Water in oil in Engine

Water in oil in engine, the worst engine nightmare afloat. Water entry into the lubricating oil of a marine diesel engine is one of the most damaging contamination events that can occur. Even small amounts of water compromise the oil’s ability to lubricate, cool, and protect internal surfaces. Left uncorrected, water contamination leads to rapid wear, corrosion, loss of oil pressure, and ultimately catastrophic engine failure. In marine environments, where seawater, freshwater cooling circuits, and condensation are all potential sources, the risk is considerably higher than in automotive engines

Understanding the mechanisms, symptoms, and corrective strategies is essential for reliable operation. If water enters engine lube oil, immediately stop the engine, identify and repair the leak source, drain contaminated oil, flush the system, replace filters, and refill with fresh oil. Conduct thorough inspections, monitor oil condition closely, and implement preventive maintenance to safeguard bearings, reduce corrosion, and restore reliability. If only it was that easy.

How Water Enters Lubricating Oil

Cooling System Leaks. The most common cause is failure within the engine’s cooling system. Typical failure points:

• Heat exchanger tube leaks: Saltwater or freshwater breaches the tube bundle and enters oil passages at the cooler interface.
• Oil cooler failure: A corroded or cracked oil cooler core can leak seawater directly into the oil galleries.
• Cylinder head gasket failure: Cooling water from the jacket space passes into the combustion chamber or oil galleries.
• Corrosion in water jackets: Pitting corrosion or cavitation damage breaks through into oil cavities.

How Water Enters Lubricating Oil

Cooling System Leaks. The most common cause is failure within the engine’s cooling system. Typical failure points:

• Heat exchanger tube leaks: Saltwater or freshwater breaches the tube bundle and enters oil passages at the cooler interface.
• Oil cooler failure: A corroded or cracked oil cooler core can leak seawater directly into the oil galleries.
• Cylinder head gasket failure: Cooling water from the jacket space passes into the combustion chamber or oil galleries.
• Corrosion in water jackets: Pitting corrosion or cavitation damage breaks through into oil cavities.

How Water Enters Lubricating Oil

Condensation.  Small diesel engines on yachts often operate for short durations. This generates milky emulsified oil (“mayonnaise”) often seen under the rocker cover.  When they do not reach full operating temperature, moisture accumulates from:

• Internal condensation during cooling
• Atmospheric humidity entering via crankcase breather
• Repeated cold starts

Seawater Backflow

This can be caused by several events or combination of events

• Improper exhaust installation (including water trap)
• Failed anti-siphon vent may allow seawater to enter cylinders. Water then drains past piston rings into the sump. (see vented loop page)
• Hydrolock event which may be part of the other problems. (see hydrolock page)

Fuel Contaminated With Water

Diesel containing free water contributes moisture to the crankcase through blow-by or via injection system failure

Effects of Water Contaminated Oil on Engine Components

Even low levels of water (above ~0.1%) severely reduce the protective properties of engine oil with the following results:

• Loss of lubrication
• Water displaces the oil film, causing:
• Metal-to-metal contact
• Scuffing of bearings and journals
• Accelerated ring and liner wear
• Corrosion

Water reacts with metal surfaces, producing:

• Rust on crankshaft, camshaft, tappets, and rocker gear
• Pitting corrosion on bearings
• Corrosive acids when mixed with combustion byproducts
• Oil degradation

Water forms stable emulsions with:

• Detergents
• Dispersants
• Additives

These emulsions:

• Become thick and milky
• Block oil passages
• Reduce heat transfer
• Increase sludge formation
• Bearing failure

Hydrodynamic bearings rely on stable viscosity. Water accelerates:

• Bearing shell erosion
• Delamination
• Tin/aluminium corrosion
• Reduction in oil pressure

Water lowers viscosity, and emulsions aerate easily, both reducing stable oil pressure at the crankshaft.

Water in Oil in Engine - Symptoms and Detection Methods

Water in Oil in Engine Visual Indicators

• Milky, creamy oil on dipstick or under rocker cover
• Oil level rising on dipstick
• Water droplets on filler cap
• Steam from breather or oil separator

Operational Symptoms

• Drop in oil pressure
• Elevated crankcase pressure
• Hard starting
• Misfire in cases of water entering cylinders

Dipstick and Smear Test. Rubbing a small amount of oil between the fingers may reveal:

• Gritty feel (corrosion particles)
• Milky texture indicating emulsification

Water in Oil in Engine - Laboratory Oil Analysis

Water in Oil in Engine professional oil sampling provides the following information:

• Karl Fischer water content measurement (A Karl Fischer water content measurement is a precise analytical technique that quantifies trace moisture in oil samples)
• Detection of sodium, potassium (coolant indicators)
• Wear metals (Fe, Cu, Pb, Sn)
• Glycol contamination
• Oil viscosity shift

