A high mileage does not necessarily indicate the need for a major overhaul. At the same time, low mileage does not at all exclude the fact that the engine requires major repairs. The most important factor can be considered the regularity of service. An engine that has been regularly and frequently changed oil and filter, which has been regularly and other required maintenance, can reliably and for a long time without major repairs and provide many thousands of kilometers of run. Conversely, an engine that has been neglected may require an overhaul very early, regardless of its age.
Excessive oil consumption is a sign that piston rings, valve stem seals and/or valve guides need attention. Before deciding that the rings and/or guide bushings are worn, make sure that the cause is not an oil leak. To determine the likely cause of the problem, perform a compression or leak test as described in chapter 2B or 2B (whichever is applicable).
Check the oil pressure with a pressure gauge installed in place of the oil pressure sensor and compare the reading with the prescribed value. If the pressure is very low, it is possible that the cause is wear on the crankshaft main and connecting rod bearings and / or the oil pump.
Loss of power, rough running, knocking or metallic noises from the engine, increased valvetrain noise, and high fuel consumption can also indicate the need for an overhaul, especially if they all occur at the same time. If full service does not solve the problem, the only solution is a major overhaul.
Overhaul of the engine involves the restoration of all internal elements in accordance with the technical parameters of the new engine. During a major overhaul, pistons and piston rings are replaced. As a rule, new crankshaft main and connecting rod bearings are installed; if necessary, the crankshaft itself can be replaced or reground to restore the necks. Valves are also subject to repair, since at the time of repair their condition is usually far from ideal. When overhauling the engine, be sure to pay close attention to the condition of the oil pump and replace it if there is any doubt about its performance. The result of the work should be an engine that, in terms of its characteristics, is not inferior to a new one, which will provide you with thousands and thousands of kilometers of trouble-free mileage.
The main elements of the cooling system, such as hoses, thermostat and coolant pump, should also be replaced when the engine is overhauled. The radiator should be carefully checked for blockage or leakage. When performing a major overhaul, it is recommended to replace the oil pump.
Before starting a major overhaul, review the entire procedure to assess the scope and requirements of the work. Check that you have the necessary spare parts and special tools and equipment. Most of the operations can be performed with normal plumbing tools, although a number of precision measuring tools will be required to inspect the components to determine if they need to be replaced.
It will almost certainly be necessary to turn to a machining or engine repair specialist, especially if the overhaul requires crankshaft regrinding or cylinder boring. In addition to performing machining, specialists can perform qualified inspection of the elements, give advice on the installation, replacement and purchase of new elements such as pistons, piston rings and bearing shells. In the cases described above, it is recommended to contact trusted organizations licensed to perform such work.
Before deciding which service and repair operations need to be outsourced to specialists, you should first completely disassemble the engine and inspect all elements (especially the cylinder block/crankcase and crankshaft). The condition of these items will be the determining factor in deciding whether to overhaul the engine or buy a remanufactured unit. Therefore, do not buy assemblies and parts and do not overhaul other elements until you have completely inspected the basic elements. You can take this rule as a basis: the main cost of a major overhaul is the time spent, and therefore do not spend it on installing worn or substandard elements.
And finally, the last note: to ensure the maximum resource and reliability of the repaired engine, assemble it very carefully and be sure to keep it clean.