Use the driver questionnaire, the troubleshooting tree, and the low power troubleshooting chart to determine if low horsepower is the problem.
The factors affecting wheel horsepower are listed in the following table.
| Factors | Considerations |
| Throttle Position | Using DDDL 7.0, verify that you have 100% throttle position. |
| DDEC Power Rating | Is the correct power rating programmed into the MCM? 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 and could have a possible Amber Warning Light and derate situation. |
| Fuel Blend (specific gravity) | Check the specific gravity in the fuel/vehicle system. A typical D2 ULSD fuel has a specific gravity raging from 0.830 to 0.85 @ 60°F”. It should be noted that No. 1 diesel fuel can reduce horsepower to 3.5 to 4 % HP 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 undersize or improperly routed/damaged fuel lines, faulty check valves, or contaminated fuel filters. Check the restriction gauge or fuel pressure to determine 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 Pipe | If the fuel tank stand pipe is plugged by a shop rag or other debris, fuel delivery will be 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. |
| 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) or/and Charge Air System 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 5280 feet (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 ATS 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 reduce horsepower as a function of altitude to protect critical engine components. Power reduction starts at approximately 8700 ft on power ratings greater than 500 hp. Altitude is determined by barometric pressure. A faulty Barometric Pressure Sensor could start a premature power reduction. |
| 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 System | Continuous movement of the steering wheel will call for continuous work by 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. |
| 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. |
| Exhaust Leaks | These leaks will cause low power and black smoke. |
| 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. |
| APT Failure | Damage to turbine causing low power |
Table 1. Factors Affecting Wheel Horsepower
