Test 41 - Check Engine / Cylinder Leak Down Test / Compression Test - Generac Power Systems GP Series Diagnostic And Repair Manual

Test 41 – Check Engine / Cylinder
Leak Down Test / Compression Test
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.
The cylinder leak down
0F77000SRV) 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
Remove a spark plug.
1.
Gain access to the flywheel.
2.
Remove the valve cover.
3.
Rotate the engine crankshaft until the piston
4.
reaches top dead center (TDC). Both valves
should be closed.
Lock the flywheel at top dead center.
5.
Attach cylinder leak down tester adapter to spark
6.
plug hole.
Connect an air source of at least 90 psi to the leak
7.
down tester.
Adjust the regulated pressure on the gauge to 80
8.
psi.
Read the right hand gauge on the tester for
9.
cylinder pressure. 20 percent leakage is normally
acceptable.
NOTE: Use good judgment, and listen for air escaping at
the carburetor, the exhaust, and the crankcase breather.
This will determine where the fault lies.
Repeat steps 1 through 9 on remaining cylinder.
10.
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
gasket should be replaced.
Diagnostic Repair Manual
tester (Generac P/N
Check Compression
Lost or reduced engine compression can result in failure
of the engine to start, or rough operation. One or more of
the following will usually cause loss of compression:
• Blown or leaking cylinder head gasket.
• Improperly seated or sticking-valves.
• Worn piston rings or cylinder. (This will also result
in high oil consumption).
Procedure
Remove both spark plugs.
1.
Insert a compression gauge into either cylinder.
2.
Crank the engine until there is no further increase
3.
in pressure.
Record the highest reading obtained.
4.
Repeat the procedure for the remaining cylinder
5.
and record the highest reading.
Results
Normal compression is approximately 150 psi. The
difference in pressure between the two cylinders should
not exceed 25 percent. If the difference is greater than 25
percent, loss of compression in the lowest reading
cylinder is indicated.
Example 1: If the pressure reading of cylinder 1 is 165
psi and of cylinder 2, 160 psi, the difference is 5 psi.
Divide "5" by the highest reading (165) to obtain the
percentage of 3.0 percent.
Example 2: Cylinder 1 reads 160 psi; cylinder 2 reads
100 psi. The difference is 60 psi. Divide "60" by "160" to
obtain "37.5" percent. Loss of compression in cylinder 2
is indicated.
If compression is poor, look for one or more of the
following 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 Generac P/N
0F6923 for further engine service information.
Section 4 AC Diagnostic Tests
57