Rough Running Engines - 3 Basic Tests to Perform
By Mark Gittelman
Not all rough running engine problems are related to electronic control systems. Therefore a diy car mechanic still needs to understand basic engine testing procedures. These tests are an important part of modern engine diagnosis. Below I will talk about three of the most important basic testing procedures. The compression test, testing with a vacuum gauge, and the power balance test can be an auto mechanics 3 best friends.
Compression test for rough running engines
Why perform a compression test? Internal combustion engines depend on the compression of the air fuel mixture to maximize the power produced by the engine. The upward movement of the piston on the compression stroke will compact the air fuel mixture within the sealed chamber raising the temperature of the charge and making easy to ignite and burn completely.
An engine with poor compression or low compression will not run correctly and cannot be tuned properly. If a rough running engine condition is present. This may be considered a symptom that suggests the cause of the problem may be poor compression. A compression gauge is used to check the cylinder sealing capabilities. This is an easy to use inexpensive tool that comes with complete instructions on how to use it.
Before conducting a compression test, it is recommended to disable the ignition and also the fuel system. You should also prop open the throttle plate into a wide open position to allow the maximum amount of air to enter the cylinder during the test. Mark the spark plug wires and remove all the spark plugs. Make the test a one man operation by connecting a remote starter button to the starter motor. This will allow you to crank over the engine by yourself.
Power balance test
Diagnose rough running engines with a power balance test. This is performed quickly and easily using an engine analyzer, because pushing a button can automatically or manually control the firing of the spark plugs. Some vehicles have the power balance test built into the engine control computer. This test is either a part of a routine self-diagnostic operating mode or must be activated by the mechanic through a scan tool. These types of vehicles will often use these functions to set engine misfire codes.
On some computer controlled engines certain components must be disconnected before attempting a power balance test. Always check the service manual for appropriate procedures. Be careful not to run the engine with a shorted cylinder for more than 10 or 15 seconds. The unburned fuel in the exhaust can damage the catalytic converter and create an unsafe situation. It is also a good idea to run the engine on all cylinders for a minute before proceeding to the next cylinder.
As each cylinder is shorted, a noticeable drop in engine speed should be detected. Little or no decrease in RPMs indicates a week cylinder. If all of the readings are fairly close to each other, the engine is in good condition. If the readings from one or more cylinders differ from the rest then a problem should be looked into on that cylinder.
Engine vacuum testing
Ideally, each cylinder of an internal combustion engine will produce the same amount of vacuum. This is why the vacuum gauge readings should be steady and give a reading of at least 19 to 22 inches of mercury.
If one or more cylinders produce less vacuum than the others the needle of the vacuum tester will fluctuate. When performing the engine vacuum tests, the intensity of the fluctuation of the needle on the vacuum gauge indicates the severity of the engine problem. For example, if the reading on the vacuum gauge fluctuates between 9 and 19. You should then look at the rhythm of the needle. If the needle stays at 19 inches of mercury, but drops to 9 and then quickly rises we know that reading is probably caused by a malfunction in one cylinder.
If the vacuum tester reads low all the time let's say a reading of 10 inches and has no fluctuation. This would be an indication of a clogged exhaust system. If you disconnect the exhaust system in front of the catalytic converter and the vacuum returns to full strength, this would indicate a clogged catalytic converter or other parts behind it.
Mark provides information and auto repair videos on engine testing procedures on his car website. He also recommends that you brush up on the basic principles of automotive engine repair.
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