Performing a Compression Check

This is the first how to instructional that I’ve written and I’ve chosen compression as a topic because I think it’s the most important check you can run on a used outboard. About 6 years ago, a young man showed up asking me to diagnose and fix his outboard. As always, I started with a compression check. In the course of about 90 seconds, I advised him that his engine had a scored cylinder. The kicker to the story is that he’d already been to a certified shop mechanic at a local marine repair business and they had rebuilt his carburetors as well as installing a new waterpump, impeller, gear oil, and plugs. In short, he’d spent about 600 dollars at that shop tuning up a blown engine. I don’t know everything but I surely know that spending 600 of his hard-earned dollars on a blown engine was not his idea of a good boating experience. The learning experience that I’d like to impart from that story is that a 30 dollar compression gauge (sold at any auto parts store) can save you a lot of heartache when looking at a used outboard or preparing to service one.

Outboards require 3 basic things, and this instructional could well serve as a primer for things you may want to check for when buying a used outboard. Those 3 things, in order of importance, are compression, spark, and fuel. If you have those 3 things and they’re working correctly, your engine will run. After those 3, most everything else comes down to the lower unit.

Compression is created within the cylinder when the piston moves forward and back. During the stroke, the piston pushes air toward the cylinder head, which creates pressure. This is measured in psi, or pounds per square inch, and we refer to it as the compression reading. 4 stroke engines make compression differently than 2 stroke engines, but the principle is the same. On a 4 stroke engine, most mechanics and dealers will perform a leak down test, to see how much pressure escapes over a set period of time. On a 2 stroke engine, a compression check is sufficient.

Steps to performing a compression check:

  1. Obtain a compression gauge. As earlier stated, you can purchase a compression gauge at most any auto parts store for about 30 dollars, give or take. They’re not expensive and you don’t need the best one on the market.
  2. Remove all spark plugs. The point of removing all of them at once is that you won’t accidentally fire the engine.
  3. Ensure that your engine is in neutral. Most newer outboards have a safety that will keep the engine from starting in gear but older outboards didn’t always have that and it’s extremely dangerous to start an engine in gear.
  4. Ensure that your battery is fully charged. If your battery is weak, your starter turns over more slowly, which will cause a low reading.
  5. Screw the end of the compression gauge into one cylinder.
  6. Go to the key and turn the engine over for 3 to 5 seconds. It doesn’t matter whether you turn it over for 3 or 5 seconds. What matters is that you turn it over for a consistent time on each of the cylinders.
  7. Go back to the compression gauge, write down the reading, and release the pressure (generally, there’s a button on the gauge or hose that will release compression). Repeat steps 5 and 6 on each of the other cylinders.

Most Common Issues that Cause Low Compression

The most common issues that will cause low compression:

  • A scored cylinder. If I have a 3 cylinder engine and the readings are 120, 120, and 15 psi, I’d pull the head and generally find a scored cylinder.
  • Broken rings: If a ring breaks, you’ll probably already have a scored cylinder anyway. Removing the head will answer that question pretty quickly.
  • Worn rings: This will usually cause low compression across the board. Rings will wear after enough hours. They can be replaced during the course of a rebuild.
  • Bad head gasket: One of the telltales for a bad head gasket is when 2 cylinders are both low. Head gaskets often burn out between the 2 cylinders. Since you’re getting low compression, you’ll want to pull the head off and inspect it. Check the head gasket at that time to ensure it’s still making a good tight seal all the way around. I’ve seen a lot of people throw outboards away over a head gasket because they thought the engine was blown.


Can I run a compression check without a control box and keyswitch being hooked up?

Yes – If you have a set of battery cables and a battery, you can ground the negative lead to the block and use the positive (red) lead to jump directly to the starter post. This will jump the starter and spin the flywheel.

How much compression should my engine have?

This question is almost too involved to answer in simple form. The most general rule of thumb that I can provide is that most 2 stroke engines need to be above 100 psi and within 10% of each other. Johnson and Evinrude made a Looper, or 90 degree engine starting in 1985 that would generally read 85-100 psi on the cylinders, which was fine. The readings still need to be within about 10% of each other, and the closer all of your readings are, the better. Older OMC outboards (Johnson and Evinrude) came off the line around 150. After they’re broken in, they’ll generally sit around 120-130. As they get below 120, even if they’re still even, it’s a sign of worn rings. Mercury 75-115 inline 3 and 4 cylinder engines (1987-2006) run around 115 when broken in. The older Mercury engines would usually be between 120 and 130. Pull start engines will run anywhere from 90-115. Like I said, this is a very involved question.

Should you always replace a head gasket if you pull the head off?

Most mechanics say yes. The common sense answer for everyday use is that it’s not always necessary if the gasket is in good shape. Good judgment should be applied here. If the head gasket looks good and it’s my outboard, I’m not replacing it. If my engine is weak after I choose not to replace it, I’ll run another compression check and proceed accordingly. If the head gasket looks old, worn, and/or tired, replace it.