Advice on Compression Test - 2004 90hp Yamaha TLRC

[Jim Hawkins]

I'm looking at buying a boat with a 2004 90hp Yamaha TLRC. When I checked out the boat my Compression tester showed 90# top cylinder, 90# middle and 60# on the bottom. I assumed the motor had issues and left.

Next I looked at another boat, again a 90HP Yamaha and again off on compression.

When I got home I checked my compression tester on my boat motor and again, low compression. I fiddled with the tester and hooked it up to my air compressor. The tester showed 120# close to the same as the gauge on my compressor. I then checked my motor again and got 110# on both cylinders as it should be. So now I am doubting both my ability to do a compression check correctly as well as the heath of my tester.

Question 1 - does it matter if the engine is cold or hot when you test compression?

Question 2 - do you really keep cranking until the gauge stops climbing? (even if that means cranking for 10 seconds or more?)

Question 3 - is it wrong that 2 to 3 seconds of cranking is enough and also that you should test each cylinder the same?

Question 4 - does a compression tester need any maintenance, like a drop of oil from time to time like an air nailer tool?

Thanks for any responses.

 

[mjf55]

Jim, My thoughts:
1- I have read that the engine should be warm when taking the test. Cold tests will result in slightly lower readings.
2- I think that the pressure should stop climbing after 2-3 seconds. At least my engine does. ( If it keeps climbing, it I think that indicates an issue but not toooo sure ). I would not crank for 10 seconds straight.
3- Answered above
4- I do not know. ( I think enough gas / oil get pumped in during the test ( well oil on a 2 cycle ) I would like to know this answer.

Just my views and what I do. Hope it helps.

 

[Jim Hawkins]

Thanks mjf55, that matches this link I found for compression testing.

http://www.linnbenton.edu/auto/perform/compress.html

I will see the boat again today and try to do a proper test.

 

[rjcamel2355]

I wouldn't worry about maintenance on a compression tester.

All gauges are going to read differently, a snapon gauge will not read the same as a china made non branded gauge (USUALLY)

You're SUPPOSED to:

Warm engine up (if it's cold the reading may show a little lower)
Advance throttle completely forward
Crank the engine over (Make sure the battery is fully charged)
3-5s, 5-10s I've never noticed a difference once it climbs to a certain point that's it.

However.....I can't say I always follow those steps...

Usually I:

Engine cold
Pull plugs, screw in the tester, crank the engine, read the gauge and move to the next cylinder.
If it's even, great....If they differ more than 15% from cylinder to cylinder I walk away.


Also, I've read some engines the bottom cylinders will have lower compression (Not saying this is true, but I have read it in numerous places) I cannot tell you why, as it doesn't make much sense to me either....

 

[Jim Hawkins]

Thanks RJ,

I was just about to do a post to close this thread when I saw your reply. What you say pretty much matches what I found.

I went back and re-tested the motor. With the engine warm (instead of cold) and the throttle wide open (instead of closed) I got right at 110# on all 3 cylinders. 110 to 120 is the compression I have gotten on every used outboard I've ever owned.

So, although this is old hat to most everyone on here, for anyone searching the forum and unable to find a procedure to test compression,

Step #1 - Warm the engine to normal operating temperature.
Step #2 - Disable the Fuel and Ignition System (some say that allowing the ignition system to stay on with nowhere for the spark to go will damage the electronics. Pulling the plastic key from the emergency shutoff cut power on my motor while still allowing the motor to turn over)
Step #3 - Remove ALL spark Plugs
Step #4 - Hold the throttle plate wide open (On my motor I can disengage the shift so I can move the throttle control forward without engaging the prop. Other wise you can move the rod on the carb(s) and clamp it open. Don't forget to take it back off)
Step #5 - Install compression tester and crank engine through five compression cycles (2 to 3 seconds)
Step #6 - Note the compression.
Step #7 - Release the pressure on the gauge and test again comparing to #6
Step #8 - Repeat for other cylinders.

Note* Keep the battery full charged - if the battery is turning the motor slower by the time you get to the last cylinder your results may be skewed.

So, I bought the boat and I'm a happy camper.

 

[gm280]

I always assumed that any compression readings were merely an idea about the condition of the engine and not so much of an accurate reading. I guess what I mean is unless you have your compression gauge calibrated before using it, it is merely a close guess and not an absolute reading. BUT, using that same gauge on all cylinders gives you an idea if the cylinders are close to each other and toward the higher end. And that is probably the reason you can't find a compression reading in any OB shop manual. It is a vague reading at best and nothing absolute. It is used to rule out a bad cylinder or cylinders and not much of anything else. If a compression reading is too low, and again I don't know where that cut off reading is, it can tell you that either the engine is shot, or the gauge is bad. So it does have its use, but certainly not a perfect absolute reading by any stretch. The speed the engine turns over can easily effect the reading. And that is why pull start engines read different (usually lower) then starter cranked engines. And that is also why you want your battery fully charged as well. . JMHO