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Solved problems · Propulsion · Crew answer

Why does my diesel engine crank but not start?

A diesel that turns over normally but never fires is usually short of fuel: a clogged filter, air in the lines, or an empty or shut-off supply. Here's the order to check it.

Symptom

The starter turns the engine over at normal speed, but it never catches, or it fires for a moment and dies. Often this follows a filter change, running a tank low, or rough weather.

Likely cause

A diesel needs clean fuel delivered at pressure with no air in it. Most no-starts trace to a clogged primary filter, air drawn in on the suction side between the tank and the lift pump (loose filter bowls, fittings, or a low tank), or a closed fuel valve. Slow cranking from a weak battery, or failed glow plugs in cold weather, can also keep it from firing.

Stop now if

You smell fuel in the bilge, fuel is spraying from a fitting, or the engine is running away or making a knocking noise. Shut the fuel off and stop. After a few failed cranking attempts on a wet-exhaust engine, close the raw-water seacock so seawater can't fill the exhaust and back up into the engine, and reopen it the moment the engine starts.

The fix, in short

  1. Confirm fuel is available. Check the tank level by dipping or sighting it rather than trusting the gauge, and make sure every fuel valve between tank and engine is open.

  2. Check the primary filter. Look in the water-separator bowl for water, sludge or a dark film, and drain it. If the element is old or the bowl is dirty, change it. A vacuum gauge on the primary filter that climbs while cranking points to a restriction between the tank and the lift pump.

  3. Bleed the low-pressure side. Follow the engine manual. Typically you open the bleed screw nearest the lift pump first, work the hand lever until fuel runs out with no bubbles, close it, then move to the next bleed point downstream, such as the secondary filter and injection pump.

  4. Bleed the injector lines only if needed. If it still won't fire, crack the injector-line nuts a turn and crank in short bursts of about 10 seconds, letting the starter cool for a minute between tries, until fuel without bubbles appears, then tighten. Injection pressure is very high, so keep skin and eyes away from escaping fuel.

  5. Find where the air got in. If the engine loses its prime again, look for weeping fuel at filter bowls, gaskets and hose fittings on the suction side, since that is where air gets drawn in.

  6. Rule out slow cranking and glow plugs. A starter that turns sluggishly may not heat the air enough to light the fuel. Charge or test the start battery and check the glow plugs before cold starts.

Written by Sailor Cruiser crew. General guidance, not an inspection of your boat.

Equipment involved

  • Fuel system: Tank, primary filter and water separator, lift pump, secondary filter, injection pump and injectors
  • Starting circuit: Start battery, starter and glow plugs

Related guides

Sources

Background reading we checked while writing this answer. The wording above is our own.

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