How To Accurately do an Outboard Compression Test

Why Compression Test an Outboard Engine

There are several reasons you may want to compression test an outboard engine. If you notice the engine is running rough (especially at idle) is one. If you are buying a used boat than certainly performing a compression test is definitely in order.

 Don’t make a decision on purchasing based on a compression test alone. There are a number of things that can point to a bad engine. Reed problems, carburetors, seals and lower units are just a few of the things that can go wrong and potentially be costly to repair.

If you are going to compression test an outboard to decide on a purchase, make sure you take that boat out on open water to take it through a trial run. Check the condition of the lower unit oil as well. Also make sure that there’s a healthy stream of water coming from the engine that would indicate the impeller is in decent shape. Having said that, here’s how to compression test an outboard engine.

Types of Compression Tests

There are two basic ways to compression test an outboard. The first is called a leak down test. This uses a special set of gauges. To perform a leak down test, you would pressurize the cylinder with compressed air. One gauge will tell you the pressure of the air compressing the cylinder. The second gauge will show you how much pressure has been lost. With this method the cylinder has to be TDC or top dead center in order to get a correct reading.

The second method to compression test an outboard uses a single gauge. With this method, you’ll be actively measuring the actual compression of each cylinder as the engine is turned over. My preference is to use the single gauge method. The equipment to do this is relatively inexpensive and easier to perform this test, which is what I’ll be describing here.


Single Gauge Compression Test Kit

How To Correctly Compression Test an Outboard Engine

Performing a compression test on an outboard engine is fairly simple. However, I’ve seen a lot of misinformation on this topic. The most common being to just pull out the plugs and do a pressure test on a cold engine with out any prep work. I have done comparison compression test on several engines In the past. The results are always interesting. I have seen a difference between 15 at 30 psi between a cold compression test and one that is done correctly.

What is a Correct Compression Test

There are a few things that need to be done to compression test an outboard  correctly. First, get the engine to operating temperature. The pistons and rings need to expand to their normal size when the engine is at it’s operating temperature. Remove all the spark plugs. Removing the plugs allows pistons to move freely without any resistance which could affect the compression test results.
Before inserting the compression test adapter into the cylinder spark plug, make sure the O ring on the adapter is in place.  I once ran a compression test with a missing o-ring and the results were pretty bad. I almost made a call to scrap or rebuild based on the compression readings I got. Once I corrected the problem I got much better numbers.
Johnson - Grounded-Sparg-Plugs.jpg
Grounded Spark Plugs

I don’t like cranking the engine with the coils not connected to ground through a spark plug. There’s a possibility of burning out a coil that way. I typically keep the spark plugs in the boot and make sure the tip of the plug is in contact with a grounding point on the engine. I use electric fencing wire to wrap around the plugs and ground the wire. It’s a lot easier than trying to hold the tips against a grounding point

Taking a Reading with a Compression Gauge

Johnson - Holding-Throttle-Wide-Open.jpg
Holding Throttle Wide Open

After the engine has run and is at normal operating temperature, insert the compression tester into the spark plug hole in any of the cylinders.  When you are ready to begin the test,  hold the throttle wide open. In this case the control for this engine is older and would engage the gears by using the control to open the throttle. So I held the throttle open at the carburetors. Again doing this allows a free flow of air so there’s no resistance that might affect the compression test.

Use the Correct Adapter

There are several adapters in a compression test kit. Two of them (with the tubes in the picture above) are to hold directly against the cylinder hole where the spark plug was. The other four are to replace various size spark plugs. Use the correct size adapter to screw into the spark plug hole. With this kit you screw the hose into the adapter. There is a quick connect on the other end that snaps into the gauge. Snap the gauge on and you are ready to test.

Ready to Compression Test
Ready to Compression Test

Once everything is ready it’s really a simple matter of cranking the engine. As you turn the engine over you’ll see the needle on the compression gauge start to climb and eventually settle on a number. It may take 4 to 6 continuous turns of the engine for the pressure gauge to give you a reading

Reset the compression gauge and repeat.  I prefer to do this at least three times per cylinder. You’re reading should be Fairly consistent the same cylinder. Make a note of the various reading As you go along. Calculate and record the average PSI  for that Cylinder, we’ll use that number later. Go on to the next seller at repeat this process. Complete all of the cylinders on the engine.

