Why does my diesel engine start, run for 5 minutes, then die and refuse to restart?
This morning my diesel started up perfectly, but after about 5 minutes of running, it just died. Since then, it's completely dead and refuses to restart no matter how much I crank it. What should I check first?
Answer
An engine that starts, runs for five minutes, and then dies is suffering from fuel starvation caused by a blocked filter or an air leak in the fuel line. The engine burns the fuel sitting in the injection pump and secondary filter, stalling as soon as air or a vacuum reaches the pump.
Check tank breather and valves
A heavy sailbag shifting in a cockpit locker can easily knock a ball valve shut. Check the main fuel valve at the tank first. If the valve is fully open, inspect the tank breather. A blocked breather creates a vacuum as fuel is drawn out. Dried salt, spider webs or a mud wasp often clog the vent. The lift pump fights this vacuum until it can no longer pull fuel. Open the fuel deck fill cap. A sharp hiss of air rushing in means your breather is blocked. Clear the vent line and the mesh screen on the hull fitting.
Inspect the primary filter
Most boats have a primary water separator and filter mounted between the tank and the engine. Check the glass or plastic bowl at the bottom of your Racor 500 or Delphi CAV unit. Water or black sludge from diesel bug means the 30-micron filter element is completely clogged. Swap the element and inspect the rubber O-rings. A standard Delphi 296 element costs about £5. A Racor element is around £12. Older CAV filters are notorious for air leaks if the central bolt is over-tightened or the square-section seals are seated poorly. An incorrectly seated O-ring is the most common fault following a recent filter change.
Hunt down air leaks
If the filters are clean and the breather is clear, air is entering the fuel line. Diesel lift pumps pull fuel under negative pressure from the tank to the engine. A loose connection will suck air in rather than leak diesel out into the bilge. Work your way systematically from the tank to the engine lift pump.
- Tighten every hose clamp using a 7mm or 8mm nut spinner. A flathead screwdriver will slip.
- Check the copper crush washers on banjo bolts. Replace them if they show scoring or hardening.
- Inspect the flexible fuel hoses for hairline cracks. Pay special attention to older boats where hoses pre-date modern ISO 7840 fire-retardant standards.
- Check the mechanical lift pump itself. The internal diaphragm can split and leak diesel into the engine oil.