Truth is, the MerCruiser, Volvo Penta, Marine Power and Pleasurecraft Marine engines found on many mid-size cruisers first saw the light of day on Detroit drafting tables. The process of adapting automotive power plants for marine use is called marinization and it has been going on for years.

Under certain marine applications, however, one of the engineering features that puts the power in big block General Motors engines can also turn them into a fish out of water. Valve overlap, a process that helps engines run more efficiently, can also cause marinized engines to ingest the seawater used to cool their exhaust systems.

BoatUS has received information about this problem from marine mechanics, several boatbuilders and dozens of owners of Volvo and MerCruiser engines installed on boats 30-40 feet. Water ingestion problems are also well documented in a Merc service bulletin issued in 2001. Besides leaving salt deposits, rusting internal parts and causing valve damage, water in an engine also makes short work of pistons and cylinders.

Water ingestion breakdowns can be a major headache. Some owners report multiple engine replacements during a single boating season, to the tune of two to three engines per side, in twin engine applications. They also report prolonged head scratching by boat and engine makers as they grapple with the problem and how to solve it.

All too often, boat owners end up footing the bill for repairs when boats and engines are out of warranty. Boat builders tell us that it’s not always easy to determine who is at fault and how to make repairs.

While BoatUS has received reports involving problems with 23 MerCruisers engines and eight Volvo Penta engines, all 1997 and newer, there have been no reports about problems with Crusader, Marine Power, Pleasurecraft Marine engines or marinized Ford engines. All damaged engines have raw water, not closed, cooling systems. If you’re not sure what type of system is on your boat, check with your mechanic. It is also worth noting that engines can ingest seawater as a result of worn exhaust manifolds and risers and that these components require periodic replacement. The reports received by BoatUS do not involve normal wear and tear or lack of maintenance.

Valve overlap refers to the brief interval when the intake and exhaust valves are both open during the start of the intake stroke, when the fuel-air mixture enters the combustion chamber. The pressure of the incoming mix pushes out the exhaust gases and, in doing so, makes the engine more efficient and powerful at high rpm levels.

The marine engine makers buy basic GM assemblies called long blocks, which contain the engine block, cylinder heads, crankshaft and pistons. Then they are fitted out with ignition-protected electrical components, as well as raw-water cooling and water-cooled and wetted exhaust systems. The blocks are also reinforced to better withstand the prolonged, heavy use common to marine engines.

Marinized engines can ingest water because, unlike cars, their exhausts rely on water for cooling. A water-filed “jacket” surrounding the manifold cools down the exhaust gas. Water from the jacket is also introduced into the exhaust pipes and expelled with the exhaust gases. That’s the water that finds its way back into the engine.

In August 2001, MerCruiser issued service bulletin 2001-13, "Gasoline Engines and Water Intrusion," which states that under certain conditions a fine mist or water droplets can be drawn backward into the exhaust passage of the exhaust elbow when the engine is idling. The droplets wet the vertical exhaust passage, and when the engine is shut off, the droplets fall and collect in the exhaust manifold runners that go to the cylinder head.

In saltwater areas, the water evaporates and leaves salt crystals in the runner. Over time, the crystals cause rust to form on the exposed surface of an exhaust valve. When the valve sticks, it will cause more water from the exhaust elbow discharge to be drawn backward into the engine.

Water ingestion is more likely to occur on through-hull exhaust systems. Sterndrive engines with through-prop exhausts are less likely to experience the condition. According to reports from boat owners and builders, damage is also more prevalent in cold saltwater areas, rather than warm, and is less likely in freshwater.

A spokesman for Pursuit told BoatUS that field inspections show that ingested water comes from either condensation that collects in the exhaust when the engine is shut down after idling or low temperature running, for example after running up a long channel. The engine aspirates this water on start-up. He said water can also get in when engines are shut down fast, for example, when throttling back for a no-wake zone after running wide open.

The MerCruiser bulletin advises inspecting inside the exhaust passage (at hose end of the exhaust elbow) to see if a salt or rust trail is present from that point backward toward the manifold. If there is, the engine could have ingested water.

Both MerCruiser and Volvo Penta have developed new exhaust systems to mitigate this problem and MerCruiser has an exhaust resonator kit that can be used to retrofit older designs.

This situation is complicated because so many different entities are involved. As far as GM is concerned, it's a non-problem. The engines they sell for marine purposes represent a drop in the bucket compared to all the engines they make for cars and trucks. The marine engine makers probably didn't anticipate that valve overlap — fine on land vehicles — would pose a problem on the water. And, the boat builders simply follow instructions given by Merc or Volvo or Crusader and are caught short when engines fail.

"This is a problem no one could see coming," Mac Privott, president of Carolina Classic in Edenton, NC, told BoatUS "We’re trying to head off the problem before it starts." He advises owners of his boats to have their big block engines compression tested even when there’s no evidence of a problem.

After having some difficulties getting MerCruiser to help owners of Carolina Classic boats, Privott said his company is now using only Volvo Penta engines — with a modified exhaust — because Volvo is more likely to help out by replacing or repairing engines.

(c) Copyright BoatUS Magazine, January 2003