Fault diagnosis

FAULT DIAGNOSIS

The main differences experienced when working on a Diesel engine, compared to a petrol engine, are as follows:

1. The diesel fuel system is driven by an injection pump and also comprises:
   • a tank and two fuel feed and return lines;
   • a fuel filter (with hand pump for bleeding fuel system);
   • injectors with inlet pipes;
   • preheating device for starting engine when cold;
   • an engine arrest device.

The injection pump is the most reliable part of the fuel system due to very low component wear and low likelihood of incorrect adjustment.
Engine failure should not immediately be attributed to the injection pump but more probably to one of the other fuel system components.

2. The combustion process within a Diesel spontaneous fuel self-ignition engine produces very high temperatures. Coupled with the fact that the fuel inevitably contains traces of sulphur, this determines:
   • greater tendency than petrol engines to produce sparks and thus greater wear on moving parts, particularly those close to the combustion chamber;
   • greater tendency of piston rings to become bonded in their seats.

It is very important to change the fuel filter every 15,000 km.
Use only specified lubricant oil grade and change engine oil every 15,000 km. (**) If vehicle is used under heavy conditions (mainly town driving, continuous mountain driving, towing of trailers or caravans, routes through dusty areas) change oil every 10,000 km.
Make sure injectors are always efficient in order not to increase production of uncombusted products during engine operation.

3. The combustion process in spontaneous self-ignition diesel engines also gives rise to higher engine noise levels (*) with the production of combustion knock clearly discernible from outside. Although this phenomenon has been greatly reduced in present-day engines with prechambers, it is still present, particularly at low speed, but tends to diminish at medium-high speeds.

NOTE If the injection pump is advanced only slightly in relation to correct timing setting, combustion knock is considerably accentuated.

4. An indirect injection Diesel engine needs a prechamber preheating device (rapid glow plugs and ECU) to facilitate fuel self-ignition when the engine is cold and hence engine start-up.
   A special solenoid is used to turn off the engine (which occurs when the fuel supply to the injection pump ceases). This opens up fuel flow in the injection pump when the ignition key is in MARCIA position and turns it off when the ignition key is turned to STOP.

(*) Engine running noise: this is due to an excessive fuel pressure gradient, i.e. the ratio between pressure developed by combustion and corresponding crankshaft angles of rotation.

(**) Adopt SELENIA oil. If VS oils are used, change every 10,000 km and every 7,500 km in the case of heavy use.