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Cranks But Won't Fire
The engine turns at normal speed and never catches. On a diesel that means one of three things is missing: fuel, air, or compression — and on a boat it is almost always fuel.
Before you keep cranking
On a raw-water-cooled engine the pump runs whenever the engine turns. Prolonged cranking pumps seawater into the exhaust, where it can back up into a cylinder and hydrolock the engine.
Crank in short attempts with diagnosis in between, not continuously.
Likely causes, ranked
| Cause | How common | Confirms it |
|---|---|---|
| Air in the fuel system | Very common | Bleeding restores starting |
| Blocked fuel filters | Very common | Filter bowl full of sludge or water |
| Fuel valve closed, or empty tank | Common | Look before assuming |
| Blocked tank vent | Common | Opening the filler cap changes things |
| Air leak on the suction side | Common | Perished separator bowl seal, loose fitting |
| Stop solenoid or cable stuck | Common | Fuel never reaches the injectors |
| Failed lift pump | Occasional | No fuel arriving at the filter |
| Water or contamination in the fuel | Occasional | Tank bottom sample |
| Glow plug failure (small engines, cold) | Occasional | Only in the cold |
| Injection pump or timing fault | Less common | Specialist territory |
| Low compression | Less common | Leak-down test |
Air is the answer far more often than anything expensive. A perished seal on a filter bowl, or a filter change that was not bled properly afterwards, accounts for a large share of these.
What you can check yourself
- Fuel in the tank, and the fuel valve open
- The stop control fully returned — a stuck stop solenoid is a classic
- Open the filler cap and try again: if it fires, the tank vent is blocked
- Drain the water separator and look at what comes out
- Change the primary and secondary filters, then bleed the system properly
- Check the separator bowl seal and every fitting on the suction side for air
Knowing how to bleed your particular fuel system before you need to is the most useful thing you can learn about your engine.
Fuel filter and water separator service · Marine fuel pump repair
When to call a mechanic
If bleeding and fresh filters do not fix it. Beyond that point you are into lift pump, injection pump, injectors and compression, which need proper testing.
What marine engine work costs in Australia
Common on these engines
Fuel system air and filter blockage dominate across every brand, especially on boats that sit and on any tank with a microbial problem.
→ Diesel bug treatment · Marine engine brands
Frequently asked questions
How do I bleed a marine diesel fuel system? It differs by engine — many modern engines self-bleed, older ones need bleed screws opened in a set order. Find out for yours before you need it.
It fired then died — what does that mean? Usually air, or fuel starvation. It ran on what was in the lines and then found nothing behind it.
It starts if I open the fuel filler cap — why? Blocked tank vent. The engine was drawing against a vacuum. Cheap fix.
How long can I crank before doing damage? Short attempts only. Beyond the starter and battery, cranking pumps water into the exhaust on a raw-water-cooled engine.
Practical summary
Use this fault guide safely
Stop now
Shut down for oil-pressure alarms, overheating, fuel spray, smoke in the engine space or loss of cooling-water flow.
Check safely
Start with visible leaks, strainers, fluid levels, alarms and loose connections—only when the machinery is safe and isolated.
Record evidence
Note rpm, temperature, smoke colour, alarm text, sea state and exactly when the symptom began.
Escalate
Give the evidence and engine model to a qualified marine mechanic; avoid repeated restart attempts.
Sources and verification
This page provides general Australian guidance. The current manual for the exact engine, gearbox or drive remains the authority for procedures, specifications and service intervals.
- 1Describe symptom
- 2Check stop conditions
- 3Verify basics
- 4Test one system
- 5Record result