Low Power Troubleshooting - (Diagnosis with Chassis Dynamometer)
1. Preparation of vehicle for wheel horsepower test (consult dynamometer manufacturer's operating instructions for specific details on correct operation).
NOTE: Always perform the Primary Engine Test procedure before vehicle is installed on chassis dynamometer.
Calculate the allowable limits that the customer can expect from his engine and present these figures to him.
Caterpillar engines are rated with the conditions that follow:
Measure and record these variables.
a. Place vehicle on the chassis dynamometer. Tie vehicle in a way that will not add any load to the drive wheels. Do not pull wheels down into dynamometer drive rolls.
Check the radiator coolant level, crankcase oil level, tire pressure, tire condition, remove rocks from the tire tread and connect exhaust system.
Recapped tires should be run on a chassis dynamometer only at the customer's own risk.
b. Operate vehicle at 60% of rated speed with moderate load until oil and coolant temperatures reach their normal range for operation.
If there is a heavy vibration, drive shaft whip, tire bounce, etc., do not continue with dynamometer test until cause of the problem is corrected. Engines that have had new internal parts installed should be operated on a run-in schedule before operation at full load. For run-in schedule information, make reference to General Instructions section of this Service Manual.
2. Put transmission in direct gear and the differential in the highest speed ratio. Operate vehicle at maximum engine speed and increase chassis dynamometer load until a speed of 50 rpm less than rated speed is reached (continuity light should be on). Maintain this speed for one minute and record the engine speed and wheel horsepower.
NOTE: If horsepower is low and poor maintenance is suspected, remove air cleaner and check horesepower again to see if a plugged air cleaner could be the problem.
3a. If the wheel horsepower is correct, find the set point (balance point) of the engine (speed at which the load stop pin just touches the torque spring or stop bar). At this point the continuity light should flicker (go off and on dimly).
If the set point (balance point) is correct, then the low power complaint cannot be validated. No further test or repairs are necessary.
If the set point (balance point) is low, see Procedure No. 4.
3b. If the wheel horsepower is below the correct value, find the set point (balance point) of the engine (speed at which the load stop pin just touches the torque spring or stop bar). At this point the continuity light should flicker (go off and on dimly).
If the set point (balance point) is correct, see Procedure No. 5.
If the set point (balance point) is low, see Procedure No. 4.
4. If the set point (balance point) is low, the high idle will have to be increased to raise the set point (balance point) to the correct rpm (the point at which the continuity light just comes on).
NOTE: If the set point (balance point) is still low and high idle has been adjusted to maximum, disengage clutch while maximum throttle position is maintained. Now observe high idle rpm and, if lower than previously adjusted, check frame-to-engine-mount. A damaged or loose engine mount may put the linkage in a bind and thus prevent maximum governor position at load conditions.
5. If the set point (balance point) was correct and the wheel horsepower was low, install the 6V9450 Engine Pressure Group and do the wheel horsepower test again as shown in Procedure No. 2.
Check the air inlet restriction and exhaust back pressure.
Air flow through the air cleaner and piping must not have a vacuum restriction (negative pressure difference between atmospheric air and air that has gone through air cleaner) of more than 25 in. (635 mm) of water. Back pressure from the exhaust (pressure difference measurement between exhaust outlet elbow and atmospheric air) must not be more than 40 in. (1016 mm) of water.
Make a visual inspection of the air inlet system, and check for damage to piping, rags or obstructions in the inlet piping, or damage to the rain cap or the cap pushed too far on the inlet pipe. If no damage is found, check inlet restriction with a new air cleaner element.
If there is too much exhaust restriction, make a visual inspection of the exhaust piping and muffler. If no visual damage can be found, the exhaust piping may have to be removed and the exhaust restriction measured again to see if problem is corrected.
