Why does my boat engine shut off when I put it in gear?

Your engine usually stops when you select gear because it cannot handle the extra load at idle, the propeller or transmission is obstructed, or the shift mechanism cuts the ignition. First establish whether it labours and stalls or switches off instantly, as this helps distinguish excess load from an electrical or shift-control fault.

Check how it stops

Secure the boat with adequate mooring lines and keep everyone clear of the propeller. Stop immediately if engaging gear produces a violent knock or grinding.

With the engine warm, note its idle speed in neutral and what happens when you select each direction:

• Revs sag, the engine shakes, then stops: suspect poor idle running or excessive propeller load.
• It stops sharply, like turning the key: investigate the shift-interrupt system, wiring or electronic controls.
• Only forward or only reverse causes trouble: suspect cable travel, a directional transmission fault or something catching the propeller.
• Both directions cause the same stall: start with idle quality and propeller obstruction before assuming gearbox failure.

Check warning lights, alarm messages and stored fault codes where available. A rev counter dropping to zero because the engine has stopped does not prove an electrical fault.

Check idle under load

An engine can sound healthy in neutral yet lack the torque to turn the propeller. Check the specified warm idle speed first. Many small marine diesels idle around 750 to 900 rpm, but the correct figure and adjustment procedure depend on the engine and installation.

Do not keep raising the idle until the stalling stops without finding the cause. Excessive engagement speed makes gear selection harsher.

• Carburetted petrol engines: a blocked idle jet, incorrect mixture, sticking choke or intake air leak can cause stalling as the load comes on.
• Petrol outboards: inspect spark plugs and check that every cylinder contributes. A weak cylinder can be hard to notice in neutral.
• Diesels: inspect the primary fuel filter or water separator, check the fuel supply and look for evidence of air ingress, particularly if starting is difficult or the engine also loses power underway.
• Injected petrol engines: investigate idle-air control, throttle-body deposits and fault codes using the manufacturer's procedure.

Ventilate petrol engine spaces and eliminate ignition sources before opening any fuel connection. On electronically controlled engines, do not alter throttle-stop screws to compensate for a fault.

Inspect propeller and driveline

Rope, fishing line or weed wrapped around the propeller or shaft can stall a healthy engine immediately. Suspect this if the problem began suddenly after passing floating debris, grounding or leaving a heavily fouled berth.