This can be caused by overheating or reduction of air flow to the engine. Engines need substantial amounts of air to operate correctly and starvation causes the engine to run erratically or stop. There are two possible causes:
1. The engine air inlet filter may be blocked.
2.The inlet vents to the engine compartment may be obstructed either by people or equipment such as liferafts, lifebuoys, ropes, etc. A blocked tank vent can also stop fuel flowing into the engine fuel system.
The engine produces blue or white smoke
This is generally associated with a worn engine. The engine is burning lubricating oil, which is entering the combustion chambers because of:
1. Worn pistons/rings.
2. Worn valve guides.
3. A leaking head gasket.
The battery is not being charged when the engine is running
Although diesel engines do not require electricity to run, they do need to produce electricity if the battery is to be recharged and the demands from the boat’s electrical system met. The most likely causes of not charging are:
1. The drive belt to the alternator is loose or slipping. This can be tightened relatively easily.
2.Wiring from the alternator to the electrical system is loose or corroded. Check all connections. NOTE: electrical disconnection of the alternator from the battery while it is running will burn out the diodes built into the alternator to convert the AC output of the alternator to direct current.
The battery is being overcharged
The output from an alternator is self-regulating, but sometimes a separate sensing wire is used. If this is broken or disconnected the alternator will produce its maximum output, regardless. This will overcharge the battery and cause gassing and boiling over, resulting in spillage of acid and battery failure. The gas produced is also inflammable. If the ammeter shows abnormally high charge rates, check the sensing wiring.
Engine revs apparently drop
The tachometer, or engine speed meter, often runs off an electrical output signal from the alternator, which is driven by a belt. So, if the engine appears to be running normally yet the rev counter indicates otherwise, the first thing to check is whether the drive belt is slipping. In many cases the belt also drives the raw water pump, so the engine will also suffer from reduced water flow and may overheat.
Excess vibration or noise
Diesel engines tend to be noisy but flexible mounts and reasonable acoustic insulation reduce this to an acceptable level. If unusual noise or vibration is experienced there are several possible causes:
1. A rope or other debris around the propeller hitting the hull as the prop rotates. This will cause the propeller to spin out of balance and vibrate.
2. A loose P or A bracket.
3. A loose or broken engine mounting.
4. Drive coupling to propeller shaft loose.
5. Engine oil lost or water in the oil causing engine damage.
A loose engine mounting can cause severe propeller shaft vibration and excessive strain on the P or A bracket and/or the stern gland. Water could then leak into the boat, at worst causing it to sink.