QC – Hot No Start (Ignition)

QC – Hot No Start (Ignition)

Symptom: Engine starts cold, but after heat soak it cranks and will not start (or starts only after cooling).

Rule: You must test it while it is failing. If it cools down and starts, the proof window is gone.

Tools: Spark tester (or known-good spare plug), multimeter, 12V test light, small jumper lead, basic hand tools.

Safety: Keep hands and leads clear of belts/fan. Avoid raw fuel vapor. Use insulated pliers if handling a plug wire. Do not hold a bare wire to ground with your hand.

0) Confirm this is ignition, not fuel

If you do not prove spark loss, you can waste hours in the wrong system. Run the fast proof checks below during the hot-fail crank.

  1. Check spark during the failure. Use a spark tester at a plug wire or coil output. Crank for 3-5 seconds.
    Result:
    • Strong, consistent spark: This page is not your problem. Go to Fuel QC for hot no-start routing.
    • No spark or weak/intermittent spark: Continue.

    Proof: QTS – Spark Output Test (coming)

1) Hot cranking voltage and cranking speed (foundation gate)

Hot no-start can be a voltage and speed problem. If the starter drags hot, ignition energy and sensor signal margin drop.

  1. Battery voltage while cranking (hot-fail). Meter across battery posts during crank.
    Pass: about 9.6V or higher while cranking.
    Fail: below 9.6V, or cranking speed obviously slow.
    If fail: you do not have an ignition proof yet. Fix power/starting first (battery, cables, starter heat soak, grounds).
    Proof: QTS – Voltage Drop While Cranking (coming)

2) Verify ignition feed stays alive in CRANK and RUN

A classic hot-fail is loss of coil/ignition feed during crank or loss of RUN feed as soon as you release the key. Heat and resistance make marginal switch/circuit problems show up hot.

  1. Key ON (RUN): Verify 12V at the coil positive / ignition power feed (or COP power feed). Use a test light or meter.
  2. Cranking (START): Watch the same point while a helper cranks.
    Result:
    • Power drops out in START: Ignition switch / start-run feed split / relay control issue. Repair the feed before chasing sensors.
    • Power stays solid: Continue.

    Proof: QTS – Ignition Switch RUN/CRANK Feed Check (coming)

3) Determine if the ECU/ignition module is triggering the coil

If coil power is present but there is no spark, the next gate is: is the coil being commanded (primary switching) or not.

  1. Back-probe coil negative / driver wire (primary control). Use a test light to battery positive and probe the coil negative (or primary trigger wire).
    Crank (hot-fail):
    • Flashing/pulsing: Primary is switching. Go to Step 4 (coil/secondary).
    • No pulsing (steady): No trigger. Go to Step 5 (crank/cam signal and module/ECU enable).

    Proof: QTS – Coil Primary Switching Test (coming)

4) If primary switches but spark is missing: coil/secondary heat-fail checks

When the primary is switching but spark is weak/missing, the coil or secondary path is failing under heat, or voltage is collapsing under load.

  1. Swap test (fast isolation): If COP or coil pack, swap a known-good coil into the suspect position and retest spark during crank (hot-fail).
    Result:
    • Spark returns with known-good coil: coil heat-fail confirmed.
    • No change: continue.
  2. Secondary path check: Inspect for cracked coil tower, carbon tracking, wet boots, or burned terminals. Verify plug gap is not excessive.
  3. Ignition supply under load: Re-check coil power while cranking. You can have “12V key on” but collapse under crank due to a bad connection hot.

    4. Proof: QTS – Coil Heat-Soak Failure Confirmation (coming)

5) If there is NO primary switching: prove crank/cam signal and ignition enable

No coil trigger means the controller does not see a valid engine position signal, or it is not allowed to fire (power/ground/enable). Heat soak failures here are common.

  1. Check for RPM signal while cranking (if you have a scan tool or tach input).
    Result:
    • No RPM: suspect crank sensor, wiring, air gap, or sensor power/ground (heat-related open/high resistance).
    • RPM present: suspect ignition module/driver, ECU output, or an ignition enable issue.

    Proof: QTS – Crank Sensor Signal Presence (coming)
  2. Heat-soak tap test (quick): During hot-fail crank, lightly tap the ignition module or crank sensor connector harness. If it flickers to life, you have a connection/component heat fault.
  3. Ground integrity gate: Verify engine ground strap and controller/module ground points. Heat increases resistance and exposes marginal grounds.

    4. Proof: QTS – Ignition Module Power/Ground Verification (coming) | QTS – Ground Drop Test (coming)

6) Decision closure – what you can prove today

  • Confirmed spark present during hot-fail: ignition is not preventing start. Route to Fuel hot no-start QC.
  • Confirmed spark absent and coil feed drops in START or RUN: repair ignition feed path (switch, relay, wiring, connector heat fault).
  • Confirmed coil feed OK but no primary switching: prove crank/cam signal and module/ECU enable. Repair signal or power/ground before replacing parts.
  • Confirmed primary switching but no spark: coil/secondary heat-fail or voltage collapse under crank. Confirm with swap and voltage drop.

Next links: Ignition Troubleshooting Hub | Fuel Troubleshooting Hub | QC – Cranks, No Start | QC – Intermittent No Spark | QC – Misfire Under Load