Hey there, folks.
Lenny Rudow here for BoatUS Magazine.
Today we're going to talk compression checks.
Everybody hears about a compression check and everyone knows that when you buy a boat
with a used outboard on it, you should get a compression check.
Well, why is that?
We have outboard technician Sean Stahl here with us today to tell us.
Why should you get a compression test?
You want to get a compression test because you basically want to see how much life the
engine still has, you want to see how much wear is on the piston and the piston rings,
and you want to see how much air is being pushed through the engine.
The engine is a giant air pump, is what it is.
And how does the compression test tell you?
What do you look for during the test?
What you're looking for is, you're looking at the PSI in each cylinder, so if one cylinder
is 120, you want to go off that cylinder and base it off -- you want them to be within
10 percent of one another.
Ah!
So every cylinder should have about 10 percent of the same amount of pressure.
Correct.
So if one cylinder is at 120, all the others cylinders should be relatively between 110
PSI to 120 PSI.
Gotcha.
OK, Sean, let's get started.
Show me what to do.
Alright.
So the first thing we're going to do is pull off this cover right here.
It's pretty easy ... sometimes.
Alright.
Then you've got four 10 mm bolts.
Take those out.
And of course different engines will have different access points to get to the plugs.
Correct.
And we're doing this on a ... ? This is a 175 Yamaha inline four.
Now they're out of the way.
The next thing we're going to want to do is pull the plugs.
And the plugs are usually a five-eighths socket.
So even though you only compression test one cylinder at a time, you pull all the plugs
out right from the start.
Correct.
And why is that?
It's just so the engine doesn't fire.
It allows the maximum amount of air to get into the engine.
Like I said before, the engine is a giant air pump.
You want to get as much air into the engine as possible.
That way you get a good accurate reading.
I'd imagine it would be quite a mess if it actually fired off.
Now what about fuel?
Isn't fuel flowing into the cylinders while you do that?
That is correct.
Usually what I normally do is on the side of the engine there's a 15-amp fuse for the
fuel pump.
I'll take the 15-amp fuse out to keep the fuel pump from pumping fuel.
Gotcha.
So there's no fuel actually moving through the motor when you're doing this?
Correct.
If you get fuel into the cylinders, it will wash out your readings.
They'll vary.
So this is your gauge here.
What are we looking at?
Alright, so this is a compression gauge right here.
As you can see it has your PSI readings on here, ranging from 25 all the way to 300.
When we're doing the compression test, it's just going to trap the air inside and give
you a good, accurate reading on how much air is being pushed through the cylinder.
And how does that attach into where the spark plug was.
So, the way it attaches is you have this piece right here.
This is the end ... it almost looks like an air hose end to it.
This end will go in here, and this end will actually go into the cylinder where the spark
plug is.
Let's do it up!
Let's see it!
So, usually what I do is I leave the end off first, then I'll twist this in to the cylinder
until it's tight.
Then just clip this on.
Excellent!
Leave it sitting there, and then I would do my compression test.
[SOUND OF ENGINE TRYING TO TURN OVER] Good!
And that cylinder is at 200.
[SOUND OF ENGINE TRYING TO TURN OVER] Good!
So, that's our compression test.
And we should mention that before you do it, the engine has to be?
Hot.
It should be warmed up ahead of time.
Correct.
So what did we learn?
All four cylinders are at 200 PSI, so they're definitely well within 10 percent of one another,
and this engine's got a lot of life left.
Excellent, which is no shocker since this is a brand new motor.
Correct.
Well, folks, we hope you've enjoyed this video, and we hope you'll leave any comments you
might have in the spot below.
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Thanks a lot!