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 |
| compromised. |
| 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 | |
| specification. | |
| 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 |
| motion. |
| 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 |
| chamber. |
| 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 |
