Checking Compression Pressure; Causes Of Low Cylinder Pressure - Hobart 6921 Series Operation And Maintenance Manual

OM-2040 / Operation and Maintenance Manual
90D20 / Generator Sets
If the cylinder has been misfiring, there will be no noticeable difference in the sound and
operation of the engine. If the cylinder has been firing properly there will be a noticeable
difference in the sound and operation when the plunger is held down. This is similar to
short-circuiting a spark plug in a gasoline engine.
(5) If cylinder No. 1 is firing properly, repeat the procedure on the other cylinders until the faulty one
has been located.
(6) Providing that the injector operating mechanism of the faulty cylinder is functioning satisfactorily,
remove the fuel injector and install a new injector.
c. Checking Compression Pressure
(1) Start engine and run until normal operating temperature is reached.
(2) With engine stopped, remove fuel pipes from the injector and fuel connectors of No. 1 cylinder.
(3) Remove the injector from No. 1 cylinder and install the proper adapter of the pressure gage (tool
J6692) in its place.
(4) Use one of the two fuel pipes as a jumper connection between the fuel inlet and return manifold
to permit fuel to flow directly to the return manifold. On some cylinder heads it is necessary to
fabricate a jumper connection from a fuel pipe.
(5) Start the engine and run at idle speed.
CAUTION
Do not crank the engine with cranking (starting) motor to obtain compression
pressure.
Record compression pressure indicated.
NOTE: A guide to minimum compression pressures to be expected is included in Sect. 2-3, Fig. 6.
(6) Remove pressure gage, install injector, and reinstall fuel lines.
(7) Repeat steps (2) through (6) for each cylinder.
d. Interpretation of Compression Pressure Records
Compression pressure in any one cylinder should not be less than specified in Sect. 2-3, Fig. 6.
Variation in compression pressures between cylinders must not exceed 25 PSI. A low compression
pressure in one cylinder indicates that something unusual has occurred and that a pressure leak has
developed.
e. Causes of Low Cylinder Pressure
Low cylinder pressures may result from any one of several causes:
(1) Piston rings may be stuck or broken. To determine the condition of the rings, remove the air box
cover and press on the compression rings with a blunt tool. A broken or stuck ring will not have a
spring-like action.
(2) Compression pressure may be leaking past the cylinder head gasket, valves and valve seals,
injector tube, or through a hole in the piston or combustion chamber.
To correct any of these conditions, consult your Detroit Diesel Service Outlet.
Chapter 3-1
Page 4
February 28/94 Revised