Diagnostic Aids

Perform this test when instructed during QUICK TEST or if directed by other test procedures. This test is intended to diagnose the following:

• Spark plugs.
• Spark plug wires.

1. 1) Check Possible Cause Of Misfire
   
   Possible causes of a misfire DTC are as follows:
   • Spark plugs.
   • Spark plug wires.
   • Faulty distributor.
   • Engine mechanical problem.
   • Incorrect firing order.

   Check firing order. Correct firing order if necessary. Restore all electrical connections and repeat QUICK TEST . If firing order is okay, go to next step.

2. 2) Check For Spark
   
   Connect a spark tester at end of spark plug wire and crank engine. If each spark plug wire produces a spark, go to next step. If no spark is produced, go to step  9).
3. 3) Check Secondary Display
   
   Connect an oscilloscope to engine. Start engine and run at 2000 RPM. Normal spark output voltage is about 25-31 k/volts. If spark output is not as specified, go to next step. If spark output voltage is okay, go to step  20).
4. 4) Check For High Firing Voltage
   
   If spark output is about 28,000 volts, but some cylinders vary more than 5500 volts, go to next step. If average spark output voltage is more than 28,000 volts, check spark plugs for wear or wide gap. Check coil wire installation and check distributor cap and rotor. Repair as necessary. Restore all electrical connections and repeat QUICK TEST .
5. 5) Check For Uneven Firing Voltage
   
   If spark output voltage variation is more than 5500 volts, check spark plugs for worn electrodes, ignition wires for high resistance, faulty fuel injector or engine mechanical problem. If spark output voltage variation is less than 5500 volts, go to next step.
6. 6) Check For Uneven Firing Voltage
   
   If spark output voltage is consistently high only in selected cylinder(s), check for defective spark plug or wire. Repair as necessary. If spark output voltage is low, go to next step.
7. 7) Check For Cylinder Misfire
   
   If spark output voltage is low or spark line slopes downward in some cylinders, check distributor cap for carbon tracking. Check spark plugs for narrow gap or fouling and spark plug wires for grounding. If spark output voltage is not as described, go to next step.
8. 8) Check For Reversed Voltage
   
   If oscilloscope displays an upside down firing pattern, replace distributor. Restore all electrical connections and repeat QUICK TEST . If firing pattern is not upside down, go to next step.
9. 9) Check Distributor
   
   Check distributor, cap and rotor for wear or damage. Ensure rotor turns without binding. Replace or service as necessary. If distributor, cap and rotor are okay, go to next step.
10. 10) Check Spark Plug Wire Resistance
    
    Remove spark plug wires. Measure resistance of each spark plug wire. Resistance should be 7000 ohms per foot. Replace wires that are not within specification. Restore all electrical connections and repeat QUICK TEST . If spark plug wires are okay, go to test HD.
    NOTE: A break in step numbering sequence occurs at this point. Procedure skips from step 10) to step 20). No test procedures have been omitted.
11. 20) Check Ignition Timing
    
    Start engine and run to operating temperature. Turn all electrical loads off. Connect a timing light to engine. Disconnect Throttle Position (TP) sensor. Ignition timing should be 13-15 degrees BTDC. Adjust timing if necessary. If timing is okay, go to next step.
12. 21) Check Idle Speed
    
    Start and run engine until it reaches operating temperature. Turn all electrical loads off. Connect NGS to fuse panel DLC. Idle speed should be 650-750 RPM. Idle speed is not adjustable. If idle speed is not within specification, check for contaminated fuel, vacuum leaks and incorrect ignition timing. Ensure throttle bore, EGR system and PCV system are clean. Repeat QUICK TEST . If idle speed is okay, go to TROUBLE SHOOTING - NO CODES - VILLAGER article.