Correct procedure for compression test

[MickLovin]

Hi Guys, I am a little concerned I haven't done my last compression test correctly. I only got 75psi on the top two cylinders, which had 125 psi when I bought the motor. The motor runs and starts fine which makes me suspicious on my latest compression findings. Is there a special way to do compression. I still had the fuel lines connected when I did it last which I wouldn't think was right.

 

[Big Fish Billy]

Re: Correct procedure for compression test

I wouldn't worry about the fuel lines. When the engine is cold, take out all the spark plugs and turn the engine over a couple times. Hook up your gauge and turn the engine over so the cylinder compresses three or four times (as long as you do them all the same), take & record your reading and watch to make sure your gauge is holding at pressure. ( some leak back thus effecting your reading, if you find yours is leaking, use a little WD-40 on the release valve until it holds pressure) Move on until all cylinders are read and recorded. Put it back together and run it up to operating temperature, and repeat the process. The hope is that all cylinders are within 10-15% of each other. (A 5 or 10 pound difference is nice, but if a high cylinder was 130, a low of 110 is acceptable) If not then the troubleshooting begins, worn rings, head gasket, etc....if an engine has been sitting a while, it is possible rings are stuck, giving a low reading, sometimes running up to temp a few times or decarbing may free them up. In your situation I would suspect/hope your gauge is leaking back.

 

[pnwboat]

Re: Correct procedure for compression test

Nothing really special. I know that some folks say that the throttle handle has to be set to WOT, but I've tried it WOT and at idle but not seen much difference. Make sure the fitting that goes into the spark plug hole is seated securely and not leaking. Also there should be a one way schrader valve in the end of the hose similar to what you see in pneumatic tires. Make sure it's not leaking or sticking.

 

[Frank Acampora]

Re: Correct procedure for compression test

Since each cylinder goes to atmospheric pressure at bottom dead center, there is no need to have the throttle in any particular position. Oh, the argument could be made that having the throttle closed will cause crankcase vacuum and slow the cranking speed, but really, there is almost no change in reading.

Just be certain all plugs are removed and grounded (so you don't damage ignition components if you use the key for cranking) and be certain that you crank at least four revolutions or until the gauge max's out

 

[MickLovin]

Re: Correct procedure for compression test

ok next question before I do another test, would it run so good if it was 75psi in two cylinders, it doesn't make sense as it starts up now after all adjustments First time and runs fine, I was getting 45MPH last run. I think I will buy a new compression tester, I don't trust someone elses.
Thanks Frank for reminding me to earth the plugs, I did earth them last test but I don't trust the gauge now, it,s not mine and and I haven't tested it on anything but my motor. I know they were all 125 except the bottom at 123

 

[Frank Acampora]

Re: Correct procedure for compression test

Tell you a story Mick: About ten yeas ago I had a 125 that one day locked up completely for no particular reason and then started right back up. It ran fine for the rest of the season. Near the end of the season I broke a prop shaft and for some reason which I don't remember I also tore into the engine. I think it was because of low compression. I found two cylinders with the top rings broken. The engine started and ran well. I don't remember if it ran at top efficiency and power though. Does this answer your question? maybe--maybe not.

 

[MickLovin]

Re: Correct procedure for compression test

ooo could do, I was hoping to re-do the compression and find it ok Hoping wishing wanting. If i was a mechanic instead of a bloody electrician, I'd be chomping at the bit to pull it apart and check it out, but to be honest I am scared LOL. Really scared lolz, I have a motor which is running fine and don't want to play mechanic anymore. You will be hearing from me soon when I recheck comp with my own gauge, and if it is low, you will probably here me weeping from America, for that matter Canada too :facepalm:
How bout I give you a plane ticket to Australia, ya bring a few tools with you and your missus and you come stay with us and visit the lovely Byron Bay and tweed heads areas.
By the way you are scaring me :grumpy:
I am definately not an internal engine guy 'sad face'

 

[MickLovin]

Re: Correct procedure for compression test

I will give you an idea of my mechanical skill, the first time I did a compression test on my 75hp Chrysler, I nearly stuffed it by using the wrong attachment on the gauge which was too long and hit the cylinder head , it's not always that bad I am a lot more maticulous now re-checking everything I do. I heard if you hit an outboard mechanic over the head with a lump of wood he will turn into an electrician, only rumours at this stage

 

[jerryjerry05]

Re: Correct procedure for compression test

Get another gauge.Don't get a cheap one.
If the compression was really at 75 on 2 cylinders it probably wouldn't turn over real well.
Something about the comp being uneven and making it hard to turn.

 

[MickLovin]

Re: Correct procedure for compression test

yeah Jerryjerry but I just was talking to a bloke who had an older 105hp and he had low compression on the top two cylinders and it is still running ok. All in all I need my own compression tester.

 

[Big Fish Billy]

Re: Correct procedure for compression test

Well here's an old Irish jingle that may apply so close to St Patty's day....if me aunty had balls, she'd be me uncle....ooops, no, that isn't it...."Mickey, me boy, if you don't want to have problems putting something back together...don't f*****g take it apart"....there we go now.... http://www.youtube.com/watch?v=q2fizeoT22g

 

[MickLovin]

Re: Correct procedure for compression test

LOL Big Fish Billy, nice vid mate, I won't be taking it apart untill the it blows up on me, other than that I will be keeping an eye on it and re-checking my comp. Love your straight forward approach Billy you sure your not an Aussie

 

[Big Fish Billy]

Re: Correct procedure for compression test

It's all in fun, if it ain't broke don't fix it.........

 

[Michael Lang]

Re: Correct procedure for compression test

Just have to cut in to relate my experiences of tearing into engines hoping to find a problem and only ending up spending more money, head gaskets are quite expensive now, I bought a used 70 hp force last year and refuse to do a compression test, the motor starts quickly and runs good, so from I am learning from this forum (leave it alone). I not talking about preventative maintenance.

 

[MickLovin]

Re: Correct procedure for compression test

Don't worry Michael, I am not a tear it down put it back together guy, I can do anything with the motor to have it running properly, but when it comes to internals, I leave it alone.

 

[jerryjerry05]

Re: Correct procedure for compression test

A compression test is a tool.
You use tools to fix/repair.
I do a comp test to check on my motors.
The results will tell me if there is anything starting to go wrong.
It will start and run fine with low comp,for a while.
Then it hits the point where it won't run or start right.
By then it might be too late.
You don't have to replace the head gasket to do a comp test.

 

[MickLovin]

Re: Correct procedure for compression test

So from what you are saying Jerryjerry, is that if I find low readings it is time to do a repair before it is too late.

 

[jerryjerry05]

Re: Correct procedure for compression test

Yes.
I had a problem (new boat)after about a year of running.
The port motor wouldn't start.
The rings were carboned after using the TCW-1 oil.
Turns out the oil was actually designed to carbon up.
I did a lot of slow trolling back then.
Did a comp test and then removed the head.
No damage to the cylinders or piston.
Replaced the rings(both motors), then they changed the oil to TCW-2 then 3 and haven't had a problem with carbon since.