6. Measure the fuel transfer pump pressure (measured at plug on top of fuel pump housing) must not be less than 20 psi (140 kPa) at full load conditions. Normal fuel pressure at full load rpm is 30 ± 5 psi (205 ± 35 kPa). If fuel pressure is low or erratic, check for the conditions that follow in the order shown:
A. Air in the fuel system ... Disconnect the fuel return line at the tank. Place this end of the line in a container of fuel to see if air bubbles are present with the engine running. If air bubbles are observed, check for loose connections or other line leaks ahead of fuel transfer pump.
B. Restrictions ... Check for a plugged fuel filter or crimped (damaged) fuel lines.
C. Too Much Fuel Returned to Tank ... Block the fuel return line to stop all fuel returned to tank. Check fuel pressure again, and, if a significant pressure increase is observed, an incorrect constant bleed orifice may be the problem. Install the correct orifice.
Do not run the engine under load for more than 5 minutes with return line blocked.
D. Damaged Fuel Transfer Pump Parts ... Check bypass valve to see if dirt or other debris could make the valve stick. Clean or replace valve if damaged. If bypass valve is good, remove and disassemble fuel transfer pump to check for wear in the gears.
7. A very small amount of air in the fuel may not affect the transfer pump fuel pressure, but could affect the performance at the nozzles. The 2P8278 Flow Tube can be installed ahead of the fuel transfer pump. This will allow a visual check of any air in the fuel when the engine is running. Another method to check for air in the fuel is to put the fuel tank end of the fuel return line in a container of fuel.
8. Install the 6V7925 Fuel Rate Indicator according to Special Instruction SEHS8346. The specified fuel rate at full load is given in the FUEL SETTING AND RELATED INFORMATION FICHE.
Do the wheel horsepower test again as shown in Procedure No. 2, but this time record the fuel rate and rpm.
NOTE: This fuel system has a constant flow of fuel that is returned to the fuel tank. This orifice must be blocked before accurate fuel rate readings can be taken. Do not run the engine under load for more than 5 minutes with return line blocked.
9. If the fuel rate was too low, install 5P4203 Field Service Tool Group and check the fuel system setting. The correct fuel system setting is given in FUEL SETTING AND RELATED INFORMATION FICHE. If fuel system setting is not correct, the adjustment procedure is shown in the Testing and Adjusting section of this Service Manual under the subject FUEL SYSTEM SETTING.
If the fuel rate was low, and the fuel system setting is correct, check the items that follow in the sequence shown:
A. Remove nozzles and check to see if they are the correct nozzles for your engine or if they have been wire brushed. Either condition will affect performance.
B. Check for any restrictions to fuel flow in lines between fuel filter and nozzles. Any sharp bend in the lines could cause a restriction of fuel delivery.
C. Check governor for worn parts. A worn bushing or worn spring seat can change governor operation. Also check to see if the correct governor spring is installed.
D. Check fuel injection pumps for wear and for the correct calibration. Put the fuel injection pump group on the fuel injection test bench. Perform a fuel delivery test to determine if the correct amount of fuel is delivered by each pump.
10. Use the 8T5300 Timing Indicator Group to check the timing of the fuel injection pump camshaft to the engine camshaft. See Fuel System of the Testing and Adjusting section of this Service Manual for the subject CHECKING ENGINE TIMING AND AUTOMATIC TIMING ADVANCE UNIT WITH 8T5300 TIMING INDICATOR GROUP.
11. Use the 8T5300 Timing Indicator Group to check the automatic timing advance unit. Check to see that the advance is smooth and that the amount of advance is correct. See Fuel System of the Testing and Adjusting section of this Service Manual for the subject CHECKING ENGINE TIMING AND AUTOMATIC TIMING ADVANCE UNIT WITH 8T5300 TIMING INDICATOR GROUP.
If rpm, timing and fuel rate are found to be within specifications, the engine should produce acceptable power (unless engine has excessive internal friction, i.e., high crankcase oil level). When all the above conditions are within specifications, and the wheel horsepower is still lower than expected, some component of the vehicle must be absorbing too much power. Do not continue with engine troubleshooting, but locate and correct the problem with the vehicle. (Caterpillar is not responsible for problems with vehicle).