What is diesel engine runaway?
The revs climb on their own, without you touching the accelerator, often accompanied by a deafening noise and thick smoke. A diesel engine can burn engine oil as fuel: if oil enters the intake in large quantities, switching off the ignition is not always enough to stop it.
Pull over somewhere safe and switch off the ignition. If the engine keeps running and the car has a manual gearbox: select a high gear, apply the brakes fully, and release the clutch to stall it. If heavy smoke is coming from the bonnet, move well away from the vehicle.Likely causes
- Massive oil leak from the turbo: destroyed bearings or seals allow oil to pass directly into the intake. This is the most common cause.
- Oil that has built up in the intercooler and is suddenly drawn in.
- Excessive crankcase pressure (heavily worn engine, blocked breather) forcing oil back towards the intake.
- Oil level far too high, following overfilling.
After a runaway
Do not restart the engine. An engine that has run away has often exceeded its maximum rev limit: connecting rods, valves and bearings may be damaged, and the engine will have lost some of its oil. Have the vehicle towed.
Repair or replace?
The turbo must be replaced, the intercooler and hoses cleaned, and the source of the leak corrected. The engine should then be inspected: if the runaway has caused damage, a reconditioned engine on an exchange basis, supplied as a bare engine and warranted for up to 24 months, avoids an uncertain repair.
