AI Mechanic

Symptom · Petrol engines, port and direct injection

Petrol engine cranks but will not start

It cranks properly, so the battery and starter have already answered for themselves. What is left is spark, fuel, compression and timing. Three of those you can check in ten minutes without removing a single part.

01Possible causes, none of them ranked yet

  • No spark at the plug

    A failed coil driver, crank sensor or immobiliser cut leaves the mixture unlit.

  • No fuel pressure

    A dead pump, a burnt relay or a blocked strainer means nothing to ignite.

  • Crank or cam sensor not signalling

    The control unit needs a position signal before it fires a coil or opens an injector.

  • Timing chain or belt jumped

    Valves and pistons out of phase kill compression and put the spark in the wrong place.

  • Flooded from repeated start attempts

    Wet plugs will not light, so a genuine fault gets buried under a second one.

  • Immobiliser not releasing

    A key or transponder fault blocks injection while everything else looks perfect.

Nothing on that list is the answer yet. A measurement decides which ones survive.

02The tests, in the order a senior tech runs them

Test 1

Pull one plug, refit it to its coil, ground the shell against the block and crank while a helper watches for spark.

tool
Plug spanner, spare plug, insulated pliers
expect
A crisp blue spark that jumps a 3 to 4 mm gap. A weak orange spark counts as a fail.

Good spark moves you to fuel. No spark on any cylinder points at crank signal, immobiliser or a coil supply, not at the coils themselves.

Test 2

Read fuel rail pressure on a gauge or the scan tool with the ignition on, then again while cranking.

tool
Fuel pressure gauge, or scan tool with live data
expect
Port injection: roughly 3 to 4 bar and holding. Direct injection: low-pressure side around 5 bar before the high-pressure pump builds.

Zero pressure with the ignition on sends you to the pump relay, fuse and pump supply voltage. Normal pressure clears fuel supply entirely.

Test 3

Read cranking rpm on the scan tool while the starter turns, and check whether the tool reports any injection.

tool
Scan tool with live data
expect
A steady 200 to 300 rpm, not a flat 0 rpm.

0 rpm with the engine visibly turning means the control unit sees no crank signal, which explains both no spark and no injection at once.

Test 4

Run a relative compression test with an amp clamp on the battery cable while cranking.

tool
Scope with amp clamp, or compression gauge
expect
Even current humps, all cylinders within about 10 % of each other.

A flat, uniformly low result across every cylinder is the signature of jumped timing rather than an ignition fault.

Test 5

Look at the immobiliser or key symbol in the cluster with the ignition on, and scan for immobiliser codes.

tool
Scan tool
expect
The symbol lights briefly at ignition on, then goes out.

A symbol that stays lit or flashes ends the mechanical hunt and moves the case to key and transponder.

Spark, then pressure, then cranking rpm. Three tests and about ten minutes narrow a no-start to one system before anything is unbolted.

Your car is not this car.

Open a Case with the vehicle and the symptom in front of you. It asks the two or three questions that narrow yours, then names one test with the value to expect, and re-ranks the list on what you measure.

Open your first Case

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