Engine Diagnosis and Testing

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Engine diagnosis on modern power plants should start with the same basic tests that mechanics have used for decades.

It's true that problems could be hiding in electronic control parts and high tech systems, but it is still recommended to start with the basics of the engine's health first.

This basic engine diagnosis tests explained below can provide a good indication of the true internal condition. Performing a compression test and manifold vacuum test is not difficult on many cars and trucks, but provides valuable results.

Add to this fact the cost of the needed test equipment is so low you get a lot of bang for the buck. After performing these diagnostic procedures the path to a solution will be more clear for most engine performance problems like rough running, lack of power and stalling.

Performing Compression Tests

Internal combustion engines depend on compression of the air and fuel mixture to maximize the power produced by the engine.

The upward movement of the piston on the compression stroke compresses the air fuel mixture within the combustion chamber. The over head valves close so that this procedure can take place at the right time.

Both the exhaust and intake valves must seal correctly if maximum compression is to be achieved. The air/fuel mixture gets hotter when it's squeezed. And these hot mixtures ignite easier.

When ignited it makes more power than the same mixture at lower temperatures. If the combustion chamber leaks, some of the air/fuel mixture will escape when it is compressed.

This results in a loss of power and waste of fuel. The leaks can be caused by burnt valves, blown head gasket, worn piston rings, slipped timing belt or chain, worn valve seats, a cracked cylinder head and more.

An engine with poor compression or even low compression will not run correctly and cannot be tuned to factory specifications.

If initial engine diagnosis suggests that the cause of the problem may be poor compression, a complete test should be performed. A compression gauge is used to check each cylinder. The reading is a good indication of the health of that particular cylinder.

The typical compression gauge indicates pressure in PSI (Pounds per Square Inch). Most compression testers have a vent valve that holds the highest pressure reading on the meter.

Opening the valve releases pressure when the test is completed. The steps for conducting the cylinder compression test are included with the gauges. Specifications can be found in service and repair manuals now available online.

How to Test Engine Vacuum

Measuring intake manifold vacuum is another fast and efficient way to diagnose the over all condition of an engine.

Manifold vacuum is tested with a specialized vacuum gauge that comes with instructions and different size fittings and adapters. The downward movement of the piston forms vacuum during the intake stroke.

If the cylinder is sealed well, a maximum amount of vacuum will form. Vacuum readings are interpreted to identify many different kinds of engine problems. Maybe the most important one is the ability of the cylinder to seal.

As well as the timing of the opening and closing of the engines valve train, and provide insight about the correct ignition timing setting. Ideally each cylinder of an engine will produce the same amount of vacuum.

Therefore, the vacuum gauge readings should be steady and give a reading of about 18-20 inches of mercury.

If one or more cylinders produces more or less vacuum the needle of the vacuum gauge will jump around. The intensity of the fluctuation indicates the severity of the problem of the leaking cylinder.

For example, if the reading on the vacuum gauge fluctuates between 10 and 17 inches of mercury you should look at the rhythm of the needle.

If the needle seems to stay at 17 Most of the time, but drops to 10 and quickly rises this reading is probably caused by a problem in one cylinder.

Fluctuating or low readings can indicate many different problems. For example, retarded ignition timing or incorrect valve timing might cause a low steady reading. A burned intake valve might cause a sharp drop in vacuum at regular intervals.

Other conditions that can be uncovered with a vacuum test are weak valve springs, faulty PCV valves, improper exhaust gas re-circulation valve operation and poor compression resulting from worn piston rings or damaged cylinder walls.

A few other possibilities include a leaking head gaskets, a manifold vacuum leak, and restricted exhaust systems will also cause low vacuum readings.

Yes it's true that your engine diagnosis could lead you to a high tech part such as a fuel injector or ignition module. But experienced auto mechanics will most often start with the basics to ensure that the engines overall health is sound before continuing with there engine diagnosis.

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