Buying a diesel car from a stranger, put the engine a thorough check. Make sure that when you run the engine was cold. Serviceable diesel plants with a twist. Please note that the cold he emits a pretty noticeable noise. Then start the engine is warmed up, the noise should significantly subside. If in this state it won't start immediately, but only after 3 to 9 attempts, so worn rings, piston.
Check whether belching from the exhaust smoke when you press down on the accelerator when the engine is warm. If a cloud of dark smoke, it is likely, wear has undergone a oil ring or a faulty injector. White smoke indicates fuel getting into the water. Put under the exhaust a sheet of paper. If it appeared soot, so the fuel is not burned completely.
The sound of the engine should be soft, uniform, no knocks. Reverse the evidence of irregularities in the operation of the valves. Try to recognise the noises in the set and reset speed, to determine the operation of the crankshaft and piston. Open the oil cap. Indication of what is happening somewhere blowby are the oil spray from the oil fill neck.
Look under the hood. Determine the integrity of the cylinder block and nuts to the injectors. Traces of white or red sealant instead of black indicate that the engine's been opened. All the bolts that fix the auxiliary devices must be in place.
To assess the condition of the ear, run the car, switch off and then switch on the ignition. Light oil pressure switch should light up in 2-3 seconds, but not before.
If the results of all these actions, you are not disappointed and you are ready to become the owner of a diesel engine, measure the compression and crankcase pressure, check injectors, cooling system.