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How To... Prevent Outboard Motor Seizing-up By Mark Shorto, The Boat Business |
Problem: Seawater ingestion to large capacity four stroke outboard motor Damage: Seawater ingestion to all cylinders, bent connecting rods, camshaft. Repairs required: Complete powerhead overhaul and substantial expense incurred. Scenario: Engine operating at near full throttle 5,000 plus rpm, throttle closed back to in-gear idle position too quickly. Outcome: Engine suddenly stops and is impossible to re-start Diagnosis: Seawater drawn up into cylinders via exhaust system causing flooding of the cylinders and hydraulic locking of the engine. Cause? Four stroke engine powerheads are similar in design and complexity to that of the modern motor car engine. However, unlike the motorcar that delivers its exhaust gasses directly into the atmosphere via the exhaust system, the outboard motor has to exhaust the gasses below sea level – into water, not air. Basic Theory The EFI engine draws the air required for combustion via a component called a throttle body. The throttle body’s main function is to meter and control the amount of air mixture that can enter the engine cylinders via the inlet manifolds and inlet valves. The two major components of a throttle body are the casing and butterfly valve. The valve is a round brass or steel disc that fits neatly into the throttle body casing and can be rotated from 0 deg (closed) at idle rpm up to 90 deg (open) full throttle. The engine being operated at high rpm requires large volumes of air to enter the combustion chamber. This air usually has the fuel injected into its path by a fuel injector positioned in the inlet manifold before the inlet valves. The air fuel mix entering the cylinder is compressed then ignited. The spent air fuel mix is then exhausted via the exhaust valves and into the ocean through the centre of the propeller. All of the above happens thousands of times per minute per each cylinder of the engine, therefore the movement or air flow is from the inlet throttle body through the engine cylinders and out of the exhaust, creating pressure and forcing water out of the engine mid section and gear case. However. when the throttle is abruptly closed and air flow into the engine cylinders has been shut off, the still high revving engine is now creating a large internal vacuum that cannot be relieved by induction air as the throttle butterflies are closed. There is minimal expansion of ignited air fuel mixture and the positive pressure of exhaust is reduced and/or momentarily goes negative, as the exhaust valves are still opening and closing as normal. The momentary negative internal pressures of each cylinder draw sea water into the cylinders via the exhaust passage in the engine’s lower unit and mid section, past the exhaust valves and onto the piston crown. It is this water that hydraulically locks the moving piston and can cause severe mechanical damage to these internal components. How to Avoid: Read and understand the owners operating handbook and seek early training advice. Have operating parameters of the engine demonstrated by your sales dealer, if unsure. Avoid throttle ‘chops’, especially whilst still in the ocean (water level is very close to the lower cylinders at rest). Recognise symptoms of water ingestion: - short term poor performance of engine - rough running after stall or shut down - any sounds of mechanical damage - loss of compression to 1 or more cylinders - seizure of engine after short-term storage - noisy valves – clicking, ticking, etc. |