Need education

[almost retired again]

I have been to all the schools that OMC had to offer over the years,and several Mercury/mercuser schools. Nowhere have I ether seen or heard of anyone doing a leak-down test on an outboard until I started visiting this board. I didn't think that it could be done because the pressure would exit through the exaust,and not be trapped in the cylinder,or crankcase. I have preformed this test several times on 4 strokes,but never on an outboard. Would someone please explain how this test is performed,and what to look for.

 

[G. Patton]

Re: Need education

Almost Retired Again..... As you, I've been a serious minded OMC technicial since the early 1960s, and I've never heard of this leak down test (on a 2 cycle) that's been mentioned. Frankly I think that it has something to do with Greek mythology <grin>.

 

[petryshyn]

Re: Need education

I've never done it on a 2-stroke, but I think it has potential. As any test, the leakage test arms us with SOME information. How we disect this information, adds to our troubleshooting ability.<br />Leakage test on a 2-stroke would be handy. If you do not have a definate compression spec, you can still tell if an engine has a potential problem (carboned up or tired)<br />Some engines have high compression when new, some don't. If an older engine had the compression of a newer looper, it may mean its worn out.(75-85psi etc.) <br />If you had a one cylinder engine that had 75 psi compression. Is this ok, or is it bad? You don't have another cylinder to compare to, and maybe no spec sheet. The leakage test would determine this for you. (25% or less leakage is OK) Leakage can mean alot.<br />If you checked an engine and found 120psi, you may be inclined to buy it. If you continued with a leakage test and found 40% leakage, would you still buy it? (maybe 140psi was the norm)<br /><br />When doing a leak test, the piston is at TDC. Any leakage will pass the rings and exit the exhaust ports. You are measuring the amount leaking by...<br /><br />While no "one test" is conclusive in engine condition, the leakage test will atleast give you an indication of cylinder to ring condition at TDC. Every bit of info helps.....<br /><br />(I hope this helped)

 

[G DANE]

Re: Need education

Its easy to see the benefits of a leakdown test, cranckcase leaks, bad axelseals, reeds failures, piston wear etc, but how do you perform it.<br /><br />You can access the belowpiston-chamber on one cylinder by removing fuel pump, but how about the others ?? How do you do this test, and with what equipment ??

 

[almost retired again]

Re: Need education

Thanks Schematic;<br />Now I understand what happening,yes I can see the advantage at TDC. How much pressure do you use,and how do you hold the cylinder under test at TDC while performing this?<br />Thanks all information helps,that's why I'm on this board

 

[petryshyn]

Re: Need education

Hi again<br />To do a leakage test, you need a special tester. Lots out there....I use a snap-on. You could make your own if you had one to calibrate to. It consists of a pressure regulator and an orfice. The gauge is calibrated in "percent of leakage" 0-100%. >Although you use shop air, the regulator and orfice reduce the pressure substantially.<br />>first you calibrate the meter to zero leakage.<br />>then position the piston to TDC. This must be right on TDC. As you suspected, if you're not bang on, the crank will rotate. You actually must center the crank position to the "piston rock" position. (the crank has some rotation near TDC where it does not affect piston position) You could also use a dial gauge on the top of piston to find TDC.(mark the flywheel at .020 BTD and mark it at .020 ATC. center the flywheel between marks....voila...TDC)Air pressure will not rotate the crank if you are BANG on....<br />>install tester hose in plug hole<br />>gauge reads % of leakage<br />>if I remember right, 20-25% is acceptable. After checking a few, you'll get to know whats normal.<br /><br />G Dane<br />Checking for crankcase leaks (reeds, seals, crankcase sealant) is dam hard to do, especially on an outboard. To do it using pressure, one must completely block off the exhaust ports.(very hard to do unless the powerhead is removed) We use a smoke generator on small engines. It works very well. It applies high density smoke under low pressure that will show up the smallest leak.(quite impressive) I've used it on chainsaws and weedeaters, but not on marine.(too hard to plug off exhaust, but is possible) <br />In most cases, leaking crankcase and/or seals, leave an oily mess on the outside area of the leak. Thats the obvious clue. If you suspect a crankcase leak, simply spray some WD-40(new or old) over the area. if it affects the running of the engine, its leaking.<br /> Reeds spit back at you through the carbs and can make a mess in the air box. <br /><br />

 

[MGuckin]

Re: Need education

Question? Isn't this test basically testing the integrity of the combustion chamber?

