It is not always easy to come to a conclusion about the advisability of a complete engine overhaul, since it is necessary to rely on a number of objective indicators.
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, apparently, is 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. Conversely, 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. 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 failures, 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 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, and cylinders are bored or honed. After the cylinders have been overhauled in a specialized workshop, pistons of repair sizes will need to be installed. The connecting rod and main bearings of the crankshaft, as well as the bearing journal covers of the camshafts, 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.
During a major overhaul, important cooling system components 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 purchased a rebuilt 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 more accurate 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.
It should be noted that the assembly of units should be carried out with the utmost care in a clean room to avoid further failures of the repaired engine and to ensure its reliable operation.