Performing Compression Tests

[EuropeanAM]

When performing compression tests, it's imperitive that the tests be done with:

-A fully charged and PROPERLY RATED cranking battery
-Engine warmed up to Operating Temperature
-Kill switch OFF
-All Spark Plugs Removed
-Plug wires on Spark Tester (or grounded with jumper leads- an added precaution)
-Throttle plate at the WOT Position
-Water supplied to the engine while cranking!

Insert compression tester into #1 spark plug opening, and crank the engine until the reading no longer rises. Record reading. DO NOT spin/crank for longer than 10 seconds at a time.

Repeat above step for each cylinder.

(This post is being added to the Commonly Asked- Commonly Used Sticky for future time-saving reference).

[Nikon Man]

Great "FYI" post Don. This info as found in the manuals was always somewhat open to interpretation when read directly from the Merc manuals, but now you've put it into easy to understand terms, plain and simple........

[Oldgeezer]

When performing compression tests, it's imperitive that the tests be done with:

-A fully charged and PROPERLY RATED cranking battery
-Engine warmed up to Operating Temperature
-Kill switch OFF
-All Spark Plugs Removed
-Plug wires on Spark Tester (or grounded with jumper leads- an added precaution)
-Throttle plate at the WOT Position.

Insert compression tester into #1 spark plug opening, and crank the engine until the reading no longer rises. Record reading.

Repeat above step for each cylinder.

(This post is being added to the Commonly Asked- Commonly Used Sticky for future time-saving reference).

Just curious. What was the most common "error/errors" that would lead to incorrect measurements? How much variation from "reality" would most see if not performed properly? Thanks for the specific method and order of importance. Is this "new" from Mercury or reiteration of what we should have been doing all along?

[Nikon Man]

Common errors include;
1. Not having the throttle blades wide open
2. Motor not warmed up (this will really change the numbers as the piston & ring seal is not at it's tightest thereby increasing blow by)
3. Plug wires grounded & kill switch off (safety more than anything)
4. A battery not "FULLY CHARGED" or a battery of less than required output/capacity, this obviously reduces the power to propley spin the motor to get the compression readings.

This is not a new method, just one that should be followed to the letter for accurate readings. Also remember that a compression test is only one part of a method of test for a motors health, the other is a "Leak Down Test", both these work hand in hand to determine a motors health.

[EuropeanAM]

How much variation from "reality" would most see if not performed properly?

+1 on NikonMan's comments above.

I have seen several instances where improper methods resulted in more than a 50% variance in ACTUAL compression readings (ie: average compression reading number, taken with the SAME GAUGE improved by 50% when performed PROPERLY).

Although that's certainly an extreme case.... there's no substitute for performing the tests correctly.

[bassburch]

-Kill switch OFF
-Plug wires on Spark Tester (or grounded with jumper leads- an added precaution)

I thought pulling the kill switch disables spark to the cylinders... making it unnecessary to ground. Is this not the case?

[EuropeanAM]

I thought pulling the kill switch disables spark to the cylinders... making it unnecessary to ground. Is this not the case?

Good question. Under NORMAL circumstances, you are correct.

However.... kill (lanyard) switches DO sometimes fail. Should this occur, there is a potential that spark might occur over/near the spark plug openings. Hence the "added precaution": SAFETY FIRST.

I once worked with a tech that found out the hard way a kill switch had failed... let's just say it raised his eyebrows (for a couple weeks).

[fnsfool]

That was a fast response! Thank You!
Battery rating?
Mine is a group 24 625 marine cranking 500 cold cranking and test was done as you listed.

[EuropeanAM]

That was a fast response! Thank You!
Battery rating?
Mine is a group 24 625 marine cranking 500 cold cranking and test was done as you listed.

ENGINE ALONE requires 800 CCA or 1000 MCA.

See the following list for "batteries that meet the requirements" for fishing applications:

http://www.bbcboards.net/mercury-3-l...ment-just.html

[logan bryan]

I've been told by a mercury marine dealer that you only take one spark plug at a time to get accurate reading. He said know body knows why the pressure readings are different than taking all 6 out at a time.

[Nikon Man]

I've been told by a mercury marine dealer that you only take one spark plug at a time to get accurate reading. He said know body knows why the pressure readings are different than taking all 6 out at a time.

You have to consider the spin rate of the motor with all 6 out is faster than with just one out, it doesn't amount to a big difference, but it does make a difference......

[EuropeanAM]

You have to consider the spin rate of the motor with all 6 out is faster than with just one out, it doesn't amount to a big difference, but it does make a difference......

+1... and the higher the compression (the bigger the difference).

Actually had a tech come to me on a 4-stroke a couple years ago with compression numbers in the 80-90 psi range.

Watched him "recheck" compression PROPERLY.... 160-168 psi on ALL cylinders.

[thomasfouraker]

Just curious, when performing basic diagnostics on a motor that will not start or run long enough to achieve operating temperature, I would assume the compression test could only be performed when COLD.

Will this still produce an acceptable reading?

Thanks!

Thomas

[Nikon Man]

Thomas,
It will produce a useable reading, not the one stated in your manual, but rather a lower number. On a cold motor you would only be able to gage the balance of all cylinders when cold to each other and not necessarily their actual compression ratio, you'll only get that from a motor warmed up to operational temps...."true compression ratio". Cold or warm they should all run close to each other.

[EuropeanAM]

To add to Gary's above comments:

What's most critical on these engines is the tolerances within the combustion chamber when the engine is WARM (up to operating temperature). THAT is the condition under which the engine is expected to run the majority of the time...

A warm engine's sleeves/pistons/rings have expanded to their "running" position, providing the most consistent and accurate readings.

[Droptop5.0]

Great stuff here and thank you for putting it together. I have a truckload of motor parts and I'm a NOOB. This might not be the best move I've ever made but I figure I have all winter to educate myself. So my question is can I to a compression test on this motor?
IMG_2474.jpg
I was told this was a good powerhead so I need to start somewhere and go through this thing. Seems like a compression test isn't a bad place to start. Serial Number is A163804. Thank you.

[EuropeanAM]

Great stuff here and thank you for putting it together. I have a truckload of motor parts and I'm a NOOB. This might not be the best move I've ever made but I figure I have all winter to educate myself. So my question is can I to a compression test on this motor?
IMG_2474.jpg
I was told this was a good powerhead so I need to start somewhere and go through this thing. Seems like a compression test isn't a bad place to start. Serial Number is A163804. Thank you.

Good topic for a separate post. You could certainly perform a compression test on that one- but give the fact it's not mounted (or rigged)... it will be very difficult to get it to operating temp.

Might also consider a LEAKDOWN test as another (viable) option.

[catfish123]

I have recently found out the hard way that a leakdown test is far more important on high performance outboards than a compression test. I will NEVER waste my time again with a compression test because compression numbers may be excellent when leakdown numbers are bad, so bad in fact that bearings are shot, pistons are scuffed, and rings are needed.

[EuropeanAM]

I have recently found out the hard way that a leakdown test is far more important on high performance outboards than a compression test. I will NEVER waste my time again with a compression test because compression numbers may be excellent when leakdown numbers are bad, so bad in fact that bearings are shot, pistons are scuffed, and rings are needed.

I would definitely have to agree. A leakdown test is a FAR better indicator of the CYLINDER'S health (cylinder, piston, and rings).

Just to clarify, it gives very little information on the condition of bearings.

[catfish123]

I agree, although more than likely, when pistons are scuffed, rings are in bad shape, there may be a chance that bearing failure caused those issues.