Car Doctor Q&A: It’s Called “Car Doctor” not “Boat Doctor”

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Despite the title of this column every week, the Car Doctor occasionally gets questions of a marine nature.

Q. What do you know about boats?

I have a 20 year old Ocean Yacht with twin V-8 engines in it. One engine runs great the other engine runs rough and “chugs” at an idle. When I put the boat away last year it was running great.

The engines look like Chevrolet 350’s with the distributor in the back of the engine, just like what I remember with my old Camaro. Where should I start?


A. Here’s what I know about a boat: The word “BOAT” is actually an acronym for “Break Out Another Thousand” when it’s time to get it fixed.

What I do know about is those engines. The engines most likely are small block Chevrolet V-8s. Chevrolet also offers the engines like the 4.3-liter V-6 and the 454 as marine engines (pictured above.)

Really, the only difference between one of these and the engine in your old Camaro could be the way it’s liquid cooled. Some engines have radiators just like your car did. Others use “raw water,” or the water the boat floats upon, to cool the engines.

Like any engine that is running rough, you need to begin to diagnose if you have a problem with one or multiple cylinders. Using insulated pliers, remove one spark plug wire at a time while the engine is running. When you come to the cylinder that when the wire is removed and the idle doesn’t change, you have found the ‘dead” cylinder.

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Another method: With the engine warmed up, spray water near the spark plug. If the water instantly turns to steam, that cylinder is firing. If the water just sits there, the cylinder is not firing.

Once you find the bad cylinder, remove the plug and check the spark plug wire. If the plug and wire look okay, then perform a compression or cylinder leak down test. This will give you an indication if the problem is a valve or piston.

My very limited boating experience is this: one of the valves is probably stuck. This can happen when the exhaust manifold or riser allows fresh water into the exhaust port. Over the winter, water from the engine cooling system can seep onto an open valve and rust the valve in place.

The fix is removing the cylinder head, checking the piston and inspecting the manifolds and replacing them as necessary.

This is where you start breaking out several thousands. Ahoy, matey.