The factors affecting wheel horsepower are listed in Table 12-1.
To begin low horsepower diagnosis, refer to section 12.2

Factors Considerations
DDEC Power Rating Is the correct power rating programmed into the ECM? Is the driver
aware of the effects that cruise power, or the lack of cruise power has
on perceived power?
Road Speed Setting Is the road speed setting causing a perceived lack of power?
Crank Case Overfilled If the crank case level is too high, there will be a loss of power due to
churning losses created by the crank shaft throws contacting the oil.
Fuel Temperature Make sure there is sufficient fuel supply (at least 1/3 of normal capacity)
in the fuel tanks. Check fuel temperature. For every 10°F increase in fuel
inlet temperature above 100°F, the engine will experience a power loss
of up to one percent.
Fuel Blend (specific gravity) Check the specific gravity of the fuel/vehicle system. A good number 2
diesel fuel has a specific gravity of 0.840 or higher @ 60°F. It should be
noted that No. 1 diesel fuel can reduce horsepower to 7% less than No.
2 fuel. Blends of No. 1 and No. 2 (common in winter) will produce less
horsepower, depending on the percent of the blend. This is a common
concern when dealing with low power complaints in cold climate locations.
Fuel Filter Restriction Check for fuel flow restrictions which can be caused by fuel heaters, water
separators, fuel flow meters, undersize or improperly routed/damaged fuel
lines, faulty check valves, contaminated fuel filters or high fuel pressure
resulting from a plugged restricted fitting or regulator valve. Replacing
the fuel filter is often the best recommendation in lieu of testing for the
filter condition.
Fuel Shut-Off Valve Position Make sure the fuel shut-off valve is fully open.
Fuel System Leak Fuel system leaks which result in aerated fuel are normally caused by a
leak at the connections and /or filters between the suction side of the fuel
pump to the supply tank and not between the pressure side of the pump
and engine.
Fuel Tank Vent Restriction A plugged fuel tank vent will create a vacuum in the tank and result in a
loss in fuel pressure at the injectors. This will reduce fuel delivery rate.
Air in Fuel Aerated fuel, caused by a fuel system leak, will result in reduced fuel
delivery and late injection timing.
Plugged or Cracked Fuel Tank Stand If the fuel tank stand pipe is plugged by a shop rag, fuel delivery will be
Pipe restricted. A cracked stand pipe will allow air to enter the fuel system and
reduce fuel flow and cause late injection timing.
Faulty Injector A faulty injector will limit fuel delivery and alter the combustion process
such that power is compromised.
Injector Codes Incorrect injector codes will limit fuel delivery.
Valve Lash Incorrect valve lash will alter the combustion process such that power is
Factors Considerations
Camshaft Timing Incorrect cam timing will alter the combustion process such that power
is compromised.
Air Flow Restriction Air flow must not be inhibited by plugged filter, or inadequate inlet air
duct shrouding.
Faulty Turbocharger A turbocharger that has wheel rubbing, oil leaks, bent blades, etc. will not
provide adequate air supply.
Temperature Controlled Fan A faulty thermo control will cause the fan to be locked on and drain power
on a continuous basis.
Air System Leaks (gaskets and seals) Air system leaks will result in insufficient air for optimum combustion.
Charge Air Cooler Leak Air system leaks will result in insufficient air for optimum combustion.
Climate (fresh air temp) The maximum allowable temperature rise from ambient air to engine inlet
is 30°F. Undersized or dirty air cleaner element, as well as damaged or
obstructed air inlet piping can cause low power. Make sure under-hood
hot air is not being taken in. Pressure drop across the air to air charge
cooler should be checked (3.0 in. Hg maximum from turbo discharge to
intake manifold). Check turbocharger boost pressure and compare to
Altitude Performance Site altitude has an effect on engine horsepower. Expect approximately 2%
loss in power when operating at an altitude of 1 mile, relative to sea level.
DDEC Parameter Settings Make sure vehicle settings such as: axle ratio, tire size, top gear ratio, etc.
are set correctly to avoid a false sense of engine performance.
EGR Valve A misadjusted or malfunctioning EGR valve will alter the amount of oxygen
available for combustion as well as introduce inert gas that does not
promote combustion.
Exhaust Restriction A damaged, undersized, or otherwise restricted muffler or exhaust system
can result in high exhaust back pressure. Refer to the engine specification
sheets for maximum allowable pressure.
Delta P Sensor The Delta P sensor, along with the exhaust temperature, determines
EGR flow rate. A faulty delta P sensor will cause the EGR system to
malfunction and alter the amount of oxygen available for combustion as
well as introduce inert gas that will not promote combustion.
Barometric Pressure Sensor The engine will transition between EGR and boost mode at an altitude of
6500 ft. Altitude is determined by the Barometric Pressure Sensor located
in the MCM. A faulty Barometric Pressure Sensor will compromise the
availability of boost pressure.
Air Compressor Leak An air compressor leak will cause the air compressor to work more and
increase the parasitic load on the engine.
Air Conditioner Leak An air conditioner leak will cause the air conditioner to work more and
increase the parasitic load on the engine.
Excessive Play in Power Steering Continuous movement of the steering wheel will call for continuous work by
System the power steering unit. This will increase the parasitic load on the engine.
Alternator Load Excessive use of vehicle electrical power will cause increased use of the
alternator. This will increase the parasitic load on the engine.
Tire Pressure Under inflated tires will significantly increase driveline resistance to rotation.
Factors Considerations
Trailer Aerodynamics/Alignment A trailer that has poor aerodynamics or has misaligned axles (causing dog
tailing) will significantly increase vehicle inertia and resistance to forward
Vehicle Payload As vehicle loading increases, vehicle inertia and resistance to forward
motion increases.
Winter-front Installation Improper installation or usage of a winter-front will result in extremely high
intake air temperatures and reduced mass flow of air into the combustion
Vehicle Application Unusual applications such as triple drive axles, PTO’s, pumps, high air
compressor duty cycle, etc., will have higher parasitic loses resulting in less
horsepower at the wheels.
Table 12-1 Factors Affecting Wheel Horsepower

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