ENGINE
TRUCK SERVICE MANUAL
Fig. 2-55, (N114215) Adjusting valves
Check Exhaust Back Pressure
1. When engine pistons must act against a back
pressure in exhaust system to expel exhaust gas; usable
output of engine is lowered; since air-fuel ratio will be
Fig. 2-63, (N10146) Vibration damper alignment marks
reduced because of incomplete scavenging of cylinder,
fuel economy is reduced and exhaust temperatures will
There should be no indication of relative rotation
increase. Although turbocharged engines are affected to
between hub and inertia member. Check for extrusion of
a lesser degree than naturally aspirated engines due to
rubber particles between hub and inertia member.
positive pressure in intake manifold, it is essential
exhaust system for all engines be designed to offer least
Check Fuel Manifold Pressure
possible restriction to exhaust flow.
Note: Air pressure on AFC Fuel Pumps can only be
2. High pressure indicates restriction caused by foreign
checked by Cummins Distributor.
objects, excessive bends or small size of piping. The
Assurance of correct governed speed is necessary
lowest pressure obtainable is desired.
before any other fuel pump checks are attempted. Use
3. If exhaust back pressure exceeds value listed below,
an accurate tachometer or revolution counter. Use of a
early engine failure and poor performance may be
dynamometer makes determining rated speed easy. If
expected.
no dynamometer is used, take a reading of the no-load
4. Check exhaust restrictions (back-pressure) as
maximum speed. Allow 10% above the rated speed as a
follows:
maximum governed speed. Example: 2100 rpm rated,
a. Using a mercury or water manometer, take readings
2310 rpm maximum.
when engine is developing maximum horsepower at
There may be some variation In maximum governed
maximum engine speed.
speed from various causes.
b. Maximum permissible back-pressure is 3.0 inch
1. Fan load.
[76.2 mm] Hg or 40.8 inches [103.6 cm] of water.
2. Engine temperature.
Cummins Distributors are equipped with special tools to
3. Air compressor pumping.
check exhaust back pressure. If back pressure is too
4. Alternator carrying high charging rate.
high check entire exhaust system for restrictions.
5. Any auxiliary load such as power-steering pump,
air-conditioning compressor, etc.
Inspect Water Pump, Idler Pulley And Fan Hub
Inspect water pump, idler pulley and fan hub for wobble
and evidence of grease leakage. Replace with rebuilt
prelubricated units as necessary.
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