Gas inboards and sterndrive engines from MerCruiser, Volvo Penta and Crusader, such as the 4.3 V6 and 5.7 V8, share the same weak points. Surveyor David Pascoe did hundreds of engine damage claims for insurers, and his conclusion is that the exhaust system, more than the engine itself, decides how long they last.
Exhaust risers and manifolds
Every marine inboard has exhaust risers. They keep water from backing up into the engine and introduce cooling water into the exhaust. Pascoe calls riser failure, "either in the design of the thing, or by corrosion", the single largest cause of engine damage and premature engine failure in the claims he evaluated.
Common gas engines use cast iron risers, which corrode. In salt water, he says, good risers last about four to five years, and at four years you should start worrying. He writes that MerCruiser "has a wonderful reputation for risers that sometimes don't last a year", and shows a MerCruiser 4.2 V6 riser with tiny water passages that little corrosion can block, and a 350 riser whose leak ruined the engine. In his gas engine article he says Crusader's main competitors have risers almost certain to go bad in seawater use within three to five years, while Crusader puts its risers on the freshwater side of the cooling system.
On a Sea Ray 250 Sundancer he reviewed, the MerCruiser risers had already been replaced once and were leaking at the gaskets again.
Where to look. Riser age and replacement records; rust, weeping and salt crust at the riser to manifold joints and gaskets; water stains below them. On a cold engine, ask about water in the cylinders or oil.
Cooling system and coolant
On freshwater cooled engines, Pascoe warns that too much antifreeze is harmful: excess ethylene glycol can gel in hot spots such as the cylinder water jackets and manifolds, causing uneven cooling that can distort the block. He calls this a major cause of cracked blocks, manifolds and heads, ring wear and melted pistons. Too little coolant lets corrosion build and does the same. Poor head gaskets that leak put water in the cylinders and corrode the mating surfaces so they need machining before a new gasket will seal.
Where to look. Coolant colour and mix, heat exchanger and manifold condition, raw water pump and impeller records, and temperatures under load on the sea trial.
Engine room details
Pascoe's reviews show the same installation faults on gas engine boats: spark plugs on the outboard banks of a Sea Ray 370 Aft Cabin's MerCruiser 502s only 2 inches from the fuel tanks and unreachable; steel parts mounted flat on bilge sections that collect water and rust. On any gas engine boat, BoatTest's rule of thumb is to run the engine compartment blower, set to exhaust, for at least 5 minutes before starting.
Recalls
The USCG recall list holds Volvo Penta campaigns for the 3.0 GLP, 4.3 GL and 4.3 GL-JF, the 3.0 GXi and the 8.1 litre gas engines, and Mercury Marine campaigns for MerCruiser sterndrives. The public pages don't give the defect; check engine serial numbers with a dealer.
What to check on a gas inboard
| Area | What to look for | Source of the concern |
|---|---|---|
| Exhaust risers | Age over four years in salt water, leaks at gaskets, rust | Pascoe, exhaust risers |
| Manifolds | Rust, blocked passages, water stains | Pascoe, exhaust risers and gas engines |
| Coolant on freshwater cooled engines | Correct mix | Pascoe, gas engines |
| Head gaskets | Water in cylinders or oil | Pascoe, gas engines |
| Installation | Access to plugs, parts in wet bilges | Pascoe, Sea Ray reviews |
| Engine compartment blower | Working, set to exhaust | BoatTest |
| Recalls | Open campaigns for each engine | USCG recall list |
Writing it up
"Port engine, MerCruiser 5.7: exhaust risers and manifolds appear original on a seawater cooled engine, with rust weeping and salt deposits at the starboard riser gasket. No replacement record. Recommend both risers and manifolds are removed and inspected, and replaced if corroded, before the engine is run further." Note whether the engine is seawater or freshwater cooled.