Page Number - 3
Form 824255
Always read instructions carefully prior to use.
Included in this Kit
• Heavy duty 2 ¾” (70 mm) gauge with high impact lens,
rubber boot, and three (3) scales of measure (psi,
kg/cm
2
, bar), attached to 10” (250 mm) hose and quick
release coupler
• Four (4) extension hoses:
• 6 ½” (165 mm) long x 14 mm thread, short
• 12” (305 mm) long x 14 mm thread, short
• 12” (305 mm) long x 14 mm thread, long
• 12” (305 mm) long x 18 mm thread, short
• One (1) adapter: 14 mm thread, short x 18 mm thread,
long
• Two (2) small engine adapters
• 10 mm thread
• 12 mm thread
• One (1) quick release air hold adapter
• Custom blow-molded case
• Field service kit
Applications
The Mityvac Professional Compression Test Kit can per-
form dry or wet compression tests to determine the amount
of pressure produced in the cylinder of an engine. It can
also be used as a tool for pressurizing an engine cylinder
to hold the valves closed while performing repairs.
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.
Always use eye protection when performing compres-
sion tests.
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 or more
pounds higher than the other cylinders, the probable cause
is carbon build-up inside the cylinder.
The
Mityvac
Professional 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, the
cylinders, and valve-train.
Dry Compression Test Procedures:
1. Refer to the appropriate service manual for the com-
pression specifications specific to the engine you are
testing.
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.
5. Remove spark plugs one at a time, marking the number
of the cylinder they were removed from, and place them
on a clean flat surface. This will aid you in identifying
problem cylinders by allowing the comparison of spark
plug appearance to the compression level of a given
cylinder.
NOTE: When testing engines with two spark plugs
per cylinder, it is only necessary to remove the
spark plugs located on the exhaust side.
6. On vehicles with standard distributors, disconnect the
coil wire (high tension lead) from the distributor cap and
secure it to a suitable ground, or disable the ignition by
disconnecting the positive (BAT) terminal from the igni-
tion coil.
7. On vehicles with a distributorless ignition (DIS), disable
the ignition system by removing the electronic ignition
(control) module fuse, or disconnect the crank angle
sensor.
NOTE: Refer to the appropriate service manual to
determine which fuse or component to temporarily
remove or disconnect.
8. Remove air cleaner from carburetor or throttle body and
secure throttle linkage in wide-open throttle (WOT) posi
-
tion.
NOTE: NEVER place anything inside the throttle
body; internal damage to the engine could result.
On vehicles equipped with port fuel injection,
remove throttle linkage covers (as necessary) and
secure throttle linkage in the wide-open throttle
(WOT) position.
9. Crank engine several times to ensure removal of any for-
