On this page
Loses Power Under Load
The engine runs fine at the dock and falls away when you ask it to work. That pattern is diagnostic in itself: almost everything that behaves this way is a restriction that only matters at high demand.
Is it safe to keep going?
At reduced load, usually. But losing power offshore is how a manageable problem becomes a rescue, so treat it as a reason to head in rather than to press on.
Shut down if it comes with overheating, smoke or unusual noise.
Likely causes, ranked — cheapest first
| Cause | How common | Confirms it | Cost |
|---|---|---|---|
| Fouled bottom or damaged propeller | Very common | Will not reach rated WOT RPM | Antifoul |
| Blocked fuel filters | Very common | Filter bowl; vacuum gauge rising | Filters |
| Blocked tank vent | Common | Opening the filler cap restores power | Free |
| Air leak on the fuel suction side | Common | Perished bowl seal, loose fitting | Cheap |
| Collapsing suction hose | Common | Fine at idle, fails at revs | Hose |
| Blocked exhaust elbow | Common | Back pressure, soot, heat | Elbow |
| Dirty air filter | Common | Inspection | Clean |
| Turbo or aftercooler fault | Occasional | Low boost | Repair |
| Slipping gearbox | Occasional | Revs rise, speed does not | Gearbox |
| Worn injectors or pump | Occasional | Bench test | Specialist |
| Low compression | Less common | Leak-down | Major |
The tank vent one is worth knowing. A blocked vent creates a vacuum in the tank as fuel is drawn out, progressively starving the engine over an hour or two. Opening the filler cap fixes it instantly — which is also how you diagnose it.
Fuel starvation at high revs · Won’t reach wide open throttle RPM
What you can check yourself
- Does the engine reach rated WOT RPM? On a clean bottom?
- Open the fuel filler cap while it is struggling — does it improve?
- Filter and separator condition; vacuum gauge if fitted
- Air filter and intake
- Does it smoke black at load? That is overload or air restriction.
- Do the revs rise without matching speed? That is the gearbox or the prop.
When to call a mechanic
Once the bottom, prop, filters and vent are ruled out. Bring the actual WOT RPM figure — it narrows the diagnosis substantially.
Fuel filter and water separator service · Marine gearbox service
What marine engine work costs in Australia
Common on these engines
Fuel starvation dominates on boats with tanks that sit; overload dominates on boats due for antifouling.
Fuel polishing and tank cleaning · Marine engine brands
Frequently asked questions
Fine at idle, gutless at revs — what does that suggest? A restriction that only matters at high demand: filters, a collapsing hose, a blocked vent, or air on the suction side.
Power fades over an hour then recovers after stopping — why? Classic blocked tank vent. The vacuum builds while running and releases when you open the filler.
Could it be the propeller? Yes. A damaged or fouled prop, or one that was never right, is a very common cause and shows as low WOT RPM.
Should I replace the injectors? Not until fuel supply, air and overload are ruled out. Injectors are the expensive guess on this symptom.
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