- Engine oil and filter have not been changed in a while
- Engine oil is of insufficient quantity
- Engine oil specification not suitable for climate condition.
- Poor quality fuel
- Diesel fuel filter is clogged, fuel pressure is inadequate.
- Injectors are worn out or clogged
- Injectors are tainted with carbon deposits
- Injector compensation codes have not been registered on the ECU
- Fuel rails are leaking
- Timing belt has stretched, crankshaft and camshaft are out of sync
- Camshaft sprocket is not aligned according to its peripheral stamp line
- Alternator is not proving the car with enough electric power.
- Rocker cover is not sealed perfectly, engine can't reach necessary vacuum (or pressure), manifold air pressure sensor isn't providing an accurate measurement
- Air intake manifold, cylinder head and exhaust gas recirculation valve assembly are severely clogged up with thick coated oily carbon deposits. Air intake is impeded, exhaust recirculation valve is stuck closed
- Injector washers are of the rubber variety and not of the highly recommended copper variety, resulting in engine "blow by" that leads to the next problem
- Oil pickup mesh screen inside sump is clogged with carbon deposits, oil circulation within the engine is impeded. Injectors require washer replacement with the certified copper washer.This action will prevent "blow by" within the cylinder head.
- Exhaust gas recirculation valve is not functioning properly. Check vacuum controller
- Exhaust gas recirculation heat exchanger is clogged, it's preventing the passage of exchaust or preventing the passage of coolant
- The engine has just been started, usually on a cold morning. The ECU is running a warm up cycle. Usually the engine runs fairly rough, though using SAE 40 instead of SAE 30 makes this warming up process less noisy
Crankshaft position sensor
This may be a common cause of misfiring. It may have been removed at some stage, the magnetic sensor will attract any metal filings or particles. A mechanic may have removed the sensor, reinstalling with small particles attached to its sensor head (the end that reads the sprocket location). The ECU then receives a less than perfect crankshaft position. Metal particles found clinging to this sensor should be removed, though its likely that the sensor would have lost some
degree of magnetism to the filings (giving a false reading) and be in need of replacement.
Reinstalling a new crankshaft position sensor
Make sure the hole that the sensor goes into is clean and free of dirt and it presses properly between the surfaces. Sometimes a new sensor has a slight imperfection on the moulded plastic, this prevents it from pressing up against its surface. Its easy to remove - just a thin bit of excess plastic.