AI Mechanic

Symptom · Common-rail diesels, roughly 2004 onward

Diesel cranks but will not start

It turns over at a healthy speed and never fires. The engine needs fuel at pressure, air, compression and a signal to inject. Each of those is measurable, so none of them has to be guessed.

01Possible causes, none of them ranked yet

  • Air in the low-pressure fuel side

    A leaking lid seal, filter housing or hand primer lets the pump pull air instead of fuel.

  • Failed or worn high-pressure pump

    The pump turns but cannot build the pressure the injectors need to open.

  • Blocked or waxed fuel filter

    Restriction starves the high-pressure pump, most often after a cold night or bad fuel.

  • Crank or cam sensor not signalling

    Without a position signal the control unit never commands an injection.

  • Leaking injectors returning too much fuel

    Fuel goes back to the tank instead of into the cylinder, so rail pressure collapses on cranking.

  • Low compression, glow system aside

    A slipped timing belt or worn bores drop compression below the point where diesel lights.

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

Read live fuel rail pressure on a scan tool while a helper cranks the engine for 5 s.

tool
Scan tool with live data
expect
At least 180 bar during cranking. Most engines will not fire below roughly 150 bar.

Good pressure clears the whole low-pressure side and points at signal, compression or injectors. Low pressure keeps fuel supply in front of you.

Test 2

Fit a clear hose in the feed line between filter and high-pressure pump, then crank and watch it.

tool
Clear hose and two clamps
expect
A solid column of fuel, no bubbles. Any bubble stream is a leak on the suction side.

Bubbles move air ingress to the top of the list and rule the pump down until the leak is fixed.

Test 3

Measure injector back-leak into a graduated tube while cranking for 30 s.

tool
Back-leak test kit or four clean tubes
expect
Roughly even volumes, a few millilitres each. One injector well above the others is the suspect.

An uneven return with low rail pressure points at a leaking injector rather than the pump.

Test 4

Check the crank sensor signal on a scope or read cranking rpm on the scan tool.

tool
Scope, or scan tool showing engine speed
expect
A steady rpm reading of 150 to 300 rpm while cranking, not 0 rpm.

A flat 0 rpm with the starter turning means the control unit sees nothing and will never inject.

Two tests usually settle it: rail pressure first, then the clear hose. Replacing a pump before either one is a guess with an invoice attached.

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

Free and complete. No account, no card. A Bay Pass after that is $12.00 for 90 days.

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