Recently I have come across a situation where piston damage is the topic of discussion. I would like to shed some light on the hazards of outboard motor piston damage particularly (2-Stroke applications), the causes and the culprits.

Pre-Ignition & Detonation is essentially piston failure resulting from abnormal burning or explosion of the fuel/air mixture in the cylinder. Rarely found in more than one or two cylinders at once. And most always caused by an owner controlled condition.

Common causes

• Excessive carbon build up on the pistons and cylinder heads
• Overly lean fuel/air ratios
• General overheating of the powerhead
• Excessive carbon which creates a natural hot spot in the cylinder

Pre-Ignition

• Is defined by abnormal fuel ignition, caused by combustion chamber hot spots
• Combustion pressure rises to early, causing power loss and rough running
• The upward motion on the piston is opposed by the pressure rise. Resulting in extensive damage to the internal parts from the high increase in combustion chamber temperature.
• The fuel/air mixture ignites before the spark plug fires, hence the term “Pre-Ignition”

Causes

1. Hot spots in the combustion chamber from glowing deposits (improper oils and fuels used)
2. Overheated spark plug electrodes (improper heat range or defective plug)
3. And other issues in the combustion chamber ex. overhanging piece of gasket, improperly seated valve, any other inadequately cooled section of material.

By far the most common cause in outboard motors is poor quality oil & fuel leaving excessive carbon deposits in the combustion chamber.

Detonation

• Or otherwise known as “fuel knock”, “spark knock”, or “carbon knock”. Is defined as abnormal combustion of the fuel which causes the fuel to explode violently.
• Detonation is the collision of two opposing flame fronts in the cylinder. Occurring after the spark has fired
• This results in overheating or damage to the spark plugs, pistons, valves and in severe cases results in pre-ignition
• Simply put detonation can and sometimes will cause pre-ignition.

Causes

A few of the most common causes of detonation in a marine application are as follows.

1. Over-advanced ignition timing
2. Use of low octane fuel (old fuel loses octane rapidly)
3. Propeller pitch too high (engine rpm below recommended maximum range)
4. Lean fuel mixture at or near wide-open-throttle
5. Spark Plugs (heat range too hot – incorrect reach – cross-firing.
6. Inadequate engine cooling (deteriorated cooling system)
7. Combustion chamber/piston deposits (results in higher compression ratio)

Travis Palmer: SAMS-SA, ABYC, AIMU
Principal Marine Surveyor
Corsica River Marine Surveys

*Credit is given to Mercury Marine for Information obtained from service manual and “Marine Piston Damage” a publication by Tom Benton, SAMS® AMS®, NAMS CMS.