3406 DIESEL TRUCK ENGINE Caterpillar

Low Power Troubleshooting


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. Advertised engine bhp (less 3% manufacturing tolerance) can then be corrected to test conditions by use of procedure and correction tables found in Special Instruction GEG01024.

a. Place vehicle on the chassis dynamometer. Tie the 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.

NOTE: 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 or inlet piping to turbocharger and check horsepower again to see if a plugged air cleaner could be the problem.

3a. If the wheel horsepower is correct, find the balance point of the engine (speed at which the rack adjustment screw just touches the torque spring or stop bar). At this point the continuity light should flicker (go off and on dimly).

If the balance point is correct, then the low power complaint can not be validated. No further test or repairs are necessary.

If the balance point is low, see Procedure No. 5.

3b. If the wheel horsepower is below the correct value, find the balance point of the engine (speed at which the rack adjustment screw just touches the torque spring or stop bar). At this point the continuity light should flicker (go off and on dimly).

If the balance point is correct, see Procedure No. 6

If the balance point is low, see Procedure No. 4.

4. Stop the engine. Remove the AFRC (air-fuel ratio control). Put a cover over the hole where the AFRC was installed. Start the engine and check the balance point and horsepower again. If both of these are now correct, the problem is in AFRC. Repair or replace the AFRC.

If, with the AFRC removed, horsepower is now acceptable and balance point is low, the problem is still with AFRC. Repair or replace the AFRC. Then adjust balance point according to Procedure No. 5.

5. If the balance point is low, the high idle will have to be increased to raise the balance point to the correct rpm (the point at which the continuity light just comes on).

NOTE: If the 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.

6. If the balance point was correct and the wheel horsepower was low, install the 4S6553 Engine Test Group and do the wheel horsepower test again as shown in Procedure No. 2. At full load rpm, measure the boost and the fuel pressure. See the RACK SETTING INFORMATION to find the correct boost pressure for a particular engine (since the engine is in vehicle, be sure to make reference to the General Notes in the RACK SETTING INFORMATION to determine the correct boost pressure with air cleaner and muffler installed). If boost is low, check connections ahead of turbine side of turbocharger for exhaust leaks and connections after compressor side of turbocharger for inlet air leaks.

7. 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.

8. The fuel pressure recorded in Procedure No. 6 (measured at plug on fuel filter base) 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.

9. 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.

10. Install the 5P2150 Horsepower Meter according to Special Instruction SEHS7050. The specified fuel rate at full load is given in the RACK SETTING INFORMATION.

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.

11. If the fuel rate was too low, install 9S240 Rack Position Tool Group and check the static rack setting. The correct rack setting is given in RACK SETTING INFORMATION. If rack setting is not correct, the adjustment procedure is shown in Testing and Adjusting section of this Service Manual under subject FUEL RACK SETTING.

If the fuel rate was low and the rack 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 servo parts (valve, piston, cylinder or sleeve). A worn bushing or worn spring seat can change governor operation. Also check to see if the correct governor spring is installed.

12. Check the timing of the fuel injection pump camshaft to the engine flywheel by use of 8S2291 Timing Pin. If timing pin does not go into camshaft notch when flywheel timing bolt is installed, see Fuel System of the Testing and Adjusting section of this Service Manual for the subject CAMSHAFT TIMING FOR THE FUEL INJECTION PUMP.

13. Install the 2P8280 or 1P3500 Injection Timing Group to check the automatic timing advance unit.

NOTE: Do not do this test with Chassis Dynamometer load on the engine. The transducers are designed for engine no-load conditions.

Check to see if advance is smooth and constant through the proper speed range. For complete procedure and correct amount of advance for a particular engine, see Special Instruction SMHS6964.

14. If at this point incorrect timing is still suspect, the flow timing method should be used since it is the most accurate method of timing. See Fuel System in the Testing and Adjusting section of this Service Manual for the subject FLOW CHECK FUEL INJECTION PUMP TIMING.

15. Measure the boost again as detailed in Procedure No. 6.

If rpm, timing, fuel rate, and boost are found to be within specifications, the engine should produce acceptable power (unless engine has excessive internal friction, ie. high crankcase oil leve). 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).

16. If boost is still low, check inlet manifold temperature. If temperature is too high, check the aftercooler for possible restrictions to the coolant. This condition reduces the amount of air in cylinder for combustion (since warmer air is less dense). If manifold temperature is correct, check the turbocharger for mechanical problems (ie., wrong nozzles, worn bearings).

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