High mileage is not a sufficient indicator of the need for major repairs, on the other hand, low mileage does not exclude the need for major repairs. The most important indicator is probably the timeliness of routine engine maintenance. With timely oil and filter changes, as well as with all other necessary maintenance work, 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 of piston rings, valve guides and valve stem seals. It is necessary to make sure that leaks are not the cause of increased oil consumption, and only then draw a conclusion about the unsuitability of piston rings and valve guides. To determine the probable cause of the malfunction, measure the compression in the engine cylinders.
To determine the scope of the upcoming work, check the compression in the engine cylinders. Also conduct tests using a vacuum gauge and determine the nature of the readings of this device.
Check the oil pressure with a pressure gauge screwed in place of the oil pressure sensor and compare the test result with the standard value. If the oil pressure is low, the cause may be wear of the main and connecting rod bearings or oil pump parts.
Loss of power, engine "dips", detonation or metallic knocks, increased noise from the valve timing mechanism, increased fuel consumption indicate the need for a major overhaul, especially if all these signs of abnormal operation appear simultaneously. If performing all the adjustments does not lead to improvement, then the only way to eliminate abnormal engine operation is a major overhaul. A major overhaul consists of 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 repairing the cylinders, which is carried out in a specialized workshop, it will be necessary to install repair pistons. The connecting rod and main bearings of the crankshaft, as well as the covers of the camshaft bearing journals, are also subject to replacement; if necessary, the crankshaft journals should be ground until normal clearances with the connecting rod and main bearings are restored. As a rule, the valves are also subject to repair, since their condition at the time of repair is usually not entirely satisfactory. During a major engine overhaul, repairs are also performed on units such as the starter, generator, and ignition distributor. As a result, the repaired engine should have the qualities of a new unit and withstand significant mileage without failures.
Attention! During a major overhaul, important cooling system parts such as hoses, drive belts, thermostat and coolant pump should be replaced. The radiator should be inspected for leaks and cleanliness of internal channels. If you bought a repair engine or an incomplete cylinder block, some suppliers do not provide a guarantee for the operation of these units without a high-quality radiator flush. During a major engine overhaul, it is also recommended to replace the oil pump.
Before starting a major engine overhaul, read the description of the relevant procedures to get an idea of the upcoming scope of work and the requirements for them. If all the rules and regulations are followed, and all the necessary tools and devices are available, a major overhaul is not difficult to perform, but it will require a significant investment of time. It will take approximately two weeks, especially if you have to contact a specialized workshop for repair and restoration of 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 performed using a standard set of tools, although precise measuring devices will be needed to check and determine the suitability of certain parts. Often, the condition of parts is checked in specialized workshops, where they also receive recommendations for replacing or restoring certain parts.
Attention! You should contact the auto service workshops only after the engine has been completely disassembled and the condition of all parts has been checked, especially the cylinder block, in order to decide what maintenance and repair operations will be performed in the workshops.
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Since the condition of the cylinder block is a determining factor in making a decision on its further repair or on purchasing a new one (or repair) cylinder block, then you should buy spare parts or perform machining operations on related parts only after carefully checking its technical condition. Make it a rule that the true cost of repair is time - then you will not have to pay for the installation of worn or restored parts.
In conclusion, we note that the assembly of any units should be carried out with the utmost care in a clean room in order to avoid further failures of the repaired engine and ensure its reliable operation.