Yanmar 4TNV98, Service Manual

Page: 238 / 246

4TNV98 & 4TNE98 Diesel Engine Section 8. Troubleshooting 236
Troubleshooting By Measuring
Compression Pressure
Compression pressure drop is one of the major
causes of increasing blow-by gas (engine oil
contamination or increased engine oil consumption
as a resultant phenomenon) or starting failure. The
compression pressure is affected by the following
factors:
1. Degree of clearance between the piston and
cylinder
2. Degree of clearance at the intake / exhaust valve
seat
3. Gas leak from the nozzle gasket or cylinder head
gasket
The pressure will drop due to increased parts wear.
Pressure drop reduces the durability of the engine.
A pressure drop may also be caused by a scratched
cylinder or piston, dust entrance from the dirty air
cleaner element or a worn or broken piston ring.
Measure the compression pressure to determine the
condition of the engine.
Compression Pressure Measurement
Method
1. Warm up the engine.
2. Stop the engine. Remove the high-pressure fuel
injection lines as an assembly from the engine.
Remove the fuel injector from the cylinder to be
measured. See Removal of Fuel Injectors on
page 147.
CAUTION
Remove or install the high-pressure fuel
injection lines as an assembly whenever
possible.
Disassembling the high-pressure fuel injection
lines from the retainers or bending any of the
fuel lines will make it difficult to reinstall the fuel
lines.
3. Turn off the fuel supply valve in the fuel supply
line. Disconnect the fuel injection pump stop
solenoid at the connector. This prevents the fuel
injection pump from injecting fuel during
4. Before installing the compression gauge (Figure
14-1, (1)) adapter, crank the engine with the stop
solenoid disconnected for a few seconds to clear
the cylinder of any residual fuel.
5. Install a nozzle seat at the tip end of the
compression gauge adapter. Install the
compression gauge and the compression gauge
adapter at the cylinder to be measured.
6. Crank the engine until the compression gauge
reading is stabilized.
Figure 14-1
7. After performing the compression check remove
the compression gauge and compression gauge
adapter from the cylinder. Reinstall the fuel
injector, high-pressure fuel injection lines, and
reconnect the stop solenoid.
See Reassembly of
Fuel Injectors on page 165.
8. Turn on the fuel supply valve and reconnect the
injection pump stop solenoid.
9.
Prime the fuel system. Check for leaks. Test the
engine. compression testing.