High mileage is not a sufficient indicator of the need for a major overhaul, on the other hand, low mileage does not exclude the need for a major overhaul. The most important indicator seems to be the timeliness of routine engine maintenance. With a timely oil and filter change, as well as when all other necessary maintenance work is performed, the engine serves reliably for many thousands of kilometers. On the contrary, insufficient or untimely maintenance can cause a sharp reduction in engine life.
Increased oil consumption indicates wear on the piston rings, valve guides and valve stem seals. Make sure that leaks do not cause increased oil consumption, and only then conclude that the piston rings and valve guides are unsuitable. To determine the likely cause of the malfunction, measure the compression in the engine cylinders (see subsection 3.1.1.2).
To determine the amount of work to be done, check the compression in the engine cylinders. Also test with a vacuum gauge and determine the nature of the readings of this device (see subsection 3.1.2.3).
Check the oil pressure with a pressure gauge screwed into place of the oil pressure sensor and compare the test result with the standard value. If the oil pressure is low, then the cause may be worn main and connecting rod bearings or parts of the oil pump.
power loss, "failures" in engine operation, detonation or metal knocks, increased noise from the gas distribution mechanism, increased fuel consumption indicate the need for major repairs, especially if all these signs of abnormal operation appear at the same time. If the performance of all adjustments does not lead to improvement, then the only means of eliminating abnormal operation of the engine is a major overhaul. An overhaul consists in restoring engine parts to the condition specified in the technical data for a new engine. During a major overhaul, pistons and piston rings are replaced, cylinders are bored or honed. After the repair of the cylinders, which is carried out in a specialized workshop, the installation of repair pistons will be required. The connecting rod and main bearings of the crankshaft, as well as the covers of the camshaft bearing journals, must also be replaced; if necessary, the crankshaft journals should be ground to restore normal clearances with the connecting rod and main bearings. As a rule, valves are also subject to repair, since their condition at the time of repair is usually not entirely satisfactory. During the overhaul of the engine, repairs are also carried out on such units as the starter, generator and ignition distributor. As a result, the repaired engine must have the qualities of a new unit and withstand a significant mileage without failure.
Attention! During a major overhaul, important parts of the cooling system such as hoses, drive belts, thermostat and coolant pump should be replaced. The radiator must be examined for tightness and cleanliness of the internal channels (see subsection 4.3). If you bought a repair engine, or an incomplete cylinder block, then some suppliers do not guarantee the operation of these units without high-quality flushing of the radiator. When overhauling the engine, it is also recommended to replace the oil pump.
Before starting an engine overhaul, review the relevant procedures to get an idea of the scope and requirements of the work ahead. Subject to all rules and regulations, with all the necessary tools and fixtures, major repairs are easy to perform, but significant time will be required. Approximately it will take at least two weeks, especially if you have to go to a specialized workshop to repair and restore parts. Check the availability of spare parts and take care of purchasing the necessary special tools and equipment in advance. Almost all work can be done with a standard set of tools, although precise measuring instruments will be needed to check and determine the suitability of certain parts. Often, the condition of the parts is checked in specialized workshops, which also receive recommendations for the replacement or restoration of certain parts.
Attention! You should contact the car service workshops only after completely disassembling the engine and checking the condition of all parts, especially the cylinder block, in order to decide which maintenance and repair operations will be performed in the workshops.
Since the condition of the cylinder block is the determining factor in making a decision on its further repair or on the purchase of a new (or repair) cylinder block, then buying spare parts or performing machining operations on related parts should be done only after a thorough check of its technical condition. Make it a rule that the true cost of repairs is time so you don't have to pay to install worn or remanufactured parts.
In conclusion, we note that the assembly of any units should be carried out with all care in a clean room in order to avoid further failures of the repaired engine and ensure its reliable operation.