Marine diesel exhaust checks

Most marine exhausts are water cooled, with water from the engine’s raw water cooling system used to cool the exhaust gas. The water and exhaust gas mix together in the injection bend, where it turns into steam before cooling and condensing back into water which then makes its way out of the hull via the exhaust system.

Exhaust system inspection

With the boat ashore, it is wise to inspect the exhaust system for corrosion damage, especially around the injection bend:

Injection bend – this is where the raw water cooling system meets up with the exhaust gases to in order to cool the gases enough to prevent the flexible rubber sections of the exhaust from melting. Check the condition carefully, looking for any signs of corrosion.
Exhaust piping – look for external signs of deterioration in the flexible rubber piping, connections and clips, checking the condition of the double clips all along the system. Any signs of white crystals indicate there have been sea water leaks.

Smoking exhaust

If you have noticed the engine exhaust smoking a lot during the sailing season this can indicate a number of potential problems with the engine that may need to be rectified. It is normal for an engine exhaust to smoke until it is warmed up so you only need be concerned if the exhaust is smoking after the engine has fully warmed up.

The colour of the smoke can be a guide to what might be wrong. White, blue or black smoke from a diesel exhaust indicates there is a problem. White smoke has two general causes: overcooling where proper combustion is not taking place or worn piston rings. Blue smoke comes from burning oil. It can be caused by worn valves or piston rings. Black smoke typically indicates partially burned fuel, worn injectors or clogged air filters. The chart below looks at possible causes and solutions. To help solve the mystery you may need advice from your boatyard mechanic.

White exhaust smoke
Possible cause	Solution
Blocked injectors	Service the injectors
Water or air in the fuel	Defective seals, possible leaking head gasket
Check the breather pipe is clear and not obstructed	Remove and clean out
Reduced cooling water flows	Check the raw water system
Blue exhaust smoke
Engine oil level too high	Check the oil dipstick. If too high, pump out some of the oil
Worn valve guides and seals	Replace oil seals
Piston ring and bore worn, giving a low compression	Get compression checked by a boatyard pro
Leaking turbocharger seal	If fitted, get the turbocharger checked by a boatyard pro
Black exhaust smoke
Blocked air filter element	Inspect and replace
Inadequate ventilation in engine compartment	Check the ventilation isn’t blocked
Blocked or damaged exhaust hose	Check the exhaust hose isn’t blocked or restricted in any way; internal inspection may be required as inner layers of exhaust flexible hose can collapse
Malfunctioing fuel injection	If the airflow is unobstructed and the engine not overloaded, poor injection could be the problem.
Over pitched propeller – engine will not reach its full rpm	Check the engine will reach full rpm in neutral; if it does not get the propeller checked / re-pitched if necessary
Accumulated debris on hull	Inspect and clean if required

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