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Rough Idle Or Uneven Running
An engine that shakes, hunts or runs unevenly at idle is not firing the same in every cylinder, or is not getting a steady fuel supply.
Is it safe to keep going?
Usually yes at cruise, if it smooths out under load. But rough running is often the early stage of something that gets worse, and if it is fuel-related it can become a stoppage at an inconvenient moment.
Shut down if it comes with smoke, overheating or unusual noise.
Likely causes, ranked
| Cause | How common | Confirms it | Cost |
|---|---|---|---|
| Air in the fuel system | Very common | Bleeding smooths it | Cheap |
| Air leak on the suction side | Very common | Perished bowl seal, loose fitting | Cheap |
| Partially blocked filters | Very common | Filter bowl, vacuum gauge | Filters |
| Water or contamination in the fuel | Common | Tank bottom sample | Polish |
| One injector faulty | Common | Cylinder cut-out test | Injector |
| Damaged or fouled propeller | Common | Vibration, not misfire — feels similar | Prop |
| Failed damper plate | Common | Rattle at idle, gone in gear | Damper |
| Loose or perished engine mounts | Common | Excess movement at idle | Mounts |
| Low compression on a cylinder | Occasional | Leak-down test | Major |
| Injection timing | Occasional | Specialist | Specialist |
Distinguish vibration from misfire. A damaged propeller, a worn cutless bearing or sagging engine mounts produce shaking that feels like rough running but is entirely mechanical. If it changes with gear engagement or shaft speed rather than with engine speed, look at the driveline.
Propeller shaft vibration · Fuel starvation at high revs
What you can check yourself
- Drain the separator; look for water and sludge
- Change filters and bleed properly
- Check the bowl seal and every suction-side fitting for air
- Note whether it is worse cold, worse hot, or constant
- Note whether it changes in gear — that points at the driveline or damper plate
- Check engine mounts for sag and movement
When to call a mechanic
If fresh filters and bleeding do not fix it. Ask for a cylinder cut-out test before injectors are replaced — it identifies which cylinder rather than replacing the set on suspicion.
Marine injector testing and replacement · Engine alignment and coupling
What marine engine work costs in Australia
Common on these engines
Air leaks and filter restriction dominate across all brands, especially after a filter change or on boats that sit.
→ Marine engine brands
Frequently asked questions
Rough cold, smooth warm — what is that? Often glow plugs on smaller engines, or an injector that seals poorly cold. Worth diagnosing before it becomes constant.
It shakes but the engine sounds even — misfire or vibration? Vibration. Look at the propeller, cutless bearing, alignment and engine mounts rather than the fuel system.
Rattle at idle that disappears in gear? Classic failed damper plate between engine and gearbox.
Should I just replace the injectors? Not on suspicion. A cut-out test identifies the cylinder; bench testing confirms the injector.
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