Glow Plug System: Testing and Inspection

Check for mechanical malfunctions such as incorrect idle speed or injection pump timing, or leaking nozzles or high pressure lines. If rough idle is still evident, refer to ``Glow Plug Resistance Check.''

GLOW PLUG RESISTANCE CHECK

1. Using multi-meter J-29125 (1982-83) or J-29125A (1984-85) set left selector switch to ``OHMS,'' right selector switch to 200 ohms and center slide switch to ``D.C. LO.'' If another ohmmeter is used, different values will result. Tools J-29125 and J-29125A were used in the development of this procedure. Their use is required if similar readings are to be obtained.
2. Start engine, allow it to reach normal operating temperature, then disconnect all feed wires from glow plugs. Turn heater to ``On'' position.
3. Disconnect alternator two wire connector.
4. Using tachometer J-26925, or equivalent, adjust idle speed screw to obtain worst engine idle roughness condition. Do not exceed 900 RPM.
5. Allow engine to run at worst idle speed for approximately one minute, then attach an alligator clip to black test lead of meter. Ground black test lead to fast idle solenoid (1983-85) or engine lift strap (1982).
6. Write down the engine firing order on a piece of paper, then with engine idling, probe each glow plug terminal and record the resistance values on each cylinder in the firing sequence. If vehicle is equipped with an electric cooling fan, record resistance values with cooling fan inoperative. Do not disconnect cooling fan circuitry. The resistance values are dependent on the temperature in each cylinder, and therefore, can indicate cylinder output.
7. If a resistance reading on any cylinder is 1.3-1.4 ohms for 1984-85 vehicles, or 1.2-1.3 for 1982-83 vehicles, check engine for a mechanical problem. Make a compression check of the lowest reading cylinder and the cylinders which fire before and after. Correct cause of low compression before proceeding to fuel system.
8. On 1984-85 vehicles, install glow plug luminosity probe, from tool J-33075, into cylinder with lowest resistance value. Observe combustion light flashes of probe. The flashes will usually be erratic and in sequence with the misfire. If not, move to the next lowest reading cylinder, until the misfire is found.
9. On all vehicles, observe the results of all glow plug resistance readings, looking for differences between cylinders. Rough engines will normally have a difference of .4 ohms or more between cylinders in the firing sequence. To correct rough engine idle, it will be necessary to raise or lower the resistance values on one or more of the offending cylinders by replacing the injection nozzles.
10. Remove nozzle from the cylinder(s) affecting idle performance. Determine the pop off pressure of the nozzle and check the nozzle for leakage and spray pattern. Refer to tool manufacturer for proper testing procedures. Install nozzles with higher pop off pressures to lower resistance values, and nozzles with lower pop off pressures to raise values. A change of 30 psi nozzle pressure will result in a .1 ohm difference in resistance. Use new nozzles on new vehicles and broken in nozzles on vehicles with 1500 or more miles, if possible. Whenever a nozzle is cleaned or replaced, crank the engine and watch for air bubbles at the nozzle inlet before connecting the injection pipe. If bubbles are evident, clean or replace the nozzle.
11. Connect injection pipe, restart engine and check idle quality. If idle quality is still not acceptable, repeat steps 6 through 10.
12. After making additional nozzle changes, check idle quality again.
13. If problem moves from cylinder to cylinder and resistance values do not change as nozzles are changed, the injection pump may be defective. Always recheck cylinders at same engine RPM. Sometimes cylinder readings may not indicate that an improvement has been made, even though the engine may idle better. A nozzle with a tip leak can allow more fuel than required into a cylinder, raising the glow plug resistance value. This will steal fuel from the next nozzle in the firing order and will result in that glow plug having a lower resistance value. If this is evident, remove and check the nozzle with the high reading. If it is leaking, it may be responsible for the rough idle. If low readings are evident on a glow plug and it does not change with a nozzle change, switch glow plugs between the good and bad cylinder. If the reading of each cylinder is not the same as before the switch, then the glow plug cannot be used for rough idle diagnosis.