1996 BMW 740iL Engine Compression Test Cost

A vehicle's engine contains a number of cylindrical cavities that are bored into the engine block. Each of these engine cylinders houses a piston that moves up and down to spin the crankshaft, much the same way someone might pedal a bicycle. Inside the cylinder is where engine combustion takes place to move the pistons up and down. The cylinders are sealed airtight, with valves that open and close to allow air and fuel to enter the cylinder, and to allow exhaust gasses to escape. When the valves are all closed, the cylinder builds up either compression or vacuum (depending on which direction the piston is moving). In both cases it is necessary for the cylinder to be sealed. If one or more of the cylinders is not sealing properly, the engine will not run properly. A compression test is aimed at determining if a cylinder has low compression - a sign that it has an inadequate seal.

To generate power, an engine utilizes the exhaust gasses created in the combustion process as a source of power to move the pistons up and down inside the cylinders. The engine in a car is known as a "four-stroke" (or "four-cycle") engine that relies on four distinct piston strokes (intake, compression, power, exhaust) to complete one full operating cycle. During the compression and power strokes, the intake and exhaust valves are closed and the cylinder is sealed airtight.

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A vehicle that has an engine suffering from low compression can be a hazard to drive if it tends to stall at idle. This can make navigating busy intersections on the road a dangerous job. The vehicle should be evaluated as soon as possible if this is the case. Otherwise, the vehicle can be driven for short distances.

The main cause of low engine compression is premature wear of the piston rings that seal the pistons against the cylinder walls or damage to the valves. Poor lubrication may result in piston ring and valve failure. With normal vehicle maintenance - including regular oil changes - sufficient engine compression should be retained for the life of the vehicle.

An engine compression test is used to determine if one or more of the cylinders in an engine is producing insufficient or low compression. A basic engine compression test is performed by allowing the engine to turn over without starting, while other tests may be done while the engine is running. All together, these engine compression tests can help a technician: locate damage to piston rings, pistons, or cylinder walls; identify bent, worn, or stuck intake and exhaust valves; diagnose a deteriorated head gasket; or even point to exhaust problems, such as a clogged catalytic converter or muffler.

To perform a compression test, a technician will make sure the oil level is normal and warm up the engine before disabling the ignition and fuel systems. The spark plug wires are disconnected and the spark plugs are removed. A compression tester is installed in one of the spark plug mounting holes. The technician will have someone turn the key to engage the starter for a short time. The gauge on the compression tester reveals the pressure in the cylinder. The technician will record the reading and repeat with the rest of the cylinders. The process is more complex for a test with the vehicle running, but it reveals more information about the condition of the engine.

Variations from one vehicle model to the next require access to vehicle-specific information and repair procedures. It is recommended that a qualified technician diagnose and repair symptoms of low compression.

The ignition system and/or the fuel system need to be disabled prior to performing an engine compression test, unless the test is being done while the engine is running. In the latter case, a professional technician should perform the test. Damage or injury can occur if excess fuel is sprayed from the engine during the procedure.

While a basic engine compression test is not too difficult to perform on some vehicles, it is quite a challenge on others. Access to cylinders located at the back of the engine compartment can be minimal. Specific vehicle data must be referenced for comparison to the compression test results. Moreover, an engine compression test alone is usually not enough information to properly diagnose poor engine performance. Other tests need to be conducted for a solid and targeted diagnosis because several conditions can lead to the symptoms associated with compression loss. These tests are usually better left to a professional technician.