Name that Spark
This ignition system uses a mechanical cam, breaker points, and a condenser to control primary coil current.
What is points ignition?
- STFT +18% at idle
- LTFT +14%
- At 2,500 RPM trims drop near 0%
- No misfire codes
What is a vacuum leak?
- Rough idle
- MAP lower than normal at idle
- STFT +18%
- Improves with RPM
- No DTCs
What is low engine vacuum from worn valves or mild compression loss?
- Primary current ramp is normal.
- Firing voltage is consistently higher than the other cylinders.
- Burn time is slightly shorter than normal.
- Misfire occurs under load only.
What is excessive spark plug gap or high secondary resistance?
An engine idles rough and sets a P0302.
Primary ignition waveform for cylinder 2 shows normal current ramp and normal firing voltage.
What should be next?
A. Replace ignition coil
B. Check injector balance
C. Replace spark plug wire
D. Replace PCM
Check injector balance?
This system still uses a distributor but replaces mechanical points with a pickup coil and ignition control module.
What is electronic ignition (HEI / module-controlled distributor)?
- STFT -22%
- LTFT -18%
- Hard hot restart
- Fuel pressure slightly above spec
What is a leaking injector or high fuel pressure condition?
- Cylinder 3 misfire counter increasing
- Fuel trims normal
- Ignition waveform normal
- Compression test shows 95 PSI on cylinder 3
What is low compression on cylinder 3?
- Current ramp slope is shallow.
- Peak current never reaches normal saturation.
- All cylinders affected.
- Battery PID shows 11.9 volts.
What is low system voltage affecting coil saturation?
An engine has STFT +22% at idle and +3% at 2,500 RPM.
Which condition is most likely?
A. Restricted fuel filter
B. Vacuum leak
C. Faulty coolant temperature sensor
D. Weak ignition coil
B. Vacuum leak?
This ignition system fires two spark plugs at the same time — one on the compression stroke and one on the exhaust stroke.
What is waste spark ignition?
- Bank 1 STFT +20%
- Bank 2 STFT normal
- Idle rough
- MAF reading slightly lower than expected
What is an intake leak affecting Bank 1 only?
- STFT +25% at idle
- Drops to +5% under load
- MAF normal
- O2 switching normal
- Engine slightly shaky at idle
What is a burned exhaust valve?
(Lean at idle because cylinder not sealing properly → oxygen in exhaust)
- One cylinder shows no current ramp.
- Battery voltage and ground at coil connector test good.
- Control wire shows no switching signal.
What is a failed PCM ignition driver?
A waste spark ignition system shows high firing voltage on both cylinders of a paired coil.
Which is the MOST likely cause?
A. PCM driver failure
B. Low battery voltage
C. Excessive spark plug gap on one cylinder
D. Shorted primary winding
C. Excessive spark plug gap on one cylinder?
This system eliminates spark plug wires and places one ignition coil directly over each spark plug.
What is coil-on-plug (COP) ignition?
- Both banks +25%
- MAF grams/sec lower than expected
- No vacuum leaks found
- O2 sensors switching normally
What is an underreporting MAF sensor?
- Long crank
- Fuel pressure normal
- Injector pulse width normal
- MAP slightly lower than expected during cranking
- Engine backfires through intake
What is incorrect cam timing (jumped timing belt or chain)?
- Firing kV is lower than normal.
- Burn time is longer than normal.
- Engine runs rough at idle but improves with throttle.
What is a low compression cylinder?
During a cylinder balance test, one cylinder shows minimal RPM drop when disabled. Fuel trims are near 0%. Ignition waveform is normal. What is the MOST likely cause?
A. Lean condition
B. Low compression
C. Rich mixture
D. Failed upstream O2 sensor
B. Low compression?
In this system, the PCM directly controls dwell and coil saturation timing individually for each cylinder using driver transistors.
What is PCM-controlled COP (smart ignition system)?
- STFT oscillates wildly
- LTFT remains near 0%
- Upstream O2 flatlines lean
- Downstream O2 mirrors upstream
- No exhaust leaks
What is a lazy or failed upstream O2 sensor?
- Rough idle
- MAP fluctuates rapidly at idle
- STFT swings positive and negative
- No DTCs
- Vacuum gauge needle oscillates
What is a sticking intake valve or valve train issue?
- Current ramp is normal.
- Firing spike is extremely high.
- Burn time nearly disappears.
- Misfire counter climbs immediately.
What is an open secondary circuit (broken plug wire or open plug)?
A vehicle sets P0172 (System Too Rich).
Fuel pressure is within specification.
Upstream O2 sensor voltage remains high.
Which condition is MOST likely?
A. Vacuum leak
B. Restricted exhaust
C. Leaking fuel injector
D. Weak ignition coil
C. Leaking fuel injector?