Compression test vs leak down test

[another todd]

Is it OK to perform a leakdown test on a two stroke (Looper) engine or would air leaking past rings damage any seals or reeds?

 

[david_r]

Re: Compression test vs leak down test

if you are capable of doing a leak down i would do it................. it tells you a lot more than a reg comp test.

a decarb would help a lot.......... you can get the instructions off the back of a can of seafoam or in the frequently asked questions section on this site.

 

[Wover98]

Re: Compression test vs leak down test

How is a leak down test done? What is required and what are the advantages over a simple compression test?

 

[Monte1961]

Re: Compression test vs leak down test

I cant see any reason to do a leak down test on a two cycle engine, a simple compression test is good enough. If it was a 4 stroke, then it would be more of away of telling you if it was an intake or exhaust valve and or rings. To answer your question about the advantage doing a leak down versus compression test, is the leak down test will tell you more about the condition of the entire engine and cylinder in one step. There for you will be able to determine if bad valve/s or a combination of all 3. Intake/Exhaust and piston rings. A two cycle only has rings. So to some this up, Bad compression in a two cycle usually means engine in need of repair any way. Besides if doing a leak down, you will have to have compressed air available while preforming test.

 

[Daviet]

Re: Compression test vs leak down test

I agree with Monte1961, on a two cycle a compression test is all you need.

 

[jeffersondodson561]

Re: Compression test vs leak down test

A compression test is adaquate for a 2 stroke. Basically if you want to know the condition of a motor a compression test will give you a good idea. If you want to know where the problem exists, a leak down will tell you. More so on a 4 stroke than a 2 stroke. To do a leak down test you need a pressure differential gauge and a compressor able of putting out at least 100 psi. Basically you use the regulator on the gauge to find TDC at a lower psi say about 20 psi, then you increase the regulator pressure to at least 100 psi while holding the flywheel in a locked position, usually with a flywheel nut/socket and a breaker bar. Every cylinder is going to have some leakage. Generally any more that 15% leakage indicates a damaged/suspected cylinder. The benefit of doing a leak down test is you can listen/determine where the air is coming from: an adjacent cylinder, intake valve, exhaust valve, worn rings, etc. As with all tests, engine should be at operational temperature. It takes a considerable amount of time to do a leakdown test, this is why it is not performed more.

 

[Faztbullet]

Re: Compression test vs leak down test

on a two cycle a compression test is all you need.

I disagree as a cylinder can be scuffed or worn out/out of round and still have good compression. The skirt of the piston is the most important part of a 2 stoke engine . I have seen plenty of engines with "good" compression that had idling problems and low crankcase pressure. A compression test is only good for finding out if cylinder/s has enough pressure to support the combustion process.

 

[SuperNova]

Re: Compression test vs leak down test

I know a two-stroke doesn't have valves to check, but in addition to worn rings, isn't there still a possibility of a blown head-gasket? If so, a leak-down might help with that diagnosis. I think that on any engine, a compression test is like a go no-go gauge and if the answer is no-go, then the leakdown is the more thorough test. I can tell you that after 22 years as a man who earns a living with tools, that I would always do the compression test first and then a leak-down if needed. Leak-down tests can be tricky for the novice to get an accurate reading and can be misleading if done improperly.

 

[jeffersondodson561]

Re: Compression test vs leak down test

I disagree as a cylinder can be scuffed or worn out/out of round and still have good compression. The skirt of the piston is the most important part of a 2 stoke engine . I have seen plenty of engines with "good" compression that had idling problems and low crankcase pressure. A compression test is only good for finding out if cylinder/s has enough pressure to support the combustion process.

This is true, it has bitten me a time or two as well...

 

[Monte1961]

Re: Compression test vs leak down test

I know a two-stroke doesn't have valves to check, but in addition to worn rings, isn't there still a possibility of a blown head-gasket? If so, a leak-down might help with that diagnosis. I think that on any engine, a compression test is like a go no-go gauge and if the answer is no-go, then the leakdown is the more thorough test. I can tell you that after 22 years as a man who earns a living with tools, that I would always do the compression test first and then a leak-down if needed. Leak-down tests can be tricky for the novice to get an accurate reading and can be misleading if done improperly.

