leak down test

[procraftmark]

I was watching a mechanic do a comp test on a motor and all the cylinders checked within tolerance limits, and then he pulled a "leak down" test on it and it showed one cyl with a 26#drop in pressure indicating i had leaking by the rings that didn't show up with a standard comp test! If this is a bettter indicator of problems then why not do this one all suspect motors?<br /><br /><br />thnx<br />mark

 

[hondon]

Re: leak down test

Most techs do if the engine has the performance history to warrant it.If you are starting right up and running great ,do you want to pay for that extra time?

 

[WillyBWright]

Re: leak down test

It's just another diagnostic tool. Most techs won't use it unless there's a need. Compression checks are usually a pretty good indicator, but they can't find some problems that a leakdown test can. But time is money. Leakdown tests take longer than compression tests.

 

[seahorse5]

Re: leak down test

A 2-stroke with scoring on the piston and cylinders can still give a good compression reading because of the residual oil. Sometimes a mechanic will see an engine with good compression, yet it won't idle right. A leakdown test will show a problem if the parts are not in good shape.<br /><br />Yamaha recommends a leakdown test during routine servicing of its 4-strokes. Many large 4 strokes that are accused of "making oil" are found to have carboned up piston rings. With pressure on the cylinder from the compressed air, leakage can be heard thru the oil fill hole if the rings are not sealing, from the exhaust if an exhuast valve is faulty, or from the intake manifold should the intake valve not seat correctly.<br /><br />A leakdown test is the only approved test for piston aircraft engines.

 

[procraftmark]

Re: leak down test

that "extra time" can save alot of grief, like i said earlier the regular comp showed good but the leak down showed it was something you were going to have to fix or something you were gonna want too get rid of! 26psi drop is major<br /><br /><br />mark

 

[WillyBWright]

Re: leak down test

I agree. Problem is the customer has to pay for the time to do it and most don't want to. On virtually every motor that comes in, I do a compression test, spark test, and I spin the prop to see if the shaft is obviously bent. Frequently the sparkplugs tell the story...or at least are a good first indicator. If everything else shows good and the sparkplugs indicate a problem, then I'll do a leakdown test...maybe. Sometimes it takes less time to simply pull the cylinder head if the problem is kinda obvious. Head gaskets for a 2-stroke aren't terribly expensive, and the heads come off quick and easy. On 4-strokes, it's a pain to remove the head, so I'm far more likely to do a leakdown on those.

 

[procraftmark]

Re: leak down test

I don't really see where the time is? it probably took 7-10 min. to do a 4 cyl. He poked a screwdriver in the cylinder to help find 'tdc', then screwed in the gauge and wala! not much more than a regular comp test<br /><br /><br />mark

 

[hondon]

Re: leak down test

Then the proceedure you saw demonstrated was'nt done like most of us do it.If you are going to do a leakdown ,do it from the bottom of the bore to the top . That gives you the best indication of that cylinders overall health.Doing this test at just TDC is just a geared up compression test.

 

[Hooty]

Re: leak down test

Just curious. How do you do a leakdown test in the bottom of the bore on a two stroke engine?<br /><br />c/6<br />Hooty

 

[seahorse5]

Re: leak down test

Hooty,<br /><br />"From the bottom of the bore" means that during the leakdown test with compressed air, you are turning the crank to test the sealing of the rings and cylinder wall as the piston travels up the whole length of its stroke until it reaches TDC.<br /><br />Years back I had an engine that wouldn't idle, yet compression was ok and a leakdown test at TDC seemed within specs. After spending too much time trying to troubleshoot the motor, I pulled the head and found that the formerly rebuilt motor had a wrist pin that moved against the cylinderwall and "machined" a beautiful groove into it. Once the piston rings passed by the groove, they sealed fine and a TDC leakdown test showed everything OK. Had I done a "full stroke" leakdown, I would have found the problem right away. Live and learn, that's how experience makes one a better technician.

 

[Hooty]

Re: leak down test

Thanks. I could see doing a full stroke leakdown test on a four stroke but on a two stroke the "full stroke" don't last past the exhaust port.<br /><br />c/6<br />Hooty

 

[rodbolt]

Re: leak down test

what I want to know is how to hold the flywheel once the piston moves off TDC.<br /> I have been smacked once or twice over the years

 

[seahorse5]

Re: leak down test

Hey Rodbolt, It's good to talk at you again.<br /><br />I use a long Snap-on 1/2" ratchet with the appropriate flywheel socket on it. With the selector lever set to "tighten" when turning clockwise I add the air from the leakdown tester after determining that the piston is on the down stroke or at the bottom. Then I use the ratchet to bring the cylinder up to TDC slowly while watching the gauges. As the piston "goes over the top" from the air pressure, the ratchet just clicks and lets the flywheel turn about 1/2 a revolution. That way you don't get "whacked" like you would if using a breaker bar.

 

[Hooty]

Re: leak down test

I just move the piston a fraction past tdc and clamp a pair of visegrips on the flywheel ring gear against the starter. Works for me. <br /><br />c/6<br />Hooty