Ignition System Diagnosis and Repair Flashcards

A spark plug gap measures 0.065 inches; specification is 0.040 inches. The effect on ignition will be:

Answer: Higher voltage demand that may cause misfires

A gap larger than specification requires more voltage to bridge, which can exceed coil capacity and cause misfires.

During an ignition coil primary voltage test with a lab scope, the technician sees no voltage drop across the coil. This indicates:

Answer: The coil driver transistor or ECM output is not switching

No primary voltage drop means the coil is not being triggered by the ECM driver, pointing to a control circuit fault.

A technician observes crossfire between two adjacent ignition wires. The MOST likely cause is:

Answer: Wires routed too closely together or damaged insulation

Crossfire occurs when high-voltage wires are routed parallel and close together, allowing inductive energy to jump to an adjacent wire.

What does a long spark line on an ignition oscilloscope pattern indicate?

Answer: High resistance in the secondary circuit

A long, high-voltage spark line indicates high resistance in the secondary circuit, which the coil must overcome to sustain the arc.

An intermittent misfire code P0301 is stored but no misfire occurs at idle. The BEST next step is:

Answer: Road-test the vehicle under the conditions that set the code

Since the misfire is intermittent and load-dependent, reproducing it under the same conditions is required before replacing parts.

A technician checks available voltage at the ignition coil positive terminal during cranking and finds only 6 volts. Specification is battery voltage. The MOST likely cause is:

Answer: High resistance in the power supply circuit to the coil

Low supply voltage at the coil positive terminal indicates a high-resistance connection or fault in the feed circuit, not the coil itself.