Automotive Engine Case Study

A customer complains that the engine has a rough idle and does not have as much power as it used to. The customer also says that the engine is difficult to start when it’s cold and runs better while cruising at highway speeds. On examining the car, it’s found that it has a four-cylinder engine with nearly 65,000 miles on it. Driving the car verifies the customer's complaint.

Diagnosis begins with a visual inspection that reveals only that the car has been well maintained. Because there is an endless list of possible causes for these problems, the first test is for engine vacuum. With the vacuum gauge connected to the manifold and the engine at idle speed, the gauge's needle constantly drops from a normal reading to 10 in. Hg. The rhythm of the drop matches the rhythm of the idle. The behavior of the gauge indicates a probable problem in one cylinder.

To verify this diagnosis, a cylinder power balance test is conducted. The results show an engine speed drop of 100 to 125 rpm when cylinders 1, 2, and 4 are shorted, and a drop of only 10 rpm when cylinder 3 is shorted.

Based on this test, cylinder 3 is identified as having the problem.

To identify the exact fault, further testing is required. The spark plugs are removed and inspected. All look normal, including the one from cylinder 3. Next, a compression test is taken. All cylinders have normal readings. A cylinder leakage test is then conducted and it too shows normal conditions. The results of the power balance, compression, and cylinder leakage tests lead to the conclusion that the cause has to be in the valve train. Some thing is preventing a valve from opening.

Removing the cam cover for a visual inspection leads to the discovery of the fault: The intake lobe for cylinder 3 on the camshaft is severely worn. A replacement of the camshaft and matching lifter will correct the problem. The worn lobe only affects the opening of the valve and does not prevent it from sealing, which is why the compression and cylinder leakage test results were normal. Cylinder power and vacuum are affected by the valve not opening fully.

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