A significant mileage is not a mandatory sign of the need for a major overhaul, while a small mileage does not eliminate such a need. The most important consideration is the frequency of maintenance. An engine that has had regular and frequent oil and filter changes, as well as other types of maintenance, will most likely last many thousands of miles reliably. Conversely, a maintenance-free engine can require a major overhaul very early.
Increased oil consumption is a sign that the piston rings, valve stem seals and/or valve guides are worn. To determine the required amount of work, check the compression ratio in the cylinders (see Section Checking the compression ratio).
Check the oil pressure: Unscrew the oil pressure switch and attach an oil pressure gauge in its place. Measure the oil pressure at normal engine operating temperature. Compare your oil pressure readings with those given in Table of sizes and adjustment data at the end of the Guide. If the readings are significantly lower than those (and if the oil and oil filter are in good condition), the crankshaft bearings or oil pump are probably worn out. On M10 and M30 engines, the oil pressure sensor is located at the top of the left rear wall of the cylinder head. On M20 engines, the sensor is screwed into the side wall of the cylinder block, below the oil filter. On M40 engines, the sensor is screwed into the rear wall of the oil filter housing.
Loss of power, rough rotation, knocking or metallic sounds in the engine, excessive valve actuation noise and high fuel consumption can also indicate the need for an overhaul, especially if these symptoms are present at the same time. If a full adjustment does not correct the situation, then the only way out is significant mechanical work.
An engine overhaul includes restoring internal components to a new engine condition. During the overhaul, new piston rings are installed and the surfaces of the cylinder walls are restored (ground and/or honed). If grinding is done in a machine shop, new oversized pistons are installed. Main bearings and connecting rod bearings are usually replaced with new ones and, if necessary, the crankshaft is ground to restore the journals. Normally the valves are also serviced as by this time they are not in perfect condition. When overhauling the engine, components such as the ignition distributor, starter and alternator can also be rebuilt. As a result, the engine should become like new and provide many thousands of kilometers of trouble-free run.
When overhauling the engine, be sure to replace critical system components, as well as hoses, drive belts, thermostat and water pump Check the radiator carefully to make sure it is not clogged or leaking (see Chapters Car maintenance and Cooling system). In addition, it is not recommended to repair the oil pump - always install a new one when restoring the engine.
Before starting an overhaul of the engine, read the descriptions of the operations themselves to familiarize yourself with the scope of work and the requirements for them.
A major overhaul is not difficult if you carefully follow the directions, have the right tools and equipment, and pay close attention to all specifications, however, it all takes time. Plan the work so that the car will be repaired for at least two weeks, especially if parts for repair or restoration need to be sent to an auto repair shop.
Check the availability of spare parts and ensure in advance that any necessary tools and equipment can be obtained. Most work can be done with normal hand tools, although some precision measuring tools will be required to check the condition of parts to determine if they need to be replaced. Often, the auto repair shop will check the condition of the parts and make recommendations about their restoration and replacement.
Before deciding what service and repair should be carried out by an auto repair shop, perform a complete disassembly of the engine and check the condition of all components, especially the cylinder block. Since the condition of the cylinder block is a major factor in deciding whether to overhaul an old engine or purchase a remanufactured one, never purchase parts or machine other components before thoroughly inspecting the condition of the cylinder block. In general, time is the main cost of a major overhaul, so wear or non-standard parts should not be installed.
In conclusion, to ensure the longest life and the least problems with a remanufactured engine, all work must be carried out carefully, in a spotlessly clean environment.