Crank-No-Start: Spark, Fuel, Compression, Sync — Worked in Order

Problem

The complaint

“Engine cranks at normal speed but won't fire. No codes that obviously point anywhere.”

For an engine to start, four things must be correct simultaneously: spark, fuel, compression, and crank/cam sync. Modern PCMs need all four inside narrow tolerance windows. A crank-no-start is always one of those four — your job is to know which.

Why guessing fails

The assumptions that burn techs

There are at least a dozen common root causes. Random guessing has a one-in-twelve hit rate.
Some failures look identical (no spark vs. no sync vs. PCM holding injectors off for security) without scan data.
Fuel pressure 'in spec' at key-on can drop under cranking — testing the wrong moment proves nothing.
Replacing a crank sensor on a no-spark complaint without scoping it costs the customer money and you credibility.

What data matters

Inputs, commands, and expected results

Inputs — what to read

Stored DTCs and freeze frame
P0335, P0340, security codes — read first, always.
RPM PID during cranking
If the PCM doesn't see RPM, it won't fire spark or fuel.
Fuel pressure during crank (not key-on)
Pressure must be present when the engine actually needs it.
Injector pulse (noid light or scope)
Confirms PCM is commanding fuel.
Spark at the plug (HEI tester)
Not 'spark at the wire' — at the gap, under compression.

Commands — what to do

Crank with scan tool recording
Capture RPM, pressure, injector PW, and trims in one shot.
Bidirectional fuel pump prime
Tests pump and circuit without relying on PCM logic.
Starting fluid test
If it fires and dies, fuel delivery. If it doesn't fire at all, spark or compression.

Expected results — what good looks like

Cranking RPM PID
150–250 RPM, smooth and stable.
Fuel pressure (port injection)
Within manufacturer spec (commonly 55–65 psi), holds after key-off.
Injector pulse during crank
Steady pulse on every injector.
Spark across HEI tester gap
Snappy blue arc — not weak yellow.
Cranking compression
Within 10% across cylinders.

Common mistakes

What sends techs down the wrong path

Parts swapping

Throwing a fuel pump at low pressure without checking voltage at the pump connector. A starved pump tests bad — and the new one will too.

Ignoring voltage drop

Not measuring voltage at the pump or coil pack during crank. A 9V supply produces 'failure' symptoms on perfectly good parts.

Skipping verification

Replacing one component, getting a start, calling it done — without recreating the original conditions (cold, hot, heat-soaked).

Skipping the scan tool

Starting with the gauge instead of the freeze frame. The PCM has often already told you which sensor dropped out.

Guided diagnostic thinking

The questions a real diagnostician asks

This is the difference between a parts changer and a diagnostician — not what you test, but the order you think about it.

1
What does the PCM already know?
DTCs and freeze frame first. A P0335 or P0340 narrows the search to a single circuit in 60 seconds.
2
Does the PCM see the engine turning?
Watch the RPM PID during crank. Zero RPM with the engine clearly spinning means crank sensor or its circuit — no spark or fuel will follow until that's fixed.
3
Which of the four pillars is missing?
Spark, fuel, compression, sync. Confirm each with a real test (HEI tester, gauge during crank, noid, compression). Don't assume.
4
If two pillars are missing, what's the common cause?
No spark AND no injector pulse usually means no RPM signal or no PCM power/ground. Don't chase them separately — find the shared cause.
5
Did I prove the fix in the conditions where it failed?
Hot soak, cold start, after sitting overnight. A crank sensor that drops out at 200°F passes a cold test every time.

Stop guessing. Start thinking.

DiagCoach helps technicians follow structured diagnostic logic using real-world test results — the same way the best techs in the bay actually work.

Start a guided diagnostic →

FAQ

Frequently asked questions

Cranks fine cold, won't start hot. Where do I start?

Heat-sensitive crank sensors are the classic. Watch RPM PID during a hot crank — if it stays at zero, sensor or its circuit.

If it starts on starting fluid, is the fuel pump bad?

Not necessarily. It proves the fuel side is the failure, but the pump itself, the relay, the wiring, or the filter could be the cause. Confirm voltage and pressure before condemning the pump.

How do I tell if timing has jumped?

Scope crank and cam together and compare to a known-good waveform, or pull a valve cover and verify marks. Don't guess.

Related diagnostics

No-Crank No-Start: Diagnose It With Logic, Not Guesses Is It Really the Fuel Pump? Confirm Before You Drop the Tank