Page Number - 5
Form 822856
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-
eign matter that may have fallen into the cylinders during
preparation for test.
10. Select the appropriate test adapter and thread it into the
spark plug hole of cylinder 1 until the o-ring on the adapt-
er seats firmly (do not tighten with a wrench).
11. If required, thread a test adapter with a hose onto the in-
stalled adapter until the o-ring seats firmly (do not tighten
with a wrench).
12. Connect the test gauge to the opposite end of the test
adapter hose.
13. Turn the tester on by pressing the ON/UNIT/OFF but-
ton.
14. Press and hold the CYLINDER button for at least three
(3) seconds to clear the memory and return the cylinder
indicator to “CYL# 1”.
15. While watching the test gauge, crank the engine as least
five (5) compression strokes or until the pressure reading
stops increasing (the peak compression value will auto-
matically be stored in memory).
16. Relieve the pressure in the tester by pressing the pres-
sure relief button.
17. Disconnect the gauge and remove the test adapter(s)
from cylinder 1.
18. Install the test adapter(s) into cylinder 2 and reconnect
the gauge.
19. Advance the cylinder number shown on the tester LCD to
2 by pressing the CYLINDER button.
20. Repeat the compression test on cylinder 2 and all remain-
ing cylinders. Be sure to advance the cylinder number on
the tester for each cylinder.
21. After testing all cylinders, review the values of each cyl-
inder by pressing the CYLINDER button to scroll through
the results.
22. If any of the cylinder readings are found to be low or
uneven, perform wet compression test.
23. When compression test is complete, return the spark
plugs to their respective cylinders and the throttle and
ignition components to their normal positions.
Wet Compression Test Procedure
The wet compression test is a way to remove the influence
of worn piston rings, pistons and cylinders from the com-
pression test. After completing the dry compression test,
squirt approximately one teaspoon of engine oil into the
spark plug holes and crank the engine several times to seal
the piston rings. Repeat the dry compression test outlined
above.
NOTE:
If the readings during the wet compression
test are greater, then air is leaking around worn or dam-
aged piston rings. If the reading is approximately the same
for both wet and dry tests then the valves, valve lifters or
the camshaft lobes are worn. Any low reading of cylinder
compression indicates worn or damaged parts.
Small Engine Compression Testing
The
Mityvac
Digital Compression Test Kit has two (2)
adapters, 10mm and 12mm, that allow it to be used on
small engines such as those found on lawn mowers, chain
saws and line trimmers. To use these adapters:
1. Disconnect the spark plug wire and remove the spark
plug.
2. Thread the appropriate size adapter onto the compres-
sion tester main hose end until the o-ring seats (do not
use a wrench to tighten).
3. Thread the assembly into the cylinder spark plug hole
until the o-ring on the adapter seats firmly (do not use a
wrench to tighten).
4. Ensure the spark plug is grounded to the cylinder head
or is insulated in a way that will prevent an electric
shock hazard.
5. Crank the engine over using the pull cord (or starter if
electric) a minimum of 5 revolutions. This will allow the
cylinder to build sufficient compression to be measured.
6. Refer to the manufacturer’s data to determine the cor-
rect reading
Using the Pressure Hold Adapter
The
Mityvac
Professional Compression Tester comes with
a pressure hold adapter. This adapter is used to perform
valve seal replacement on an engine without removing the
cylinder head(s). To use the adapter:
1. Remove the spark plug from the desired cylinder.
2. Select the appropriate extension hose that will fit the
spark plug hole.
3. Using the valve core tool (from the field service kit in-
cluded), remove the valve core from the end of the hose
and lay it aside.