 

[MGuckin]

Re: Need education

Integrity probably wrong word. Testing the ability to seal the chamber at a given pressure for a given time? (blow-by)Would test rings, seals and head gaskets correct? If the testing pressure was known, couldn't you just pump up the cylider at TDC and monitor the pressure drop? Why the need for a special tester?

 

[petryshyn]

Re: Need education

You would be surprised how fast a normal cylinder leaks down. If you applied 100psi, it would leak down in a couple three seconds. No way to accurately gauge or compare it to other cylinders. The leakage tester is actually a flow meter. It measures the constant leakage and gives it out in a very precise percentage.

 

[MGuckin]

Re: Need education

Understood and agreed but how is the constant or calibrated reference point and loss time frame determined?

 

[petryshyn]

Re: Need education

There is no time issue. A regulated pressure is applied to an orifice. The pressure on the other side of the orifice will depend on the amount of pressure that is allowed to escape to atmosphere. That amount of pressure read is a direct relation to leakage by the rings......thats why the pressure gauge is marked in percent..... <br /><br />

<br />When the gauge reads zero percent, it actually is reading maximum pressure on the gauge.....<br /><br />

 

[MGuckin]

Re: Need education

Ok I understand.<br />Thanks,<br />Mike<br />

 

[novacane]

Re: Need education

you guys killin yourselfs over noithin leak down test are great for a 4 stroke valve engine i used<br />1 from time to time its a snap-on but in general i would recomend not doing this test on a 2 stroke<br />first off, engine cranks and runs smooth accerlates fine no abnormal noises whats the point<br />, if you supect a worn cylinder on a single cyl<br />engine bet i can rip off the head quicker than you guys can do a CORRECT leak down test and lets face it a picture tells a thousand words as im no 2 stroke tech only A.S.E MASTER AUTOTECH <br />i do understand theary of engines the way i see it alot of guys and i could be wrong may not get the<br />piston @tdc as some one said theres appox 10 degress of crank rotation , piston dwell as in most engines very slighty from engine to engine<br />so to get that perfect a dial indicator insterted through spark plug hole would be only real accuate way to check piston dwell @tdc hmm just a thought leave the leak down test to the valved motors<br />if memory serves me correct on a 4 stroke 20 percent is normal thinking a a 2 stroke would be less 3 sealing rings vers 2 and no valves to leak by hmm

 

[MGuckin]

Re: Need education

Hopefuuly I won't ruffle too many feathers but I gotta say this.<br />novacane,<br />I was curious, I have never done this test on any motor. Never redone a 2 stroke but have built numerous motors for automotive racing applications. If I can learn anything, at all, that seems new to me or I do not understand, I will ask for clarity and if somebody can help, that is great and if not that is ok also. Sorry, I am not an A.S.E MASTER AUTOTECH, just a flunky backyard mechanic that is trying to learn about outboards and has been getting alot of help from here and hopefully I may have helped a few. Not always correct but I try and don't knock others for questions. Incedently, gonna toot my horn, as I said I'm not certified, but, I have built and race motors for auto racing in the 1400 hp alcohol class ( unsponsored) and I never did a leak down test but s**t, if it will help why not. I always built them as tight as I could and ran em hard. Most Detroit types are built with an efficiency ratio of 35%. Outboards are not. And I do wish to learn what they do.

 

[ebbtide176]

Re: Need education

all you guys have great experience to share, pls keep it up

 

[petryshyn]

Re: Need education

Obviously, the L.test is not applicable to every engine problem out there, but a conscientious technician should be eager to add ANY test to their trouble-shooting bag of tricks. No one test tells it all. Even a premature tear down can leave you with doubts, once the engine is apart.<br /> <br />My posts on this topic are not to convince or encourage everyone to do a L.test on every engine, but to simply shed some light on the possibilities of its usage. A good tech will understand what this test is actually measuring, and then decide when and where it’s applicable. <br /><br />I certainly welcome all questions and criticism, providing they are constructive or even contribute. I do find it a little frustrating though, when I invest time to answer a legitimate question, and it is scrutinized by someone who hasn’t taken the time to read all the posts concerned, to see what context the topic is in......

 

[MGuckin]

Re: Need education

ebbtide176,<br />Yup, I did run dragsters and yes I do have a machining background. Imagine what I might be able to with an outboard if I understand it. I am learning thanks to this board but I haven't the nerve to tear into it yet. Schematic, I am 1 of many that do appreciate all that you help with. No matter how many times you have to explain, you keep at it untill your explanation or advise is understood. <br /> <br />Mike