Test JB: Ignition Misfire Under Load: Testing

1) Check Firing Order

Ensure firing order is 1-2-3-4-5-6. Correct firing order if necessary. Reconnect all electrical connections and repeat QUICK TEST . If firing order is okay, go to next step.

2) Check For Spark At Plugs

Connect a spark tester at end of spark plug wire and crank engine. Repeat for all spark plugs. If each spark plug wire produces spark and engine starts, go to next step. If each spark plug wire produces a spark and engine is a no-start, go to TEST HC , step 1). If no spark is produced, go to step 9).

NOTE: If this portion of testing is to provide accurate results, it is essential that calibration of engine analyzer be maintained. Refer to equipment manufacturer's operating manual for procedure to calibrate engine analyzer. If this is not available, an estimate of calibration can be made by connecting spark tester to a properly operating ignition system and measuring firing voltage of spark tester only. DO NOT include firing voltage to cap-to-rotor gap. Spark tester firing voltage should be approximately 28 k/volts plus or minus 10 percent.

3) Check Secondary Display Average Voltage In Range
Connect an oscilloscope to engine. Start and operate engine 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 as specified, go to step 20).

4) Check Secondary Display Average Voltage Above Range

If average spark output voltage is more than 28 k/volts and variation between cylinders is more than 5.6 k/volts, check spark plugs for wear or wide gap at all cylinders. Check for problems that may cause excessive cap-to-rotor gap. Repair as necessary. If average spark output voltage and variation between cylinders is 28 k/volts or less and variation between cylinders is 5.6 k/volts or less, go to next step.

5) Check Secondary Display For Evenness

If spark output voltage variation is more than 5.6 k/volts, check spark plugs for wide gaps, worn electrodes, improperly installed cap or rotor, faulty fuel injector or engine mechanical problem; such as bad valves, low compression or vacuum leaks. If spark output voltage variation is 5.6 k/volts or less, go to next step.

6) Check Secondary Display For High Spark

If spark output voltage is consistently high only in some cylinder(s), check for defective spark plug or wire. Repair as necessary. If spark output voltage is low, go to next step.

7) Check Secondary Display For Low Spark

If spark output voltage is low or spark line slopes downward only in some cylinders, check distributor cap for carbon tracking. Check spark plugs for narrow gap or fouling, and check spark plug wires for grounding. If spark output voltage is not as described, go to next step.

8) Check Secondary Display For Reversed Voltage

If oscilloscope displays an upside down firing pattern, replace distributor. Reconnect all electrical connections and repeat QUICK TEST . If firing pattern is not upside down, go to next step.

9) Check Distributor Assembly

Check distributor, cap and rotor for wear or damage. Check module for breakage, cracks or oxidation. Ensure rotor turns easily without binding. Replace or service as necessary. If distributor, cap, rotor and module are okay, go to next step.

CAUTION: DO NOT puncture a spark plug wire when measuring resistance. Measure only as directed.

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.

10) Check Spark Plug Wire Resistance
Remove spark plug wires. Using DVOM, measure resistance of each spark plug wire. Resistance should be 7 k/ohms per foot. Replace wires that are not within specification. If spark plug wires are okay, go to test TEST HD , step 4) for fuel injector and fuel pressure testing.

20) Check Base Timing

Start engine and warm to operating temperature. Allow engine to idle. Turn all electrical loads off. Connect a timing light to engine. Disconnect Throttle Position (TP) sensor. Ignition timing should be 13-17 degrees BTDC. Adjust timing if necessary. See IGNITION TIMING in ON-VEHICLE ADJUSTMENTS - TRUCKS article. If timing is okay, leave engine running, TP sensor disconnected and go to next step.

21) Check Idle Speed

Connect scan tool to DLC. Ensure engine is at normal operating temperature and all electrical loads are off. Using scan tool, access RPM PID from PID/DATA MONITOR & RECORD menu. Idle speed should be 650-750 RPM. Adjust idle speed if necessary. See VILLAGER under GASOLINE ENGINES under IDLE SPEED in ON-VEHICLE ADJUSTMENTS - TRUCKS article. If idle speed is within specification, return to SYMPTOM DIAGNOSIS under SYMPTOMS in TROUBLE SHOOTING - NO CODES - VILLAGER article.