Note: although this is written primarily for 4cycle gas engine testing, 2cycle gas and diesel follow pretty much the same sequence of procedures.   Also, many diesel engine companies do not publish specific compression readings so the values, in the chart, at the end of this section will be too low for use with a diesel, you'll have to do the calculation.

1. Be sure the crankcase is at the proper level and the battery is properly charged. Operate the engine for a minimum of 30 minutes at 1200 rpm, make sure that the choke has opened, and the engine is at normal operating temperature. Putting a load on the engine for at least 10 minutes, can increase the accuracy of your test. Turn the ignition switch off; then remove all the spark plugs (injectors on the diesel). If the engine has closed cooling, (green anti-freeze) relieve pressure in the cooling system. (carefully)

2. Set the throttle plates and choke plate in the wide open position. On 4 cycle gas engines: Try not to pump raw fuel into the intake with the accelerator pump.

3. Install compression gauge in No. 1 cylinder.

4. Install an auxiliary starter switch in the starting circuit. Ground the coil wire on gas engines. Next, using the auxiliary starter switch, crank the engine (with the ignition switch off) at least five compression strokes and record the highest reading.

Count the approximate number of compression strokes required to obtain the highest reading.

5. Repeat the test on each cylinder as was required to obtain the highest reading on the No. 1 cylinder.

If the readings degrade from front to rear, re-check the battery and possibly re-run the test starting at cylinder #1. In this way you can have a set of two tests to compare an average.

TEST CONCLUSION

2 Cycle Engines:
Your test results need to be within 15 psi range to be considered an engine worth using. For example; a 3 cylinder with readings of 130, 135, and 120 would be considered a good engine. Some might argue that there should be no more than 5psi difference between high and low cylinders. But, obviously, the closer the readings are to each other, the better.

4 Cycle Engines:
The indicated compression pressures are considered normal if the lowest reading cylinder is within 75% of the highest. Refer to the following example and chart.

Seventy-five percent of 140, the highest cylinder reading is 105. Therefore, cylinder No. 7 being less than 75% of cylinder No. 3 indicates an improperly seated valve or worn or broken piston rings.

If one, or more, cylinders read low, squirt approximately one (1) tablespoon of engine oil on top of the pistons in the low reading cylinders. Repeat compression pressure check on these cylinders.

1. If compression improves considerably, the piston rings are at fault.

2. If compression does not improve, valves are sticking or seating poorly.

3. If two adjacent cylinders indicate low compression pressures and squirting oil on the pistons does not increase the compression, the cause may be a cylinder head gasket leak between the cylinders. Engine oil and/or coolant in the cylinders could result from this problem.

Note: An engine compression test is easy to do but, your results need to be tempered with the knowledge of testing conditions that reduce the accuracy of your test. As the battery gets weaker, starter gets hotter, and the engine cools down, your results from any one cylinder will change. It is important to do the test a quickly as possible, with as little cranking as needed to complete the test.
Circumstances arise where a more accurate leak down test should be utilized before you start tearing the engine down. Readings that are at the extremes of the chart, (meaning over-all very high or very low readings), may indicate conditions inside the cylinders that a compression test won't depict properly.

You can do the math yourself  but, the following quick reference chart should be used when checking cylinder compression pressures. ( to avoid mathematical errors) The chart (below) has been calculated so that the lowest reading number is 75% of the highest reading.

Special note: Although the "actual" compression readings are not all that important, keep in mind that very few engines will run with a compression under 100 psi. However, there are some engines that will test with readings in the 90 psi range on a regular basis. If your not sure if you have one of these engines, contact your servicing dealer before you get excited.

Although 75% range is tolerable for an older motor, keep in mind that most modern engines are designed at much closer tolerances. Also, on a V8 engine, if  7 cylinders test at 150psi and only one tests at 113psi, this is not a good sign...

Example: 4 Cycle Engine

After checking the compression pressures in all cylinders, it was found that the highest reading obtained was 196 psi. The lowest pressure reading was 155 psi. The engine is within specifications and the compression is considered satisfactory.