Using a Compression Guage 

Description How to Perform a Compression Test, and Understand the Readings
Author elcamino121 Date Tue Jan 23, 2007 6:25 pm Type Tech Article
Category Documents
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Compression Tester

Compression testing can reveal many internal engine problems, very quickly. Generally speaking, the biggest indicator of a problem is a low reading on 1 or 2 cylinders. Low readings can be the result of a blown head gasket, burned or leaky valves or guides, cracked head, or even a cracked block
Compression Testing Procedure

1. Warm engine to operating temperature.

2. Remove all spark plugs and gaskets. Ground all ignition leads. The battery and starter motor must be in good condition, so the engine can be cranked at equal speeds for all cylinders.

3. Attach compression tester to one spark plug hole.

4. Open throttle fully, crank engine for 3-4 revolutions, or until no increase in pressure on the gauge is noted. If the throttle is not open fully, there may not be enough air passed through the engine to allow accurate gauge readings.

5. Record the reading. Repeat for each cylinder.

6. The compression readings should not vary more than 10% between any 2 cylinders.

7. Compare the individual cylinder readings to the manufacturers specifications.

Compression Test Analysis

Compression specifications are of importance in determining the general condition of an engine. The specification set by the manufacturer is an average compression reading taken on a great number of engines of the same model and make. The pressures for individual engines of the same model vary from one engine to another, even when they are tested under identical conditions. Additionally, in a garage, there may be a great number of variables, that can enter into the picture; the cranking ability of the starting motor, the condition of the battery, the stiffness of the engine, the mileage on the engine, the temperature and viscosity of the oil, all which can affect cranking speed and thus affect the reading on the compression gauge. Lastly, if compression trouble exists in the engine, it will nearly always show up as an unbalance between the cylinders.

The actual specification reading in lbs. is not as important as the difference in readings between cylinders. On high compression engines (150 lbs. and higher) the pressure should not vary more than 15 lbs. from the highest to the lowest cylinder. On low compression engines (under 150 lbs.) the pressure should not vary more than 10 lbs. from the highest to the lowest cylinder. An engine with a difference greater than these guidelines, is considered to be unbalanced, and will run poorly.

It is possible for the compression pressures to be high in all cylinders and yet all cylinders to be uniform. This is because carbon can accumulate quite evenly in all cylinders. Compression pressures are considered to be too high when it is impossible to remove the “ping” from an engine by adjusting the timing or by using a higher-octane fuel. When an engine “pings”, damage is done to the bearings and there is a marked loss of economy because power is delivered to the piston before it reaches the top of its stroke. On high Compression Engines, a carbon coating of only .015” thick can raise the compression high enough to cause uncontrolled pinging.

Engine Unbalance

An engine is unbalanced even when only one or two cylinders are considerably higher or considerably lower than the others. A low cylinder can be caused by leakage through the head gasket, through a poorly seating valve, or through piston rings, which are worn and do not fit the cylinder walls. Leakage through the head gasket to the outside of the engine can be recognized by a “PFFFT” sound when running the engine. If the head gasket is blown between two cylinders, the compression pressure will be low in the two adjacent cylinders and there may be water in the cylinders or in the crankcase oil if the engine is water-cooled.

To determine whether a low cylinder pressure reading is due to a poorly seating valve or due to leakage past the piston rings, pour a small quantity of motor oil into the spark plug hole and retest. The oil will create a temporary seal between the piston rings and the cylinder wall. Any increase in pressure over the first test reading will indicate a better sealing of the piston rings. If there is no increase in pressure, or if it is very slight, then the leakage is past a poorly seating valve.

When one or two cylinders are considerably higher than the rest, it is nearly always caused from the accumulation of carbon on the top of the piston and on the underside of the cylinder head.

User comments 
mark75 Posted: Tue Jan 23, 2007 7:36 pm    Post subject

Great article. Now I know how to do it right.

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