Pressure and Cooling System Tests

• Cooling system pressure test
• Oil cooler integrity test
• Cylinder compression or leak-down test to detect gasket leaks

Water in Oil in Engine Troubleshooting the Source

If Water Content Is Moderate (Emulsion Present) then you need to investigate the following:

• Head gasket
• Oil cooler
• Heat exchanger
• Condensation from short-run operation

If Seawater Is Detected (High Chloride Levels). The possible causes are as follows:

• Oil cooler internal failure
• Exhaust backflow
• Seawater pump seal failure in rare cases

If Freshwater Coolant is present the indicators are as follows;

• High potassium/sodium from coolant additives

If there are combustion gases in coolant the causes are possible as follows:

• Cracked head
• Cylinder liner cavitation
• Failed gasket
• Core plug corrosion

Water in Oil in Engine - Corrective Actions 

STOP THE ENGINE IMMEDIATELY! Running with water in oil accelerates damage exponentially. The following is from my own lived experiences!

• Drain oil and remove water. You will have to use an oil extraction pump. In most cases the engine drain plug is inaccessible. If the engine was not hot the emulsified oil may be to thick to pump out using an extraction pump with small bore suction pipe. Consider lowering the viscosity of this emulsified oil with the addition of diesel oil or kerosine, I found the kerosine worked best.
• Drain the engine oil sump fully. Use the extraction pump if the drain plug cannot be accessed.
• Replace the oil filter. I bought several after market equivalents at half the Volvo filter cost, as you need to do multiple filter changes after each recovery phase.
• Flush with flushing oil or mineral oil. I used very low viscosity 0-10W which is used in some modern vehicles, and allowed easy extraction with the pump. 
• Cranking over. To get oil over all internal components with oil I cranked over the engine in a serious of short 5 second runs. Isolate the fuel supply valve, isolate the seawater pump. 
• Oil Change. After several crank over runs the oil filter was changed and oil checked for signs of water.
• Engine Oil Refill. Oil refilled with cheaper equivalent grade oil. 
• Engine Run 1. If root cause has been identified and corrected, open fuel and seawater supply valves, then start the engine. Run for 5 minutes on low load and observe oil pressure gauge, and monitor engine for any issues. If running well increase and run at 50% load for 30 minutes to allow all engine parts to reach normal temperature. Monitor oil pressure and exhaust smoke colour. Stop engine.
• Engine Oil and Filter Change. Replace the oil filter. Pour out into a container oil from filter and check oil status, if emulsified, and anything else. Remove the oil and replace with manufacturers recommended oil.
• Engine Run 2. Start engine and run back to temperature under load, observe oil pressures. If all good stop engine.
• Root Cause Analysis. Identify the root cause.  

Root Cause Analysis.

• Replace or rebuild oil cooler
• Replace head gasket
• Repair heat exchanger tubes
• Correct exhaust riser or anti-siphon faults
• Replace cracked head or block (rare but possible)
• Multiple Oil Changes as outlined above

Following system repairs follow these steps which are as above.

• Perform 2–3 short-interval oil changes along with filters
• Re-test oil for water
• Monitor oil pressure closely

Water in Oil in Engine - Preventative Measures

Routine Inspections

• Check dipstick daily during operation
• Inspect anti-siphon vents monthly
• Monitor coolant loss
• Look for milky residue under filler cap
• Maintain Proper Exhaust Installation
• Ensure vented loop high point is above waterline
• Confirm correct mixing elbow height
• Use proper waterlock dimensions
• Avoid excessive cranking that can flood exhaust with seawater
• Service the Oil Cooler and Heat Exchanger
• Clean and pressure-test every 1–2 years
• Replace corroded cores early
• Use correct sacrificial anodes if required
• Run the engine at load (60%)

Allow engine to reach and maintain:

• 80–90°C cooling temperature
• At least 30 minutes run time
• This eliminates condensation buildup.

Use Quality Oil. High-quality marine diesel oil with good detergency and rust inhibitors helps manage trace moisture.

Water in oil in Engine

Water ingress into lubricating oil is a critical fault in any marine diesel engine. Whether from cooling system leaks, seawater intrusion, or condensation, even small quantities of water rapidly reduce lubrication quality, trigger corrosion, and lead to severe mechanical damage if ignored. Early detection—via dipstick inspection, oil sampling, and pressure testing—combined with prompt corrective action greatly extends engine life and reliability. By maintaining proper exhaust installation, servicing the heat exchanger system, and running the engine to full temperature, yacht owners can significantly reduce the risk of water in oil in engine contamination and preserve the health of their marine diesel engines.