TRUCK SERVICE MANUAL
turbochargers, since this varies automatically with speed and
5. Air restriction readings may be taken at air cleaner
load of engine. If turbocharger runs too fast, it indicates the
outlet connection plug. If reading is taken at air cleaner,
engine is being overfueled.
Overfueling may cause
restriction must not exceed 20 inches [508 mm] of water.
Note: Engines with a properly operating restriction gauge
mounted in the air cleaner outlet need not be checked in the
If noticeable vibration develops in turbocharger, shut down
preceding manner Refer to applicable Engine Maintenance
engine and find cause. Vibration might be caused by damage
Manual for further details.
Exhaust Back Pressure
Inlet Air Restriction
Exhaust back pressure is a significant indication of exhaust
If inlet air restriction exceeds 25 inches [635 mm] of water
system conditions and piping layout. High pressure indicates
under conditions stated below, air flow to cylinders will not be
restrictions caused by foreign objects or excessive bends in
sufficient and a loss of power will occur. Excessive exhaust
exhaust piping or use of piping smaller than exhaust outlet of
smoke and excessive exhaust temperatures will also
turbocharger. The lowest pressure obtainable is desired.
accompany loss of power.
If exhaust back pressure exceeds 2 inches [50.8 mm] of
1. Check inlet air restriction by attaching a vacuum
mercury, early engine failure and poor performance may be
gauge or water manometer in air intake piping. Adapter must
expected. To check exhaust back pressure,
be perpendicular to air flow and one pipe diameter upstream
from turbocharger in a straight section of tubing. Fig. 104.
1. Point of measurement must be as close as possible
to turbocharger outlet flange in an area of uniform flow such as
a straight section of pipe at least one pipe diameter from any
changes in flow area or flow direction. Where it is impossible
to locate point of measurement in a straight section, it is
permissible to measure on side of a bend where flow is
uniform and equivalent to flow along centerline. Do not
measure on inside or outside radius of bend as flow is not
uniform at these points. Fig. 105.
Fig. 104, TA2. Checking air inlet restriction
2. Operate engine until normal operating temperature is
3. Operate engine at rated speed and full-load; take
reading from vacuum gauge or manometer.
4. If air restriction exceeds 25 inches [635 mm] of water:
Fig. 105, TA3. Checking exhaust back pressure
a. Clean or replace dry-type cleaner element.
2. At point selected, weld a 1/8 inch [3.18 mm] pipe
b. Replace damage air piping, rain shield or housing.
coupling to exhaust tubing. Drill through tubing with a 1/8 inch
drill. Remove all burrs. Mount 90 deg. Weather head fitting
c. Remove excessive
to coupling, then use 3 ft. [0.9 m] of 1/8 inch I.D. copper
restriction in air piping.
tubing (to resist heat) plus 10 ft. [3 m] of 3/16 inch [4.8 mm]
I.D. soft rubber hose to manometer. The manometer may be