Compression and Leak Down Testing: Squeeze Play

If you’re looking at a used boat with an older engine, or an engine that you think might not have been well-maintained, it makes sense to run a couple of crucial tests on it. First, a bit of background:

Engines make power by vaporizing liquid fuel, mixing it with air at the right ratio, compressing it in the cylinders, and igniting it. That is an oversimplification, but perhaps the most important of those, in terms of dollars required for repairs, is the part where you compress it. If the engine isn’t compressing the fuel and air as it was engineered to do, it won’t run well. Which means slow. Which means disassembly. Which means big bucks.

If you’re mechanically inclined, you can use your own compression-testing kit, or you can hire a marine mechanic to go through the process.

Engine compression can leak out in any or all of four places: the valve seats, spark plug holes, piston rings, or head gaskets. Two tests, compression and leak-down, will help you identify where a potential problem lies. Once you know what the problem is, you can either negotiate a lower price or walk away. Here’s what those terms mean. You can either learn to do the tests yourself or have a qualified shop do the work.

Before you do a compression test, the engine needs to be warmed to operating temperature. Run the engine on a set of “earmuffs,” providing cooling water to the engine.

A leak-down test is performed with the engine not cranking. Simply put, each piston is placed at top-dead-center and the cylinder is pressurized through the spark plug hole with the leak-down tester. The gauge measures the amount of air getting past the rings, valves, or other passages, and provides a reading compared with zero leakage.

Typically, readings at or below 10 percent are within spec. If there is greater than 10-percent leakage, that’s when the fun begins. Now you have to sleuth out what’s leaking. You can often determine that by listening and feeling for air passing in the exhaust or intake manifold. That would indicate bad valve(s) or seat(s). Excessive air passing through the crankcase breather could indicate bad rings, head gaskets, or both. Bad head gaskets can sometimes push water out the seam between the head and the block.

For a compression test, the engine itself provides the pressurization. For each cylinder, remove the spark plug and screw in the fitting that the gauge connects to. With the coils disconnected so the engine doesn’t start, crank the engine for about 10 seconds. The gauge on the tester will register at the highest indicated compression reading. Here again, all readings should be within 10 percent of one another.

In most instances, you do the compression check first, and if it indicates a problem, you do the leak-down test to pinpoint the problem.

Armed with the information these tests provide, you can now make a better decision on whether the that used boat you’re looking at really is a good deal, or something you’d rather not buy after all.

Brett Becker