Vehicles using OM629.91x engine list:

Engine OM629.91x

Symptoms:
- Rough idling and loss of power
- Excessive smoke from the exhaust, especially black or blue smoke
- Engine warning light illumination
- Increased oil consumption and possible oil leaks
- Difficulty starting or misfires

Diagnosis:
- Check for clogged or malfunctioning diesel particulate filter (DPF), common in these models
- Inspect fuel injectors for wear or clogging causing poor fuel atomization
- Examine turbocharger for shaft play or boost leaks
- Perform compression test to detect cylinder wear or head gasket issues
- Scan engine control unit (ECU) for fault codes related to sensors or emissions systems

Fixes:
- Clean or replace the diesel particulate filter (DPF) to restore proper exhaust flow and emissions control
- Replace faulty fuel injectors or perform ultrasonic cleaning to improve fuel delivery
- Repair or replace turbocharger components if boost pressure is insufficient
- Address oil leaks by replacing seals and gaskets, especially valve cover and oil cooler seals
- Update ECU software if available to improve engine management and emissions performance

Prevention:
- Use high-quality diesel fuel and adhere to recommended oil change intervals with manufacturer-approved lubricants
- Regularly drive the vehicle at higher RPMs to allow DPF regeneration and prevent clogging
- Perform routine maintenance on fuel and air filters to ensure clean intake and fuel supply
- Monitor engine warning lights promptly and address issues early to avoid severe damage
- Avoid prolonged idling and short trips that prevent proper engine temperature and DPF regeneration

These practical steps help maintain the OM629.91x engine’s reliability and performance in the specified Mercedes-Benz E 420 CDI and S 420 CDI models.