How to Compression Test an Outboard Motor | BoatUS
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How to Compression Test an Outboard Motor | BoatUS

November 8, 2019

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. And of course, don’t forget to subscribe to
the BoatUS YouTube channel. Thanks a lot!


  • Reply MarlonM June 23, 2018 at 1:00 am

    Hi.. Thank you for the informative video. I learned a lot especially the part of removing all spark plugs before performing the compression test. I do however, have a question regarding wear and tear and life of the motor. I know the first reading is the benchmark, but how do I know that reading is within acceptable levels? Thank you!

  • Reply Jimbo Bojim June 23, 2018 at 6:12 pm

    Can anyone out there tell me WHY you need a boat? One reason? Anyone? Your hobby of polluting our freshwater supplies is not more important than my grandchildrens future! Private ownership of boats should be banned.

    Bernie Sanders 2020!

  • Reply Gary Bartold July 19, 2018 at 2:05 am

    Do you need to disable the ignition circuit when performing a compression test?

  • Reply PACOLOCO559 September 17, 2018 at 4:57 am

    Hi I have a 2001 90 hp mercury. Last oil change was 2 years ago I do a lot of winter trolling. Last time I checked my oil level it was a half inch high. When I drained the oil milky water came out of the crank case. I would say no more than 6 ounces. I've read that these motors build up internal condensation. Before I confirm this I want do a compression test. Have you ever seen this problem before? This is my first time servicing this motor. I usually take it to a shop.

  • Reply Perry Matherne October 1, 2018 at 9:02 am

    Hey Lenny, where can I purchase a compression tester for my outboard?

  • Reply taz man March 29, 2019 at 10:48 am

    You stumped him a few times there..

  • Reply Doug Webre June 7, 2019 at 12:54 pm


  • Reply mikey joe August 29, 2019 at 2:38 am

    Turn the motor over five revolutions for each cylinder as well. Very important

  • Reply Sam Sam September 6, 2019 at 10:26 pm

    Tnx for the informative video ,I have a question plz, whats the normal pressure for yamaha 85hp mix oil and fuel engine and is there a difference in the air pressure gage if is stander engine or 50 fixed or 25 fixed ?👋

  • Reply Jeretta Jennings September 24, 2019 at 10:37 pm

    What should a25 horse mercery 4 stroke have for compression

  • Reply BANFISHING October 12, 2019 at 2:42 am

    nice video @BoatUS i am thinking about going to school for marine mechanics might as well since i love fishing so much

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