Troubleshooting Two Common Issues in Marine Auxiliary Engines

Introduction
Engine type: 6EY18ALW
Maker: YANMAR
This document briefly introduces two common issues and their solutions for reference by marine engineers.

I. Carbon Deposits, Corrosion, and Lubricating Oil Contamination in Cylinder Heads
1. Fault Description
Three YANMAR AEs on a vessel exhibited poor combustion performance. Under single-engine operation at normal load, exhaust temperature deviations between cylinders exceeded 40°C, with the highest cylinder exhaust temperature reaching 470°C.
Additionally, the lubricating oil in the sump of all three engines was contaminated at an abnormally rapid rate. The oil required replacement within less than 500 hours due to high differential pressure alarms from the self-cleaning filters.
During pre-start cranking, a mixture of fuel and lubricant was expelled from some cylinders. The vessel was forced to operate two engines in parallel during normal sailing and three engines during port maneuvers or high-load conditions.
2. Fault Investigation
The crew initially attributed the issue to fuel leakage and poor combustion. Despite frequent inspections, replacements of fuel injectors, fuel injection pumps, and adjustments to valve clearance and injection timing—even overhauls of turbochargers and air coolers—the problem persisted.
Since the cylinder heads had been replaced relatively recently (within 1,200 hours), the crew did not suspect inlet and exhaust valves issues or disassemble the heads for inspection. 
Later, a gas leak was detected at one cylinder head’s sealing surface, accompanied by higher exhaust temperatures. The head was removed, revealing severe carbon buildup in scavenge and exhaust passages. Further disassembly showed that three out of four valve stem seals (two exhaust, one intake) had detached from their guides and moved with the valve stems (Figure 1).

Figure 1. Valve Stem Seal Assembly in Cylinder Head

Subsequent inspections of the other two engines revealed similar issues:
One engine had 5 cyl. heads with detached seals.
Another engine had all six cylinder head seals detached.
This explained the excessive oil expulsion during cranking and rapid oil contamination. Detached seals failed to prevent oil ingress into cylinders and instead acted as reciprocating pumps, forcing more oil into combustion chambers.
Severe carbon buildup and wear on valve stems and guides necessitated replacement of all affected components.

3. Root Cause Analysis
The detachment of valve stem seals was traced to improper assembly. Two installation methods were identified:
1.Installing seals onto guides before valves:
New seals fit tightly on valve stems. Inserting stems often dislodged seals, which were rarely repositioned.
2.Installing valves first, then seals:
Seals must be pressed into guides until a "click" confirms proper seating. Manual pressure alone often fails to secure seals, leading to eventual detachment under high-speed valve motion.

4. Corrective Measures
Method (2) is recommended. Use a new valve guide or a long sleeve to evenly press seals into place until audible confirmation.
For in-service repairs, reposition detached seals without removing the head:
oRotate the piston to top dead center.
oRemove valve springs using specialized tools.
oAllow the valve to rest on the piston, then reinstall seals.
WARNING:
Avoid forcibly removing installed seals, as sharp edges on valve stem grooves can damage seals. Always replace seals during head refurbishment.

II. Air Cooler Core Jamming and Disassembly
1. Issue Description
The YANMAR 6EY18ALW air cooler employs a compact two-stage (high/low-temperature) design. However, disassembling the core for cleaning frequently damages the secondary stage, even when performed by manufacturer technicians. Reluctance to disassemble leads to severe fouling and increased maintenance costs.
2. Root Cause Analysis
a.Design flaws: Tight clearance between the secondary core and housing promotes rust-induced jamming.
b.Lack of dedicated disassembly tools and unclear manual instructions.
c.Neglected maintenance: Most vessels fail to clean cores at the recommended 4,000-hour interval, exacerbating rust and fouling.

3. Solutions
a.Preventive measures: Clean and apply anti-rust paint to the housing interior before reinstalling cores.
Apply anti-seize compound (e.g., graphite powder) on sealing surfaces.
b.Custom tools: A puller-type tool significantly reduces damage during disassembly.
c.Disassembly procedure:
oFlush the core with 50°C water and detergent for 1 hour.
oTap the housing gently with a wooden mallet to loosen deposits.
oAttach the puller tool and apply gradual force while tapping the housing.
d.Post-disassembly care:
Clean and polish core rails, apply anti-rust paint, and ensure thorough drying before reassembly.