Direct injection diesels with mechanical injection pumps, like the 300Tdi have very few requirements to start and run.
Apart from a battery and an operational starter motor, the only electrical device needed is the fuel shut off solenoid (legal requirement). The 300Tdi has glow plugs, but because it is a direct injection diesel, they are not needed except for extreme cold conditions (rare in Aus).
The solenoid is near the rear of the fuel injection pump and should be energised when the starter key is in the run position. There is one wire with a spade terminal at the solenoid. You may be able to verify that it is operating by listening for it to operate or checking with an alternator when the key is turned on and off. Not a very common problem, but easy to check and eliminate.
The engine needs air, and fuel which is ignited by engine compression. One of these 3 is the most likely fault.
Fuel problems, or more likely lack of, can be:
- Contaminated fuel - water or fungus in fuel. 1st check is to drain the sedimentor on the inside of the RH chassis rail near the rear wheel. There fuel drained from the sedimentor should be clean. There may be a little water and sediment as the sedimentors job is to remove these before the fuel passes to the fuel filter. The sedimentor should be drained at regular service intervals.
- Blocked fuel filter - the fuel filter has to remove extremely small particles before the injection pump. It should be replaces at regular service intervals, or if you pick up a load of bad fuel. Generally performance will drop of because the filter can not pass enough fuel when the engine requires a higher fuel rate to produce high power output. Fungus will fully block a filter very quickly.
- Blocked fuel lines - fungus or bad sedimentation. It should be obvious when the sedimentor is drained if there is fungus (like gray/black snot) or bad sedimentation.
- Leaking fuel lines - the very slightest leak will allow air into the fuel lines and will stop a diesel. Any of the plastic lines between the fuel tank and the fuel injection pump can crack. And the crack can even be small enough that you will not detect fuel leaking out when the vehicle is stopped, but still allow air to be sucked in. Leaking fittings (sometimes not tightened properly) are another source for air to be sucked in - those on the fuel lift pump (left side of engine) sometimes develop a leak. The flexible line between the lift pump and fuel filter can fail because Land Rover made it too short and not flexible enough for how much the engine moves.
- Leaking fuel injectors - they leak into the cylinder while the engine is stopped and allow air into the injectors.
- Faulty fuel lift pump - can be valves, diaphragm or fittings. If the diaphragm fails fuel can leak into the engine sump and contaminate the oil. Sometimes the lift pump can be faulty and the injection pump is still able to pull fuel through the system from the fuel tank, but not be able to when the vehicle is facing up hill or the level in the tank is low. Sometimes only performance suffers but it can make starting difficult.
If the engine compression is too low, the air in the cylinders will not be compressed enough to reach the temperature needed for ignition.
Usually the compression reduces over a longish period as the engine wears and the engine doesn't start as easily as it should and performance drops off. A normal 300Tdi will start before the crankshaft is rotated a complete turn by the starter motor (assuming good battery and starter motor).
Causes of low compression include, worn rings or valves, damaged pistons or blown head gasket.
Another possible reason for your engine not starting is if the timing of the injection pump is out - timing belt jumped a tooth or 2 (not very common). Of course if the timing belt has broken, the engine wont start, but then you would hear the pistons hitting the valves when the engine was turning over.
