High mileage does not necessarily indicate the need for a major overhaul, while low mileage does not negate the need for a major overhaul. 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 maintenance, is likely to provide many thousands of miles of reliable service. Conversely, a maintenance-free engine may 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 amount of work required, check the compression ratio in the cylinders (see section Checking the compression ratio).
Check the oil pressure: Unscrew the oil pressure sensor 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 Manual. If the readings are significantly lower than those given (and if the oil and oil filter are in normal condition), then 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 running, knocking or metallic sounds in the engine, excessive valve train noise and high fuel consumption may also indicate the need for a major overhaul, especially if these symptoms are present at the same time. If a full tune-up does not correct the situation, then the only solution is extensive mechanical work.
An engine overhaul involves restoring the internal components to a new engine condition. During an overhaul, new piston rings are installed and the cylinder wall surfaces are restored (are ground and/or honed). If the grinding is done in a machine shop, new pistons of a larger size are installed. The main and connecting rod bearings are usually replaced with new ones and, if necessary, the crankshaft is ground to restore the journals. The valves are also usually serviced, since by this time they are not in perfect condition. During a major engine overhaul, components such as the ignition distributor, starter and generator may also be restored. As a result, the engine should become like new and provide many thousands of kilometers of trouble-free mileage.
When overhauling an engine, it is important to replace critical system components such as hoses, drive belts, thermostat and water pump. The radiator should be carefully checked to ensure that it is not clogged or leaking (see Chapters Routine car maintenance and Cooling system). In addition, it is not recommended to repair the oil pump - always install a new one when rebuilding the engine.
Before starting a major engine overhaul, read the descriptions of the operations themselves to familiarize yourself with the scope of work and the requirements for them.
Overhauling is not difficult if you follow directions carefully, have the right tools and equipment, and pay close attention to all specifications, but it does take time. Plan to have the car in for at least two weeks, especially if parts need to be sent to a repair shop to be repaired or rebuilt.
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 common hand tools, although some precision measuring instruments will be needed to inspect the condition of parts to determine if they need to be replaced. Often, a car repair shop will inspect the condition of parts and make recommendations for reconditioning or replacement.
Before deciding what service and repairs should be performed by a repair shop, completely disassemble the engine and inspect the condition of all components, especially the engine block. Since the condition of the engine 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 engine block. In general, time is the main cost of an overhaul, so do not install worn or substandard parts.
Finally, to ensure the longest service life and the least problems with the rebuilt engine, all work must be carried out carefully, in a spotlessly clean environment.