A diesel needs three things to start: enough cranking speed, enough heat from compression (helped by glow plugs or an intake grid heater in the cold), and fuel delivered at high pressure. Most no-starts come from weak batteries, a failed grid heater or glow plugs in cold weather, air in the fuel after a filter change, or a fuel supply problem such as a failing lift pump or clogged filter.
Diesel cranks but won’t start
| Clue | Likely cause |
|---|---|
| Slow cranking | Weak batteries or poor cables; diesels need strong batteries; see battery voltage chart |
| Won’t start in the cold, white smoke | Grid heater or glow plugs not working, or a weak battery |
| Won’t start after a fuel filter change | Air in the fuel system; prime it |
| No smoke at all while cranking | No fuel: lift pump, filter, high-pressure pump or rail pressure fault |
| Hard start when hot | Worn injectors or high-pressure pump, low supply pressure |
| Just clicks | Batteries, cables or starter; see car won’t start |
Won’t start after a fuel filter change
Air trapped in the filter housing stops a common-rail engine from building rail pressure. Prime the system as the manual describes; on many trucks that means cycling the key several times so the lift pump runs, or using a hand primer, before cranking in short bursts. Check the new filter’s seals for leaks; see fuel filter.
Cold-weather starting
- Wait for the wait-to-start light so the grid heater or glow plugs can warm the intake or cylinders.
- Use a block heater below freezing.
- Use winter-blend fuel or an anti-gel additive; gelled fuel clogs filters.
- Keep batteries fully charged and test them before winter.
Avoid starting fluid on engines with grid heaters or glow plugs: the heater can ignite it in the intake and cause damage.
Fuel supply and rail pressure
On common-rail diesels, the computer will not fire the injectors until rail pressure is high enough. A weak lift pump, a clogged filter or a worn high-pressure pump leaves the engine cranking without starting; codes such as P0087 and P0191 point there. See diesel lift pump and CP3 pump.
Low compression
A diesel relies on compression heat to ignite fuel, so worn rings or valves make cold starting hard and add white smoke and blow-by; see diesel blow-by and compression test.
When the engine is worn out
Hard starting with heavy blow-by and low compression means an engine near the end of its life; see how long diesel engines last, 5.9 Cummins problems, 6.7 Cummins problems and our Cummins engines.
More guides: diesel hard to start when cold · glow plug symptoms · diesel fuel gelling · how to bleed a diesel fuel system · water in diesel fuel.
Trouble codes: P0090 Fuel Pressure Regulator Circuit · P0091 Fuel Pressure Regulator Circuit Low · P0092 Fuel Pressure Regulator Circuit High · P0380 Glow Plug Circuit A · all OBD2 codes.
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