The automatic timing advance unit is a component in vehicles with diesel engines. Of course gas and diesel engines both work on the internal combustion principle, but they are quite different, and require different means of controlling the flow of fuel during operation.

Gas combusts much more quickly than diesel. With diesel fuel, combustion can take place long after the timing reaches TDC (top dead center). When this happens, lag occurs and adversely affects performance. To prevent lag, the diesel fuel has to be injected before TDC. This is the function of that automatic timing advance unit – basically, it makes sure that no matter what the engine speed, the fuel is delivered in time for combustion to take place before TDC. The unit is located on the fuel pump, and operated by means of the primary drive gear on the engine.

Whenever you are driving your diesel vehicle, the automatic timing advance unit has to do its job. If it doesn’t, the engine will not get a consistent fuel supply. There is no specific point at which you should replace the automatic timing advance unit – essentially, it lasts as long as it lasts. It could last the life of your vehicle, or it could start to go bad, or even fail entirely with little warning. Signs that your automatic timing advance unit needs to be replaced include:

  • Sluggish engine
  • More black smoke from exhaust than is usual with diesel operation
  • White exhaust smoke
  • Engine knock

Performance issues can make for dangerous driving, so if you think that your automatic timing advance unit is failing or has failed, consult a qualified mechanic to further assist you in replacing the failing part.

This article originally appeared on YourMechanic.com as How Long Does an Automatic Timing Advance Unit Last? and was authored by Valerie Johnston.


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