To begin diagnosing a diesel engine problem, first identify what the problem is. Three of the most common diesel engine problems include hard starting, defective glow plug, and continuous use of thickened or wrong engine oil.
A hard start problem is often related to the vehicle's electrical or fuel system. Start with the battery, and check its charge. If the battery is low, recharge it using a trickle charger. If the engine still doesn't start even when the battery is at full charge, check the fuel filters and lines, and clear them of any obstructions. Replace the fuel filter as needed, then mix fuel conditioner additive into the diesel fuel that's already in the tank. Make sure the additive is approved by your car manufacturer, as some manufacturers prohibit their use. Otherwise, drain the tank and replace it with No.1 diesel fuel. For best results, install an aftermarket fuel heater.
Use a voltmeter to check each glow plug and confirm its voltage reading when the ignition is turned on. Replace any plug that shows out-of-specification voltage.
If the oil has thickened, change the oil. It may help to switch to a lighter oil such as a 10W-30 or CG-4 rated synthetic.