Check Engine / Cylinder Leak Down Test / Compression Test; Check Oil Pressure Switch - Guardian 4700 Repair Manual

Section 7
DIAGNOSTIC TESTS
TEST 38 - CHECK ENGINE / CYLINDER LEAK
DOWN TEST / COMPRESSION TEST
GENERAL:
Most engine problems may be classified as one or a
combination of the following:
❏ Will not start.
❏ Starts hard.
❏ Lack of power.
❏ Runs rough.
❏ Vibration.
❏ Overheating.
❏ High oil consumption.
DISCUSSION:
The Cylinder Leak Down Tester checks the sealing
(compression) ability of the engine by measuring air
leakage from the combustion chamber. Compression
loss can present many different symptoms. This test
is designed to detect the section of the engine where
the fault lies before disassembling the engine.
PROCEDURE:
1. Remove the spark plug.
2. Gain access to the flywheel. Remove the valve cover.
3. Rotate the engine crankshaft until the piston reaches top dead
center (TDC). Both valves should be closed.
4. Lock the flywheel at top dead center.
5. Attach cylinder leak down tester adapter to spark plug hole.
6. Connect an air source of at least 90 psi to the leak down tester.
7. Adjust the regulated pressure on the gauge to 80 psi.
8. Read the right hand gauge on the tester for cylinder pressure. 20
percent leakage is normally acceptable. Use good judgement,
and listen for air escaping at the carburetor, the exhaust, and the
crankcase breather. This will determine where the fault lies.
RESULTS:
❏ Air escapes at the carburetor – check intake valve.
❏ Air escapes through the exhaust – check exhaust
valve.
❏ Air escapes through the breather – check piston
rings.
❏ Air escapes from the cylinder head – the head gas-
ket should be replaced.
CHECK COMPRESSION:
To check engine compression, remove the spark
plug. Insert an automotive type compression gauge
into the spark plug hole. Crank the engine until there
is no further increase in pressure. The highest read-
ing obtained is the engine compression pressure.
Page 50
MINIMUM ALLOWABLE COMPRESSION PRESSURE
COLD ENGINE – GH-220
If compression is poor, look for one or more of the fol-
lowing causes:
❏ Loose cylinder head bolts.
❏ Failed cylinder head gasket.
❏ Burned valves or valve seats.
❏ Insufficient valve clearance.
❏ Warped cylinder head.
❏ Warped valve stem.
❏ Worn or broken piston ring(s).
❏ Worn or damaged cylinder bore.
❏ Broken connecting rod.
❏ Worn valve seats or valves.
❏ Worn valve guides.
NOTE: Refer to Engine Service manual No.
0C1103 for further engine service information.
TEST 39 - CHECK OIL PRESSURE SWITCH
DISCUSSION:
Also see "Operational Analysis" on Page 16. The Low
oil Pressure Switch is normally-closed, but is held
open by engine oil pressure during cranking and
startup. Should oil pressure drop below a safe level,
the switch contacts will close to ground the Wire 85
circuit. Engine controller board action will then initiate
an automatic shutdown.
If the switch fails closed, the engine will crank and
start, but will then shut down after a few seconds.
If the switch fails open, low oil pressure will not result
in automatic shutdown.
Figure 7-32. – Oil Pressure Switch
60 psi