Page Number - 4
Form 822856
Always read instructions carefully prior to use.
Safety Information
• Read and understand all safety precautions and operat-
ing instructions contained in this manual.
• Always wear eye protection when performing a compres-
sion test.
• Do not exceed the maximum rating of the tester; 300 psi
(20 bar) (2,000 kPa)
• Prior to testing, set the gearbox to neutral on all vehicles
or machinery having a standard transmission, or park for
automatic transmissions.
• Prior to engaging ignition, ensure that the fuel supply is
disabled using manufacturer’s recommended method.
• Make sure that all connections are secure prior to test-
ing.
• Take precautions to avoid contact with hot engine sur-
faces.
Included in this Kit
• Digital Compression Tester comprising of test gauge with
rubber boot, 10” (250 mm) long, high pressure hose, and
female quick release coupler.
• (4) - Extended hose compression test adapters:
o 14 mm standard reach thread x 6.5” (165 mm) long
o 14 mm standard reach thread x 12” (305 mm) long
o 14 mm long reach thread x 12” (305 mm) long
o 18 mm standard reach thread x 12” (305 mm) long
• (4) - Plug style compression test adapters:
o 18 mm long reach male thread x 14 mm standard
reach female thread
o 16 mm male thread for Ford Triton engines x 14 mm
standard reach female thread
o 10 mm male thread x 14 mm standard reach female
thread
o 12 mm male thread x 14 mm standard reach female
thread
• Air Hold Adapter
• Custom blow-molded case
• Field service kit
Functional Overview
The Mityvac Digital Compression Test Kit can perform wet
and/or dry compression tests to determine the pressure
produced in the cylinders of a gasoline engine. It can
measure, display, and store compression values of up to
300 psi (20 bar) (2,000 kPa) for up to 12 cylinders. It can
also be used as a tool for pressurizing an engine cylinder
to hold the valves closed while performing repairs.
When using this tester to perform a compression test on a
cylinder, the current peak compression value is displayed
in large numbers at the center of the LCD. This value
is automatically transferred to memory, and displayed in
smaller numbers at the bottom right of the display. When
the pressure relief button is depressed, the current com-
pression reading returns to zero, but the peak value is
retained in memory and continues to be displayed in the
bottom right corner of the LCD. Repeated tests can be
run on the same cylinder, and the new current peak value
will be displayed by the large numbers. This new peak
value will not be retained in memory unless it surpasses
the value of the previous test, or the CLEAR button was
pressed to erase the old value from memory.
Peak compression values can be stored for up to 12 cylin-
ders. Each is displayed on the LCD along with the cylin-
der number. During and/or after completion of testing all
cylinders, the peak values can be reviewed by pressing the
CYLINDER button to scroll through the memory. Individual
peak values can be erased from memory by pressing the
CLEAR button when the appropriate cylinder and value are
displayed. Pressing the CLEAR button for 3 seconds at
any time will clear the memory for all cylinders and return
the display to cylinder 1.
How to Perform a Comression Test
Precautions & Diagnostic Notes:
WARNING: DO NOT use the ignition switch during the
compression test on fuel-injected vehicles. Use of a
remote starter switch to crank the engine is recom-
mended. Fuel injectors on many late model vehicles
are triggered by the ignition switch during the cranking
mode, this could result in a fire hazard or contamina-
tion of the engine’s oil with fuel.
An engine in good operating condition will produce a
certain amount of pressure in each cylinder. Normally,
the cylinders should be within 10 percentage points of
one another and within the manufacturer’s specifications.
The pressure should rise smoothly on each stroke of the
engine, until it reaches a peak.
If the pressure reading fails to rise, or it remains the same
for several strokes of the engine and begins to rise, the
likely cause of the problem is a sticking valve.
If two adjacent cylinders show pressure readings of 20 or
more pounds below the other cylinder readings, suspect a
blown head gasket.
If a cylinder shows a pressure reading of 15 PSI (1 bar)
(100 kPa) or more pounds higher than the other cylinders,
the probable cause is carbon build-up inside the cylinder.
The
Mityvac
Digital Compression Test Kit can perform two
tests: the dry compression test and the wet compression
test. The result of performing these tests will provide an
indication of the condition of the piston rings, cylinders, and
valve-train.
Dry Compression Test Procedures:
1. Refer to the appropriate service manual for the compres-
sion specifications specific to the engine you are test-
ing.
2. Start engine and allow engine to run until it reaches nor-
mal operating temperature (usually about 15 minutes.)
Turn engine OFF.
3. Install an auxiliary starter switch in the starting circuit.
4. While wearing eye protection, use compressed air to
carefully remove dirt and debris from the area around
the spark plugs.
