Compression Test Kit
39224
ASSEMBLY AND OPERATING INSTRUCTIONS
3491 Mission Oaks Blvd., Camarillo, CA 93011
Copyright © 2004 by Harbor Freight Tools®. All rights reserved. No portion of this
manual or any artwork contained herein may be reproduced in any shape or form
without the express written consent of Harbor Freight Tools.
For technical questions and replacement parts, please call 1-800-444-3353
7. Use eye and ear protection. Always wear ANSI approved impact safety goggles
when working on gasoline engines. Be sure to work in a well ventilated place, or
pipe exhaust gasses to the outdoors. Carbon Monoxide Warning: Guard
against exposure to Carbon Monoxide, which is an odorless, colorless gas
produced by gasoline engines. Carbon Monoxide exposure may cause
serious injury or death.
8. Do not overreach. Keep proper footing and balance at all times. Do not reach
over or across running machines.
9. Maintain tools with care. Keep tester threads sharp and clean for better and
safer performance. Follow instructions for lubricating and changing accessories.
The gauge hose and adapters must be kept clean, dry, and free from oil and
grease at all times.
10. Stay alert. Watch what you are doing, use common sense. Do not operate any
tool when you are tired.
11. Check for damaged parts. Before using any tool, any part that appears damaged
should be carefully checked to determine that it will operate properly and perform
its intended function. Any part that is damaged should be properly repaired or
replaced by a qualified technician. Do not use the tester if it leaks or the pressure
release valve does not function properly.
12. Replacement parts and accessories. When servicing, use only identical
replacement parts. Use of any other parts will void the warranty.
13. Do not operate tool if under the influence of alcohol or drugs. Read warning
labels on prescriptions to determine if your judgment or reflexes are impaired
while taking drugs. If there is any doubt, do not operate the tool.
14. Maintenance. For your safety, service and maintenance should be performed
regularly by a qualified technician.
15. WARNING: Wear protective gloves and ANSI approved eye protection when
working on a hot engine.Keep hands away from the fan and other moving parts,
and protect yourself from electrical shock or burns. Be aware that working on a
gasoline engine is inherently dangerous, and suitable precautions must be taken.
Warning:The warnings, cautions, and instructions discussed in this instruction
manual cannot cover all possible conditions and situations that may occur. It
must be understood by the operator that common sense and caution are factors
which cannot be built into this product, but must be supplied by the operator.
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Unpacking
When unpacking, check to make sure the following parts are included.
Compression Gauge with Quick Release Chuck Molded Plastic Case
14”Vinyl Hose
Extension AluminumTube
ExtensionThread Head
Angled Extension AluminumTube
If any parts are missing or broken, please call Harbor Freight Tools at the number
on the cover of this manual.
Operation
A compression tester can give you valuable information about the engine’s top end
(pistons, rings, valves and gaskets).
If you are not experienced in doing this type of diagnostic test, the work should
be done by a qualified technician.
NOTE: The engine must be at normal operating temperature, and the battery must be
at full charge to develop correct readings. WARNING: Wear protective gloves and
ANSI approved eye protection when working on a hot engine.Keep hands away from
the fan and other moving parts, and protect yourself from electrical shock or burns.
Be aware that working on a gasoline engine is inherently dangerous, and suitable
precautions must be taken.
1. Clean the area around the spark plugs using compressed air. It is important to
prevent foreign materials from falling into the cylinders once the spark plugs are
removed.
2. On engines with a single plug per cylinder, remove all spark plugs. Note the
position of the wires so that the correct wire may be replaced on each plug at the
end of the job. On engines with two spark plugs per cylinder, remove only one
plug per cylinder.
3. Block the throttle linkage in the wide open position.
4. Disable the ignition system. On distributor ignitions, remove the coil wire from
the distributor.On electronic ignitions, remove the connector from the coil pack.
5. For fuel injected vehicles, or vehicles with an electric fuel pump, disable the fuel
pump circuit.This may easily be done by removing the fuse for the fuel pump.
6. Install the compression gauge in the number one spark plug hole.You may want
to use any of the supplied adapters.The gauge attaches and removes using a
quick-release chuck. Be sure the installation is secure and pressure tight.
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7. Crank the engine over several compression strokes and watch the gauge. On a
healthy engine, the compression will build up quickly. Low compression on the
first stroke followed by a slow build up of pressure indicates worn piston rings.
Low compression which does not build up indicates leaking valves, a blown
head gasket or cracked head. Carbon deposits on the valves can also cause
low pressure. Record the highest compression obtained.
8. Repeat this procedure for each of the remaining cylinders and record the results.
9. On a low compression engine, add approx. 1/4 oz. motor oil to each cylinder
through the spark plug hole and repeat the procedure.
10. On a low compression engine, if the compression increases after the oil has
been added, worn rings are definitely indicated. If the compression doesn’t
increase significantly, the leakage is occuring at the valves or head gasket (or
cracked head).
11. If compression is low in two adjacent cylinders, it is
likely that the cylinder head between them is leaking.
If coolant is found in the combustion chambers or
crankcase, this possibility is confirmed.
12. If the compression on one cylinder is 20% or so lower
than the others, and the engine has a rough idle, it
is likely that the valve is stuck or the camshaft has
a worn lobe.
13. If the compression is unusually high compared to
the engine specifications, it is likely that the cylinders
are carbonized. A qualified mechanic should remove
the engine head and remove the carbon.
14. If the compression varies greatly (over 15%) from
the highest compression cylinder to the lowest, or
the average compression is significantly lower than
the specification, the engine should be looked at by
a professional for a leak-down test or other service
to identify and repair the problem.
Quick
Release
Chuck
Pressure
Release
Valve
Maintenance
Very little maintenance of this tool is required.Keep this tool clean and free from dirt,
grease or grit. Store it in its case when not in use. Periodically check the threads for
damage. Release the pressure from the gauge before storing.
Note: Replacement parts are not available.
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