EVERYTHING YOU NEED TO KNOW ABOUT OUTBOARD COMPRESSION TESTS
What Is a Compression Test?
Any combustion engine can be put through a basic test like an outboard compression test. The simplest test is the one that can tell the tester whether the engine is viable.
It will demonstrate whether or not the engine’s powerhead is good. Similar to other combustion engines, the engine generates power through the upward movement of a piston within a cylinder. bringing a fuel and air combination to a specific point of compression before lighting it on fire.
The combination will push the piston back down when it erupts within the cylinder. In order to keep all of the pistons moving up and down and producing the engine’s power, the cylinders’ pistons must be driven up in a certain order. The engine’s intended use determines what it can be used for. It will rotate the transmission in an automobile, which will subsequently cause the wheels to spin.
In the context of boats, it will rotate the lower unit’s gears, which convert this motion to the propeller. The boat moves forward as a result. The results of the compression test indicate the state of the engine’s mechanics. Whether or not the engine is creating the proper amount of compression to run properly.
What Do You Need to Perform a Compression Test?
Depending on the engine type and brand you are attempting to test. will decide which tools you require.
To remove any form of plastic or covers that shield the ignition coils, you’ll need a basic set of sockets and a wrench. To unshield the spark plugs and remove the coils, you’ll need your socket sets. The right deep hole socket to remove the spark plugs is the next item you’ll need. Here’s when a few specialized tools will come in handy. Once you reach this stage, you actually just need these two items.
A compression tester and a means of starting the engine are required. The majority of compression testers include multiple hoses with various fittings on the ends of them that are compatible with the majority of spark plug thread sizes from practically all manufacturers. You can proceed as long as it fits your engine. They are rather typical. Which brings us to the second item you will require. What you are able to do at this point will depend on the engine’s manufacturer and brand. You will need a remote key switch or a starter button if you are alone. This will enable you to start the engine and obtain the compression test results.
Another way is to have someone turn the key over, again depending on what kind of engine you are doing this on. You will need to be careful because some engines have features that will continue turning the engine over. Even when you don’t want too!
How Do You Do a Compression Test On an Outboard Motor?
The compression test is actually quite easy to perform. Remove any of the coverings that were placed over the top of the ignition coils that we discussed earlier. The ignition coils will become visible when you are inspecting your engine. The fasteners holding the coils in place should be removed.
It’s fantastic if there are no covers or bolts! Simply remove the spark plug wires from the plugs and insert the plugs into the appropriate socket. Take out every spark plug. You can now take your compression tester and insert the hose into any desired starting cylinder. Take your remote start button and hook up to the starter solenoid on the engine. Now you can jump the solenoid using your button and turn the engine over while looking at the gauge.
Turn the engine over again until the top of the compression gauge levels out. The gauge ought to stop rising after just five to ten engine revolutions. It’s not necessary to keep pressing the button to find out how much more you can obtain. The next five pounds or so won’t matter—the top is the top!
This raises another concern with the administration of the test.
Should You Do an Outboard Compression Test Hot or Cold?
Another one of those “controversial” topics is this one. Whichever side is right or wrong, they will battle to the death. They are both advantageous because of their unique qualities. based on my own judgment and my expertise performing potentially thousands of compression tests. that running the compression test with the engine a little warm is unquestionably preferable.
However, in the meanwhile, what will you do if the engine fails? It won’t get heated if it doesn’t run. Do you start testing other engine parts and forego the compression test if the engine won’t heat up? This is particularly relevant when conducting intentional compression tests. When you are being paid to troubleshoot and figure out a problem. You will find that in most cases, the whole story hasn’t been told to you.
It may be said, “The engine won’t start, but it was running fine.” forgetting to mention that the engine was half underwater and they were backing down on a sailfish. Consequently, the hot and cold stuff may go either way. Warming up the engine and introducing oil into the cylinders improves valve and ring sealing. will alter your compression test’s outcomes. Thus, if the test is conducted with the engine hot. The best readings are what you’ll get. Even so, running the compression test on a cold engine will still yield results that are almost accurate.
What Is a Good Compression Test Reading or Result?
