving injectors of consistent fuel.
- Less commonLow fuel injection pressureA failing high-pressure pump can't deliver the precise pressure modern diesel injection needs.
What to do
- Get the check engine light scanned; diesel systems set specific codes for injector, glow plug, and fuel pressure issues.
- Check and replace the fuel filter/water separator if it's due for service.
- Have glow plugs tested if the rough running is worse on cold starts.
- Bleed the fuel system if air is suspected, following your owner's manual procedure.
- Have injectors tested and cleaned or replaced if fuel system checks don't resolve it.
- Use quality diesel fuel and keep the tank from running very low to reduce the chance of drawing in sediment or water.
Tools and parts
- OBD-II code reader
Diesel fuel filter/water separatorSearch: “diesel fuel filter for [year make model]”
Glow plugsSearch: “glow plugs for [year make model]”
When to call a pro
Diesel fuel systems need specialized diagnostic equipment; a shop familiar with diesels can test injector pressure and pattern accurately.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.