Stage 1: Fuel Delivery
Is fuel actually reaching the rail?
- Listen for the fuel pump prime (2-second hum) when you turn the key to ON
- Check fuel pump relay and fuse before assuming the pump itself is dead
- Verify fuel pressure at the rail with a gauge (TJ 4.0L should hold spec pressure with key on, engine off)
- Inspect the fuel filter/pump strainer if the Jeep has sat unused for a long stretch
Stage 2: Spark
Is the coil actually firing?
- Pull a plug, ground it against the block, and crank — look for a strong blue spark
- Check the crank position sensor connector for corrosion or a loose pin (common failure point)
- Inspect the coil pack and plug wires for cracking, especially if the engine bay runs hot
- Verify the ASD (Automatic Shutdown) relay is actually energizing — no ASD signal means no spark and no injector pulse, even with good fuel pressure
Stage 3: Cam/Crank Sync
Does the PCM see a valid sync signal?
If fuel and spark both check out individually but the engine still won't fire, the PCM likely isn't getting a clean, correctly-timed signal from both the cam and crank sensors at the same time. This is the stage that gets skipped most often because each sensor can test "fine" in isolation while the sync between them is still off.
- Confirm both cam and crank sensor connectors are fully seated and pins aren't backed out
- Check for correct air gap and sensor orientation if either was recently replaced
- Look for a scan tool code specific to cam/crank correlation, not just a generic sensor fault
- Rule out a compromised harness section between the sensors and PCM — chafed wire or a bad splice can pass a static test but fail under cranking vibration
Parts Worth Having On Hand
Corrosion at this connector is one of the most common causes of an intermittent or total no-start on the 4.0L. Worth checking and replacing early since it's inexpensive relative to the diagnostic time it saves.
Check Price on Amazon →A weak or failing ASD relay can mimic a fuel or spark problem, because it powers both circuits. If you're chasing your tail between fuel and spark theories, test this relay directly before going further.
Check Price on Amazon →A basic code reader isn't enough for Stage 3 — you need live data to watch cam and crank signals in real time while cranking. This is the tool that finally isolates a sync problem instead of guessing at it.
Check Price on Amazon →Related Guides
Already ruled out fuel and spark and landed on an electrical gremlin? See the TJ Problem Guide for other 4.0L wiring failure points, or check Best Blower Motor Resistors for TJ if your electrical issues started with the HVAC system.