General remarks
It is not always easy to come to the conclusion about the feasibility of a complete overhaul of the engine, since it is necessary to be based on a number of objective indicators.
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 section 2).
To determine the amount of work to be done, check the compression in the engine cylinders (see section 2). Also test with a vacuum gauge and determine the nature of the readings of this device (see section 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.
Comment. 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 Chap. 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 associated procedures to get an idea of the scope and requirements of the work to be done. 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.
Comment. 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.
Engine diagnostics with a vacuum gauge
Vacuum measurement is a reliable and relatively cheap way to diagnose an engine. According to the readings of the vacuum gauge, you can get an idea about the condition of the piston group, the tightness of the gaskets of the cylinder head, the intake and exhaust manifolds, the correct adjustments of the engine power system and the exhaust gas exhaust, the exhaust gas throughput, the condition of the valves (their sticking or burning) and valve springs, as well as check the correct adjustment of the ignition timing and the preservation of the valve timing during engine operation.
Unfortunately, the readings of the vacuum gauge are difficult to interpret and the results of the analysis of readings can be erroneous, therefore, it is advisable to combine vacuum diagnostics with other methods.
The initial factors by which the readings of the vacuum gauge are analyzed and the most accurate conclusions about the state of the engine are made are the absolute reading of the device and the nature of the movement of the instrument pointer (dynamics of indications). The scale of most vacuum gauges is graduated in mm. rt. pillar. As the vacuum increases (and, accordingly, the pressure drop) the instrument reading increases. For every 300 m above sea level, the absolute readings of the vacuum gauge will differ by about 25 mm. rt. Art.
Attach a vacuum gauge directly to the suction manifold (See photo), but not to other holes through which vacuum is created, separated from the manifold by a channel of a certain length (e.g. to openings in front of the throttle valve).
Warm up the engine completely before testing. Block the wheels and put the car on the handbrake. When the gear lever is in neutral position (or in the Park position on vehicles with automatic transmission) start the engine and let it idle.
Attention! Before starting the engine, carefully check the condition of the fan blades (the presence of damage or cracks on them). When the motor is running, do not put your hands too close to the fan, keep the appliance at a sufficient distance from the fan and do not stand in line with the rotating impeller.
Check the vacuum gauge reading. On a serviceable engine, the vacuum gauge should show a vacuum of 430-560 mm. rt. Art., and the arrow of the device should be practically motionless.
The following is a description of the nature of the readings of the vacuum gauge and the methodology for determining the condition of the engine based on them.
1. Too low a vacuum usually indicates a leak in the gasket between the intake manifold and the throttle chamber, the vacuum hose, as well as too late ignition or the wrong opening and closing of the valves. Before removing the toothed belt covers and checking the alignment of the timing marks, check the ignition setting with a stroboscope and eliminate all other possible causes, guided by the test methods described in this Chapter.
2. If the readings of the vacuum gauge are 75-200 mm. rt. Art. below normal and are unstable (arrow twitches), then this indicates a leak in the gasket at the inlet of the suction manifold or a malfunction of the injector.
3. If the arrow regularly deviates by 50-100 mm. rt. Art., then the cause is not the tightness of the valves. To confirm this conclusion, check the compression in the engine cylinders.
4. The needle is irregularly deflected towards low readings, or trembling indicates low vacuum. The likely cause is increased resistance to valve movement, or interruptions in the operation of the cylinders. Check cylinder compression and inspect spark plugs.
5. If at idle the arrow fluctuates rapidly within 100 mm. rt. Art., and the operation of the engine is accompanied by smoke from the muffler, then the valve guides are worn out. To verify this conclusion, it is necessary to test the combustion chambers for leaks (with air pumping). If the arrow fluctuates rapidly and at the same time an increase in engine speed is observed, then it is necessary to check the tightness of the intake manifold gasket, the elasticity of the valve springs. Such indications can also be due to burnt valves and interruptions in the operation of the cylinders (ignition failures).
6. Weak fluctuations of the arrow (within 20-30 mm. rt. Art. round trip) indicate unstable ignition operation. Check all foreseen settings and adjustments, if necessary, connect an ignition system analyzer to the engine.
7. In case of large fluctuations of the arrow, check the compression in the cylinders, or carry out leak tests, as the causes of the malfunction may be an idle cylinder, or a leak in the cylinder head gasket.
8. If the readings of the device change slowly over a wide range, then check the cleanliness of the pipelines of the forced crankcase ventilation system, the correct adjustment of the combustible mixture, the tightness of the gaskets of the throttle body, or the intake manifold.
9. Open the throttle sharply, and when the engine speed reaches 2500 rpm, release the throttle. The shutter should slowly return to its original position. The gauge reading should drop to almost zero, then rise and exceed the control reading corresponding to stationary idle by about 125 mm. rt. Art., after which the vacuum should be restored to its previous level. If the vacuum is restored slowly, and there is no excess of the control reading when the damper is suddenly opened, then the cause may be worn piston rings. If the vacuum recovery is extremely slow, check the cleanliness of the exhaust tract (usually muffler or catalytic converter). The easiest way to do this is to open the exhaust tract in front of the suspicious area and repeat the test.