Primary Engine Test For Low Power
1. The customer must be asked many questions to determine whether his complaint is valid, or whether his diagnosis of an actual problem is correct.
Some of the questions that must be asked are as follows:
a. Does poor performance occur when the vehicle is operated at steady speed on a level road surface, or when vehicle is pulled up a grade?
A positive response to either or both of the above conditions would indicate a low power (steady state) problem. Begin with Low Power Diagnosis.
b. Does the poor performance always occur under the same conditions or is the problem intermittent (happens only occasionally)?
This is a very important line of questioning to pursue. Any constant performance problem can normally be identified and the problem corrected. If an intermittent problem exists, the mechanic must be aware that the condition is only occasional, and must run certain tests several times in an attempt to force the malfunction condition. If the condition is not duplicated, the diagnosis that no problem exists will be incorrect, and the vehicle operator will again be confronted with the problem somewhere out on the road.
c. Was the engine running rough or misfiring when the poor performance was noticed?
A positive response to this question will indicate the need to isolate the bad cylinder(s) and correct the problem. See section Cylinder Misfire.
2. Check the crankcase oil level and the coolant level of the radiator. Start the engine and get to normal operating temperature. A slightly lower rpm (15 rpm below low limit) should be expected for the engine in vehicle than the rpm shown in the FUEL SETTING AND RELATED INFORMATION FICHE. This is caused by the parasitic loads of the engine accessories involved.
3. With the engine running, the throttle must have enough travel for the governor control lever to break over (go past the normal governor stop for high idle position) a small amount when the throttle pedal is fully depressed. If full travel is not available, disconnect throttle linkage from governor lever. With throttle linkaged disconnected, full travel of governor lever will indicate linkage problems, and the linkage will have to be adjusted. Limited travel of the governor lever will indicate a problem within the governor.
4. Only a mechanic with the correct training should change the set point. The procedure is given in the Service Manual under the subject GOVERNOR ADJUSTMENTS.
5. If the set point cannot be made correct with the high idle adjustment screw, there is a problem inside the governor. Disassemble the governor and check for damaged parts or wrong parts installed in the governor. Some common problems are worn bushings, worn spring seat, or a broken or wrong governor spring.
6. Before 8T500 Circuit Tester is installed, be sure to test the light for correct operation. Test light must come on when the clip of the wire is placed against the probe of the light (replace batteries or bulb if light does not come on).
NOTE: If light comes on and stays on when attached to governor, then insulation is bad or installed wrong in torque spring group or brass terminal. This must be corrected before test is performed.
7. With the continuity light installed, quickly push accelerator pedal all the way to the floor. If the fuel control shaft and governor functions properly, the continuity light will come on during this free acceleration until high idle is maintained.
If the light comes on, this is an indication that the mechanical movement of the governor and fuel injection pump parts operate properly. The fuel setting should now be checked. If the light does not come on during free acceleration, check for a problem inside the governor.
8. Install 5P4203 Field Service Tool Group. Check the fuel ratio control and the fuel setting dimensions. Compare the results with the dimensions given in the Fuel Setting and Related Information Fiche.
If the fuel ratio control or the fuel settings are not correct, they must be adjusted to the settings given in the Fuel Setting and Related Information Fiche. Only a mechanic with the correct training should adjust the fuel ratio control and fuel settings. The procedure to check and adjust the fuel ratio control and the fuel setting is in this Service Manual under the subject Fuel Ratio Control Adjustment and Fuel Setting.
9. If the test light still does not come on, this means that the fuel control shaft is not moving due to a problem inside the governor or a problem inside fuel injection pump housing. The governor should be checked for worn parts. The fuel injection pump housing should be checked for a worn fuel control shaft or its bore, or a fuel injection pump installed with sleeve not engaged with the lever on the fuel control shaft. The problem must be found and corrected.