Diesel Engine Won't Start: Common Causes and How to Diagnose Them

When a diesel engine will not start, it is tempting to blame the injection pump right away. In practice, most starting problems have simpler causes and are solved by following an order of checks: power first, then fuel, then pressure and sensors.

What does a diesel engine need to start?

A diesel has no spark plugs; four conditions must be met at the same time:

1. Enough cranking speed: a battery and starter that turn the engine with the necessary force.
2. Adequate compression in the cylinders, so the air gets hot enough.
3. Fuel that is available, clean and at the right pressure.
4. Pre-heating when the engine is cold, through glow plugs.

In modern engines the ECU and its sensors must also allow starting and control injection.

Step 1: what does the engine do when you turn the key?

The first question summarizes the diagnosis:

Battery and starter

If the engine does not crank or cranks lazily, check the battery and terminals first. A diesel demands a lot of current, and a battery with a low charge or corroded terminals may turn the engine enough to hear the starter but not enough to fire. Starter motors also wear out over the years.

Fuel and supply

When the engine cranks well but does not fire, fuel is the first suspect:

• A clogged fuel filter restricting flow.
• Water in the fuel, collected in the water separator. See water in diesel fuel and how often to change the diesel fuel filter.
• Air in the lines, from a leaking joint, a damaged pipe or after running out of fuel.
• A weak lift pump or transfer pump.
• A tank that is nearly empty or has a blocked pickup.

A classic sign: the vehicle starts after several strokes of the primer pump, or starts and stalls after a few seconds.

Injection pressure

On Common Rail systems, the ECU does not authorize injection until rail pressure reaches a minimum value. If pressure does not build, the engine cranks but does not fire. The most frequent causes are:

• Injectors with excessive return flow, letting pressure escape back to the tank.
• A worn high-pressure pump.
• A faulty pressure regulator valve or rail pressure sensor.

This is checked by reading rail pressure with a scan tool while cranking, and by measuring injector return flow. More on the system in what is Common Rail and bad diesel injector symptoms.

On engines with a mechanical pump, a failing rotary or in-line pump (internal pressure loss, faulty shut-off solenoid) can also prevent starting.

Glow plugs

Glow plugs heat the chamber to help ignition when the engine is cold. If one or several fail, the engine starts poorly in cold weather, takes time to settle and gives off white smoke at first. See diesel glow plugs: symptoms and testing.

Sensors, ECU and others

A faulty crankshaft or camshaft position sensor can stop the ECU from knowing the engine position, so it does not inject. A key not recognized by the immobilizer, damaged wiring or an ECU with faults can also block starting. A scan tool is the key instrument here: fault codes and live data say a lot.

What to avoid

Starting sprays are also best avoided on a modern diesel: they can damage the engine through uncontrolled combustion.

When is it the pump or the injectors?

If the battery is fine, there is clean fuel in the filter, no air in the lines and the glow plugs work, yet the engine still does not fire, the likely cause is in the injection system. A scan-tool diagnosis and bench tests of the pump and the injectors identify which of them is the origin. Learn more about injection pumps and Common Rail systems.