 |
|
|
|
|
| |
How to check your engine's compression |
|
|
Frequently asked questions regarding Engine Compression
As a technician, I am frequently asked questions related to Engine Compression. My responses have been formulated below and will help any mechanic, (D.I.Y. or Professional) to fully comprehend the importance of Compression. The information below will guide you through a basic compression test and diagnostic routine.
For your own safety, it is advised that you purchase and follow the correct auto repair literature before attempting this process. The following only describes general information and is not vehicle specific. We do not express or imply any vehicle specific directions in this article.
Q. What is compression in an engine and how does it work?
A. Compression is a process in which air, or an air/fuel mixture is confined and pressed into a smaller volume within the area of an engine’s cylinder. This process forces all of the molecules to be "pressed" together under high pressure.
In the case of a gasoline engine, moderate compression is required, 140-160 PSI (Pounds Per Square Inch). Some engines may require as much as 220 PSI depending on their size and application. (The manufacturer gives specific Compression Specifications.) If the compression in a gasoline engine is too high, it can cause a problem known as pre-ignition or detonation. This can be very destructive, causing damage to the internal parts of the engine. However, a diesel engine requires very high compression, usually 350 PSI or more because it relies on this compression process to ignite the diesel oil. Diesel engines are much heavier and louder compared to the gasoline engine. This compression process combined with an air/fuel mixture and a source of ignition is what produces the necessary power to operate vehicles of all types.
Q. When should the compression be checked on a vehicle?
A. Generally speaking, the compression should be checked in any instance when an engine
is running roughly or is lacking power. Most manufacturers and technicians believe it is important to perform a compression test every time a tune up is done as part of preventative maintenance (consult your vehicle owners manual for tune up intervals). By performing a Compression Test, internal engine malfunctions, i.e.: bad valves, piston rings or excessive carbon build up, can be detected before they cause irreparable damage. It benefits the owner to be aware of these problems as they can make an informed decision whether to invest in repairs or sell the vehicle.
Q. How is the engine compression checked?
A. Engine compression can be checked in different ways for different vehicles. In this section, I will only cover the general information required for testing a gasoline engine. Diesel engines require specialized equipment and are not covered in this article.
Compression on a gasoline engine can be tested in two ways. The first method involves using a manual, hand-held compression gauge, (for example the Mityvac“ Compression Test Kit, PN 05520).
1. Make sure the engine has been warmed up before beginning the test, to insure that the oil has been warmed up. A cold engine will not test correctly.
2. Disable the Ignition Module or Coil.
3. Insert the compression tester into one cylinder spark plug hole at a time.
4. Hold the throttle to full open position to ensure the engine gets adequate air intake.
5. Crank the engine continually for at least five to ten full revolutions to obtain an accurate reading on the compression tester.
6. Record the reading for each cylinder. If any of them vary 10% or more from each other a problem may exist in one or more cylinders. If the variance is greater than 10%, specialized testing equipment may be required to fully diagnose the problem.
7. If all cylinder readings are within 10% of each other, no further testing is required and compression is considered optimal.
The second method of testing involves the use of an electronic engine analyzer. The analyzer "shorts" one cylinder at a time with the engine running and calculates the RPM drop. Once all cylinders are measured, a reading is given that will show which cylinders are working the hardest (have the most compression) and which are working the least (have the least compression). For the do it yourselfers, the easiest method is the manual compression test.
Q. What if the compression is too low or too high?
A. If the compression results are too high or too low you might consider consulting a professional technician. Modern vehicles are complicated to test and I have witnessed disastrous outcomes when a Do-It-Yourselfer has attempted this test. If however, you are fearless in your endeavor to diagnose the problem, then use the following techniques:
1. Consecutive low compression in all cylinders could mean that the problem of fuel washed cylinders exists. This means that the engine has had too much fuel introduced into it and all of the oil has been washed off the cylinder walls. The oil creates a sealing effect between the piston and ring assemblies and the cylinder walls of the engine block. Without this thin layer of oil, the engine compression would be allowed to escape into the crankcase. This is common with an engine that has a "flooding " problem. If the engine seems to run normally but is weak and puffs a small amount of bluish smoke, it could be an indicator of worn piston rings and cylinder walls. In either of these events, use a small oilcan and squirt a little oil into each cylinder, then repeat the compression test. If the compression dramatically increases then you have found the problem(s) listed above. If the compression readings do not change, then it would indicate a timing problem between the camshaft(s) and the
crankshaft of the engine. The timing chain or belt would need to be checked for proper "timing".
2. If you find the compression reading is very low or "0" in one cylinder, it is highly probable that internal engine damage exists such has:
a. The piston could have a broken connecting rod or a hole in it.
b. A valve could be stuck or leaking.
c. There could be a broken valve spring or a bent push rod.
d. The camshaft has excessive wear and is not opening the valve(s).
3. If the compression is low or "0" on two adjacent cylinders it would indicate:
a. There is a "blown" or weak sealing surface at the head to block mounting area, which basically means a bad head gasket.
b. Another possibility is that the camshaft is broken in an area that operates valves for two adjacent cylinders.
4. When the compression is found to be too high in one or more cylinders, this would be an indication of excessive carbon build up in the engine. This can only be corrected by performing a chemical de-carbonizing process on the engine (specialized service), or by removing the cylinder head(s) and physically removing the carbon that is attached to the cylinder portion of the head(s) and the tops of the pistons.
|
|
|
| |