@SuperNova: I don't want to get in to a pissing match with you, but an engine without valves doesn't require a leak-down test to inform you that engine is worn and in need of repair. Weather it's rings, sleeves or head leaking, you will at one point or the other have to pull engine down and do repairs to fix it. Now as for possible blown head gasket, upon inspection you will see where it has been leaking or burned. As a man who has been in the business of high performance engines for the last 30+ years, only, I mean only in rare cases did we ever do a leak-down test. Now I am not talking about throwing a mile cam in to a engine and calling it high performance. I am talking very high end engines worth in excess of $13.000.00 to $30.000.00. Now that doesn't make me a no-it-all, but I know BS when I hear it. I being a Millwright for now the last twenty years, and having to work on various engines, gas, diesel and electric, two and four stroke, single to three phase electric, oh and you wanna talk about tools? I couldn't tell you the amount of $$$$$$$$$ I have invested. Snap-Off, Mac, Cornball, Craftsman. I have lots of experience in engine modifications and repair. So, trying to sell me or anyone that doing a leak-down test should be done on a two stroke is ludicrous. I don't mean to be mean, but lets just skip the leak-down test on two stroke engines. It's an unnecessary step! Peace happy boaters.

 

[jeffersondodson561]

Re: Compression test vs leak down test

A leak down is required on newer engines with DTS, to get an accurate compression test on a 4 stroke the throttle valves need to be wide open. On two strokes with individual throttle plates it is not that important because of the metered holes in the throttle plates. And BTW, I have never called any manufacturer for warranty with compression results, they want leak down results...

 

[Faztbullet]

Re: Compression test vs leak down test

So, trying to sell me or anyone that doing a leak-down test should be done on a two stroke is ludicrous. I don't mean to be mean, but lets just skip the leak-down test on two stroke engines. It's an unnecessary step! Peace happy boaters.

This is a test I do quite often on customers engines that have a problem/s that dont show up in other tests. I agree that it it unnecessary on some engines below 40hp but most of the looper V-4 & V-6 it is a great test, especially if the VRO is not cycling properly at idle rpm. If you own a Hi-performance outboard it is a have to, especially the Merc's with the single ring Mahle pistons. These type(2 stroke) engines will run but wont perform if leakdown is more than 10%. For a example, when I raced at KDBA (ky drag boat association) I had a 16.5 ft Mantra tunnel with a 505ci big block and all the bells and whistles. After about 9 races lost rpm and speed, performed leakdown and found 6 of 8 cylinder where 15-25% but compression was 165psi on all. Torn it down and found rings broken on all 8 cylinders.

 

[SuperNova]

Re: Compression test vs leak down test

@SuperNova: I don't want to get in to a pissing match with you, but an engine without valves doesn't require a leak-down test to inform you that engine is worn and in need of repair. Weather it's rings, sleeves or head leaking, you will at one point or the other have to pull engine down and do repairs to fix it. Now as for possible blown head gasket, upon inspection you will see where it has been leaking or burned. As a man who has been in the business of high performance engines for the last 30+ years, only, I mean only in rare cases did we ever do a leak-down test. Now I am not talking about throwing a mile cam in to a engine and calling it high performance. I am talking very high end engines worth in excess of $13.000.00 to $30.000.00. Now that doesn't make me a no-it-all, but I know BS when I hear it. I being a Millwright for now the last twenty years, and having to work on various engines, gas, diesel and electric, two and four stroke, single to three phase electric, oh and you wanna talk about tools? I couldn't tell you the amount of $$$$$$$$$ I have invested. Snap-Off, Mac, Cornball, Craftsman. I have lots of experience in engine modifications and repair. So, trying to sell me or anyone that doing a leak-down test should be done on a two stroke is ludicrous. I don't mean to be mean, but lets just skip the leak-down test on two stroke engines. It's an unnecessary step! Peace happy boaters.

Alrighty, then.....your spelling and grammar speak so much more eloquently than your diatribe.....'nuff said..

The O.P. 's original question was not about the relative merits of doing a leak-down test, but rather whether or not he ran the risk of damaging the reeds or any seals by doing so....and that question has already been answered, I believe.....