Usage:
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.
- b. Does the poor performance always occur under the same conditions or is the problem intermittent (happens only occasionally)?
2. Check the crankcase oil level and the coolant level of the radiator. Remove the boost air line from the AFRC (air-fuel ratio control) during the Primary Engine Test. This will prevent the AFRC from becoming pnuematically activated during the checks. If activated, the AFRC could give an indication of a problem when there is none.
3. A slightly lower rpm (15 rpm below low limit) should be expected for the engine in vehicle than the rpm shown in the RACK SETTING INFORMATION. This is caused by the parasitic loads of the engine accessories involved.
4. 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 linkage 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 indicte a problem within the governor.
5. Only a mechanic with the correct training should change the high idle setting. The procedure is given in the Service Manual under the subject GOVERNOR ADJUSTMENTS.
6. If high idle rpm can not 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.
7. Before 8S4627 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.
8. With the continuity light installed, quickly push accelerator pedal all the way to the floor. If the fuel rack 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 in an indication that the mechanical movement of the governor and fuel injection pump parts operate properly. The vehicle will now have to be tested on a chassis dynamometer to determine the cause for low power.
If the light does not come on during free acceleration, the shutoff solenoid must be removed and the free acceleration test run again.
9. If the light comes on with the shutoff solenoid removed, the problem is with the solenoid or its adjustment. Make reference to the Testing and Adjusting section of this Service Manual for the adjustment procedure, or replace the shutoff solenoid.
If the light stays off, the AFRC (air-fuel ratio control) will have to be checked.
10. Remove the AFRC and install a cover in its place to shut off the pressure oil supply hole to the AFRC. Repeat the free acceleration test again.
If the continuity light now comes on, the problem is with the AFRC. Check for smooth operation of the control shaft, broken springs, or damaged diaphragm, or replace the AFRC.
11. If the light still does not come on with the AFRC removed, the governor circuitry should be checked. With the AFRC removed, the torque control group is now accessible. Attach clip end to brass terminal and touch probe against torque control group.
If test light stays off when governor circuitry is checked, the metal tab of the torque control group is not making contact with the brass terminal screw in the cover. The tab is probably bent and must be straightened to make contact with the brass terminal.
After the circuit has been repaired, repeat the free acceleration test steps up to this point again.
12. If the test light comes on, the circuitry is all right. This means that the rack 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 servo parts (ie. valve, piston, cylinder or sleeve). The fuel injection pump housing should be checked for worn fuel rack or its bushings, or a fuel injection pump installed with the rack gear in the wrong tooth position. The problem must be found and corrected.