A rough idle that smooths out at speed almost always points to fouled injectors, a failed sensor, or a vacuum leak. Don't ignore it — it usually gets worse.
What to Check
1
Spark plugs and wires (gas)
Fouled plugs misfire most at low RPM. Pull and inspect — coloration tells you a lot.
2
Idle air control valve
Sticky IAC valves on EFI engines cause hunting idle. Clean with throttle body cleaner.
3
Vacuum / intake leak
Cracked intake gaskets pull air around the injectors and lean the mix at idle.
4
Fuel injector clogs (diesel)
Diesel injectors get gummed up by ethanol-contaminated dock fuel. Bench testing reveals which one is misbehaving.
When to Call Us
We have OE-grade scan tools for Mercury, Yamaha, Volvo, Cummins, and Yanmar — diagnosis at your slip in 30 minutes.