What Compression Results are Normal

What are considered good results when you compression test an outboard? I’ve seen a few differing opinions on this. The readings for your cylinders should be better than 100 psi. Even that is fairly low and a sign that there’s a lot of wear on the engine. My preference is 120 PSI or better. Another important measurement is the PSI difference between the cylinders. There should not be more than a 10% difference. In the case of this engine, cylinder #1 read 120 PSI. Cylinder #2 gave a reading of 115 PSI.  Using the #1 cylinder as the base gives us a difference of 4.2%. 120-115=5, 5/120=.0416. This engine is acceptable under that criteria. I’ll run this engine on the Boston Whaler 13 restoration i’m working on and see what it does.

What Causes Poor Compression

There are several causes of bad compression on an outboard engine. The first and most obvious would be the head gasket. An engine with a blown head gasket probably wont run. If it does, it will probably sound like an artillery group letting loose. This is probably not common and if there is a blown head gasket you are looking at a rebuild or replace. Wear and tear on an old engine will cause a low compression situation.

Wear and Tear on the Cylinder 

The cylinder walls and pistons will wear over time. This is especially true if the engine has overheated or had a bad mixture of oil to gas. The rings on the cylinder will also wear. Under normal operating temperatures, the rings will expand and make contact with the cylinder walls.The rings can however also be malformed or wavy. This can cause “blow-by” or in other words, compression loss.

Piston Ring Wear

The rings should be able to move freely in the grooves on the piston. This enables the piston to maintain pressure during the compression cycle. What is probably the most common cause of poor compression is carbon buildup. Carbon build up on the rings can cause them to stick. If the rings stick, normal expansion is compromised as is the compression for that cylinder.


What if the Compression Test Fails?

Then you’re out of luck. Not really. I’ve been able to get some life back in older engines. I had an 06 e-tec (one of the best two cycle outboards ever made in my opinion) that hadn’t been well cared for in the past. I say that because when I got the engine it had regular TW-3 oil but was configured to run the Evinrude Synthetic (which typically runs at a 100:1 ratio). In any event, a cold compression test gave me 90-65-90 across all three cylinders. A hot engine with WOT gave me 120-65-120. I thought it was ready for a rebuild.


Fogging The Outboard

Instead, I gave it a good fogging with sea foam and repeated the fogging every week or so. The next time I took it out I ran it wide open throttle for five minutes. The engine ran fine for a while and then quit. It was completely dead. I trailered it back home and for lack of anything else to do at the moment, I fogged it again. I gave it a couple of days and tried to start it again to no avail. So I repeated the fogging. About the 3rd time I did this the engine came back to life! I ran it on the muffs for a while and then took it to the lake. That engine ran like a sewing machine.

Restoring Compression

When I got it back home I ran a compression test again (a correct one) and got 125-115-125 out of it. My guess is that there was enough carbon on the rings that caused them to stick. More likely than not, the carbon broke loose and gummed up the works somewhere along the line. So there is possibility of bringing an old motor back to life. Sea foam isn’t terribly expensive.  I think its certainly worth the effort of repeatedly fogging an engine with bad numbers. It’s definitely cheaper than a new engine or rebuilding a power head!

Will an Outboard Run With Bad Compression

Probably but not guaranteed. If it runs, it will probably run rough, especially at idle. There’s not much you can do if your engine has bad compression. You can live with it until it dies. Or, you can go for a power-head rebuild. If the budget allows for it, a replacement engine is also an option. If its a rough running outboard because of compression problems, it’s not trustworthy. There’s nothing worse than being stuck on a lake, or worse, in the ocean with a dead engine. I would definitely look to replace or repair an outboard with these problems.


Automotive Note

The procedure described here will work with automobiles as well. The key difference will be in grounding the spark plugs. That won’t be necessary in a car. For an older vehicle, simply disconnect the output from the coil going to the distributor. For a newer vehicle with coil overs, disconnect the low voltage leads going to the coil. In some vehicles, there is a single connector going to all of the coils on that side of the engine, making it even easier.

If it’s within your budget, I would recommend this compression tester, it’s a better grade and consequently provides better accuracy and consistency. Otherwise, the less inexpensive one should work fine.

Here’s an affiliate link to the one I used in the article.

Watch the YouTube Version of this Article Here: