Practical Sailor's mechanic calls the Yanmar GM series the Toyota of sailboat diesels: tolerant of corrosion, neglect and poor running habits. He has seen GMs still running with cracked pistons and saltwater in the cylinders. The same article is blunt about the weak point that causes most of that damage.
The exhaust elbow
Yanmar's original elbows, "especially the mild steel L-style and cast iron U-bend style", fail in several ways, and they're one reason the author ends up pulling cylinder heads. They corrode from the inside out, out of sight, until the elbow breaks apart or water starts running backwards into the exhaust manifold. The water jacket also cuts the dry exhaust passage in half, so the elbow clogs with soot and carbon and the engine loses power. He shows a U-shaped elbow almost fully blocked with carbon and almost fully blocked at the raw water inlet, and a 2GM with a cracked piston crown after water had been running back into the cylinder for some time.
His advice for an original mild steel or iron elbow: remove it every year or two and inspect it inside. One fix is a cast stainless elbow.
Where to look. Ask when the elbow was last removed and whether it's original. If you can, see it off the engine: corrosion and carbon inside, how much of the passage is left, and the raw water inlet. Outside, rust, weeping and salt crust at the joint to the manifold.
Valve seats and compression
The GM series starts on compression alone, with a much higher compression ratio than engines that use glowplugs. In damp, salty engine rooms Practical Sailor sees many GMs struggle to start because of corroded valve seats, and traces the chain from bad elbow to water intrusion, weakened valve seats and weak compression.
Where to look. A cold start: how long it cranks, and whether it fires cleanly. If starting is poor, a compression test before purchase.
The air filter
The breather vents into the intake, and if oil mixes with the foam air filter, the foam disintegrates and gets sucked into the engine. The article's advice to buyers is simple: "pop open the air cleaner and check". Owners should check or replace it every 100 hours or at each oil change.
Engine mounts
Yanmar's original mounts are soft, letting the engine move a lot, and are "notorious for delamination from their steel supports" because the rubber degrades in contact with oil or diesel. The mild steel brackets corrode over time. Many owners fit stiffer replacement mounts.
Where to look. Each mount for separated or swollen rubber, oil soaking, rusted brackets, and engine movement when put in gear.
Fuel knock
GM engines are loud, with a noticeable "fuel knock" from the design of the pre-combustion chamber and injector. On these engines that knock is normal; record it, and listen for anything beyond it.
What to check on a Yanmar GM
| Area | What to look for | Source of the concern |
|---|---|---|
| Exhaust elbow | Original or replaced; internal corrosion and carbon; when last removed | Practical Sailor |
| Starting and compression | Hard starting, poor compression | Practical Sailor |
| Air filter | Oil soaked or disintegrating foam | Practical Sailor |
| Engine mounts | Delaminated rubber, rusted brackets, movement | Practical Sailor |
| Raw water inlet at the elbow | Blockage | Practical Sailor |
Writing it up
"Engine exhaust elbow: original cast iron U-type, heavy external rust and salt deposits at the manifold joint, no record of removal. Recommend the elbow is removed and inspected internally before the vessel is used, and replaced if corroded or obstructed, to protect the engine from water ingress." Note the engine serial number and hours in the particulars.