How to check compression on 1988 150 black max

[saltfisher]

I been keeping an eye on a boat i looked at last week, and it scared me when he said the motor smokes on startup for a few minutes untill warm. So im going to check it out again. Its a 1988 merc black max 150 horsepower. V6. I have a compression tester for a car if that will work? Or is there a marine compression tester for boat motors i need to get? Also its been ahwile since i did a compression test on a car, but is it the same as a boat motor? Basiclly unplug the coil so the motor doesnt run, and just pull one sparkplug at a time and replace it with the compression tester? Then does it matter how many times it cranks over? or should i count a X amount number of cranks for each cylinder? Also what PSI am i looking for, and whats within reason as far as compression diferences between cylinders? Anything to watch out for?

Thanks

 

[Barnacle_Bill]

Re: How to check compression on 1988 150 black max

A little smoke is normal. Especially on a Merc. LOL An automotive compression tester will work just fine. Pull all the plugs and trip the emergency kill switch. That way you won't have any spark. How high the compression is isn't as important as the difference between cylinders. 10% difference or less is good. Just guessing but you'll probably see around 110-120 lbs.

 

[saltfisher]

Re: How to check compression on 1988 150 black max

Yeah i was told some smoke is normal also. he said it smokes pretty good untill it warms up (he described it as looking like camp fire smoke for 2-3 minutes), then when the engine is running, or at idle after warm it doesnt smoke hardly at all, maybe a little bit. I figure it may be a easy fix if the compression is good, or may just be the nature of the engine. Its clean as a whistle inside and out. I heard of people saying about check valves going bad and sticking making the motors smoke more then normal. I guess they are located in the fuel/oil lines? i dont know.

Also, how long should i crank the motor when doing the compression test? Just crank untill the needle stops on the guage?

 

[Barnacle_Bill]

Re: How to check compression on 1988 150 black max

I've always let each cylinder cycle 10 times. May not need that much but its always worked for me.

 

[Windykid]

Re: How to check compression on 1988 150 black max

Its going to smoke on start up. When at idle, cylinders will cut off and oil will mix with the exhaust. Makes a lot of smoke. Just worry about the compression test.

 

[corrado4]

Re: How to check compression on 1988 150 black max

I check in my manual which I just bought for the boat I purchased wich also has a black max 150 and it says there should be no more then 15psi differece in the compression. Basically the difference shouldn't be more then 15psi.

Oh and mine smokes a bit at start up then does go away. But I also have old gas and oil which is getting removed asap and replace with fresh fuel and oil.

 

[saltfisher]

Re: How to check compression on 1988 150 black max

So im guessing even if it does smoke quite a bit maybe a little more then usuall, and compression tests out ok, its nothing to worry about, or something minor?