I’ve had to respond to inquiries from younger mechanics and boaters on numerous occasions over the years, so you may be thinking. which, in my opinion, is fantastic since it demonstrates someone’s openness to learning. Depending on the type of engine you are working on, different compression test results will be obtained. The primary distinction between Two-Strokes and Four-Strokes is substantial.
As we mentioned at the opening of this piece. The recommended compression range for two-stroke engines is 110–130 PSI. Even with compression levels between 80 and 90, the engine can still function flawlessly! We’ll speak about poor performance shortly.
There are now more moving parts on the Four-Strokes. Unlike the Two-Strokes, they lack ports. Rather, they have a significantly greater compression reading and intake and exhaust valves that must also close.
Results from a four-stroke test should fall between 180 and 210 PSI. These readings are excellent. It’s crucial to remember that while performing this test, some engines may only register 150 or 160 PSI.
To widen the throttle in neutral, just squeeze the throttle handles on the control box to release the throttle plates. As a result, the system will be able to receive more air, and the measurements should rise to around 180. These are the optimum outcomes for an engine that is both mechanically sound and reliable.
Compression Testing A Yamaha, Evinrude, Mariner, Mercury, Suzuki, or Honda
I want to throw this in here because it really doesn’t matter what manufacturer or brand engine you have. The compression test and test results are going to be the same. Some people will say differently, but when we get down to it. An engine is an engine. If it isn’t making compression, it’s not a good engine. All of the different brands are just going to have a slightly different process to go through, in order to perform the compression test.
That process has to do with how you get to the spark plugs to get them out. Then, where is the start solenoid so we can get to turning the engine over. That’s it; the rest is the same. The results are going to have the same ranges, regardless of what brand engine you are doing the test on.
When Should a Compression Test Be Performed?
This is a question that a lot of people go back to the idea of hearing the word “compression test” and immediately thinking. This is going to be expensive, and the engine is trashed. Which isn’t really the case. It is simply a test to check out the integrity and viability of the engine. It is a good idea to run a compression test on engines that have a lot of hours, or years on them. When you are doing a regular service on a 15-year-old engine with 1500 hours it isn’t a bad idea to check the compression just to see if the engine is still in good shape or not. Then there is also the time when you are doing troubleshooting. If you are dealing with an older engine with a lot of hours and it isn’t running. I like to check the spark and compression first, right off the bat. You don’t want to be the guy that spends 6 hours troubleshooting an engine just to find out that it has low compression. That is never good for anybody! So for any mechanic or aspiring technician out there, it is a good habit to get into.
What Causes Low Compression in an Outboard?
Something that a lot of people wonder about. There has to be a reason that causes an engine to have low compression, and there is. There is just a lot of things that can cause this. Depending again on the Two or Four-Stroke aspect of the engine. Two-Strokes generally get low compression when the piston rings fail. Once the rings fail on the piston, air will just blow right past them. Leaving the engine without the ability to run.
On a Four-Stroke, the same applies, but there is also the issue of the engine having an issue with the valve seats. If some valve carbons up or burns up, then the air will escape past the valve and loss the compression as well. Sometimes this is a simple fix and other times it is catastrophic. So if you do see low compression results, try to warm the engine up if it will run and check again. You can also run a hot mix of engine cleaner and try and burn off any carbon that is on the valves and get the compression back that way. Outside of simple fixes like that. It is tearing into the engine and investigating what is going on or just getting the engine running and run it until it explodes!
How Much Is a Compression Tester VS Having a Compression Test Done?
Sometimes having a professional’s opinion is a great idea. At the same time, you don’t need to go out spending money on services that you don’t really need. Such as when you do your own service, and you would like to check the compression while doing so.
Compression testers can be extremely affordable. We talked a little earlier about getting a compression tester at any auto parts store for sometimes under $50. These testers are really good and surprisingly durable. I bought a compression tester for $45 about seven years ago. I used that tester for about 4 and a half years and did probably 400 hundred or so tests with it! When it finally quit working, I broke down and spent almost $300 on a Snap-On, I’ve gone through 5 stem valves, and the screen is cracked, and that is only in the first six months of owning it! So, the price tag really shouldn’t be the deciding factor in which one you decide to use.