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What Outboard Compression TEST RESULTS Tell You!
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Aug 16, 2024
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0:00
A compression test will tell you about the health of an engine
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But what exactly does it tell you? Understanding what it means is important for any boater because it's something that you will be looking at and talking about anytime you sell a boat or buy another boat
0:26
I want to give a quick breakdown of a combustion engine before going into a
0:30
what the compression numbers mean, just for those that aren't familiar with this kind of stuff
0:36
On a combustion engine, there are pistons inside of the engine that come up and compress the air
0:41
and fuel mixture inside of the cylinder. That mixture is then set on fire when the piston reaches top dead center or when it is
0:49
all the way at the top of the cylinder. That explosion then sends the piston down and continues this cycle of moving up and down
0:57
the power that the engine uses to spin the propeller and move the boat
1:02
There are many things that factor into the engine being able to make this compression
1:07
On two strokes, it's pretty simple. You have rings on the pistons, then you have the head that goes on the top of the block
1:17
There are then ports that allow the fuel and air in and ports for the exhaust to allow the
1:21
burned up mixture out. On a four stroke, these ports aren't there
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Instead, you have valves. These valves open and close to allow these mixtures in and out
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And that is the basic functionality of a combustion engine. We won't go into any more detail in this video because we need to discuss the compression numbers
1:43
Performing the actual compression test is extremely easy. We just need to remove the spark plugs from the engine, then all you need is a compression tester and a way to turn over the engine
1:54
If you do not have a remote start-up. button like this and you are going to have someone turn the engine over by using the key switch
2:01
you want to pull the lanyard because it will cut off the spark from the ignition coils
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Otherwise you will also need to have a sparkboard. That is because when you use the key switch, the engine will produce spark as well, where
2:13
the remote starter button will not. And you do not want to be back here with the ignition coils firing away all over the engine
2:20
You could get shocked and above that with the spark plugs out of the engine there will be fuel and air mixtures being pushed out of the engine pushed out of the cylinders that could catch on fire which is something else that you do not want to happen
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And that is why you also want to unplug the fuel pump as well
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It's important to just be mindful of these two things because you can set an engine on fire or shock yourself if you aren't careful
2:46
Now there are two really common types of compression testers. Accurate and what is used on four strokes is the compression tester that screws into the cylinder
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This gives you the exact compression of the engine cylinders But when it comes to two strokes they do not produce the same amount of compression as a four stroke
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So these compression testers have a rubber boot on the end of them and are really quick to use
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For these we just have to push the gauge against the cylinder where the spark plug goes and turn the engine over
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These aren't as accurate as the screw-in testers, but they are super quick to use when there isn't anything wrong with the engine
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and you just want to get a good idea of the health of the engine. For the two-stroke, the compression is going to range anywhere from about 100 PSI to 130 PSI
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depending on the age, hours, brand, and model of the engine that you are testing
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The main thing that you want to look for is that the readings for all the cylinders are about the same
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You don't want to see a difference of more than 10% between the cylinders and 10% is just what I go off of
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I know you can find people saying 5%, 15% or even 20% differences, but for me, I like to stay around the 10% difference
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So if you're doing a compression test on a two-stroke and the cylinders range from 100 PSI to 110 PSI
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I wouldn't be concerned about there being any issue with that engine. Now, if you see that most of the cylinders are at 115 PSI and one cylinder is down at 85 PSI
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that is a problem. Depending on the age and where of the engine, there could be issues with the piston rings being worn out
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and you will want to dig a little deeper into why that cylinder is that low
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Running it for too long like that could end up blowing up that piston and destroying the engine
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With a four-stroke on the other hand, you will see a wider range of compression readings
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You can see anywhere from 150 PSI to 220 PSI again depending on the age hours model and brand Normal readings though for most engines are going to be between 180 and 200 PSI
5:00
You will see 150 to 160 for a lot of engines though, but that is because of the throttle
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plates being closed. On a mechanically controlled engine, if you just put the handles in wide open throttle, you will
5:12
generally see those numbers go up to 180 or so. Then some will have a much higher compression into the 200s
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With a 4-stroke, you are looking at having valve issues as well as the piston rings and the cylinder head like you do on a 2-stroke
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This is where carbon build-up comes into play. A lot of people don't realize it, but a boat engine is designed to be operated at much higher RPMs than a car
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You could be doing 50 down the highway and only turning about cosn
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r p.m.m.s in your car, whereas in your boat you'll be cruising around 4,500 to 5,000
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RPMs and then up towards 6,000 RPMs when wide open throttle. This is important because if you spend a lot of time idling or, say, trolling
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where the engine is spending hours at 1,200 to 2,000 RPMs, this is when carbon can get built up inside the cylinder and on the valves
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The best way to describe it is to say that the engine isn't getting hot enough to burn off this carbon
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And over time, the carbon can build up on valve seeds and the valve stems and on the piston
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and this can cause issues. Things like poor running conditions and a lot higher fuel consumption rate than normal
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it can also cause long-term damage to the valves and cause the engine to need major work
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or a powerhead replacement sooner than you would expect. You can help burn this carbon off by running the engine at wide open throttle for a little bit as you come back in from the day
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And it doesn't have to be too long either, just five to ten minutes at wide open throttle will help to burn off the carbon that built up
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You can also use stuff like Mercury Quick Clean or Yamaha Ring Free every few fillups
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and that will keep the inside of the engine clean and have less carbon deposits building up
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Both are great products and work very well, and I would try and go with the OEM fuel additives
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Simply because there are so many different aftermarket products and they all have different chemical chemistries that make them up some of which can have adverse side effects that you don know about
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But back to our compression test readings, some cases of carbon buildup can cause the valves to not see properly
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and give you lower compression readings. There are also other treatments out there to burn off the carbon as well
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without having to pull the head and clean out the valves manually. Things like Yamaha's Yamaha Internal Ineons
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engine cleaner, also known as Yick, but because of how powerful this chemical is, they don't allow
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it to be sold publicly and has to be done through an authorized shop. But on older engines that
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don't have warranty, you can take that ring-free or quick clean and just mix up an external
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fuel tank with a heavier mix of the cleaner, then run a couple of gallons of that through the
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engine and it can help to burn off the carbon, bringing the compression readings back up to normal
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This must be done carefully though because you can damage the valves and the deposits can burn up parts of the valves as well
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If the readings won't come back up, then the next testing procedure that would need to be done is what is called a leakdown test
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This test is where you put the piston up there at that top dead center position on the compression stroke
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Then apply air pressure from a leakdown tester to see what percentage of the air leaks out of the cylinder
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to help determine what is going on with the engine. You can usually hear whether the air is going out of the exhaust valve
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or you can take the oil fill cap off or take the dipstick out
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and feel the air coming from the crankcase telling you that the piston rings are where the leak is
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Or you can also hear it coming from the throttle plates or air intake telling us that the intake valves are leaking
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You'll usually hear a hissing sound or you could even feel the air sometimes
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Giving you the understanding and the results to know whether or not you want to spend any of your money on this boat
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we'd like to know if you've ever had an engine blow up on you or seen it happen in the comments section below
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Don't forget to subscribe to the channel and check us. out some of our other videos here. Thanks for hanging out with us, and we look forward to seeing you
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next week
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