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Leak down test noob

 
Old 07-08-2009, 07:20 PM
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Leon Speed
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Default Leak down test noob

I picked up a leak down tester with dual gauges, but instructions are not included. Just to make sure I get it right I assume the test goes as follows:
- set gauge to 0
- turn cylinder # 1 to TDC
- connect with 100 psi?
- turn crank 90 degrees CW
- test second cylinder in firing order
- repeat until all 8 are done

Check the individual results (<20% is ok?) and for consistency accross the cylinders.

Is that correct?

Thanks
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Old 07-08-2009, 08:01 PM
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tveltman
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I bought my leakdown tester at Harbor Freight. It says that 100 PSI is a maximum, so you might consider dialing it down to 90 PSI just to be safe if there are no instructions (I'd guess it's probably the same unit regardless of where you bought it). The dual gauge setup will show you only relative leakdown, I couldn't think of a way to get mine to show absolute. You will have to apply pressure to the first cylinder, then zero the gauge to it, then proceed through the firing order as you stated. Maybe on some fancier models (read: not my el-cheapo harbor freight unit) you can get it to read absolute leakage. The cylinders should all be within 10% of one another, I believe, and absolute leakdown should be less than 20%, probably more like less than 10%. Supposedly (and I've not confirmed this) well-tuned race-built engines have leakdown of about 3%, good stock sports car engines are around 5-7%, and normal cars are down to around 10%. That's hearsay, so don't take my word for it, but I'm pretty certain that they shouldnt be outside of 10% variation.

When you do the test, remove the coolant expansion tank cap, the airbox, and the dipstick. You will hear a slight hissing from the dipstick, and this is normal. If you hear super hissing, coupled with high leakdown, that points to needing new rings. If you hear hissing from the MAF, that is generally leaky intake valves, and if you hear a howling sound at the tailpipe, that is a leaky exhaust valve. You may see bubbles in the coolant tank, and this can indicate blown head gasket. I suppose that excessive hissing from the dipstick could mean the same. If the head gasket has blown between cylinders you will hear the hissing coming out of the adjacent spark plug hole (you should remove the spark plugs before you do the test). That's all I can think of, good luck with it!

EDIT: I think you can test for absolute leakdown if you put a cap on the end of the tester and pressurize it. You can then set the gauge to zero, and you'll get absolute leakdown. You may not be able to find a cap, IDK, but if you know someone with a lathe, you can get one turned for the purpose. If I was near my lathe, I would turn you out one and mail it to you, but sadly, I am several states away.

Last edited by tveltman; 07-08-2009 at 08:11 PM. Reason: Flash of "insight"
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Old 07-08-2009, 08:48 PM
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dr bob
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Dual gauge setup shows leakdown as a percentage. With the two gauges, the leakdown is (upstream pressure minus downstream pressure) divided by upstream. This gives you a percentage number that's pretty simple for any specific upstream pressure. If you set the upstream at 100, the leakdown percentage is a simple upstream minus downstream calculation in PSI.

Testing is ALWAYS done at TDC on the compression stroke for the cylinder you are testing; any other place will cause air pressure to try to spin the crank. You can look at cam specs to see when valves open if you want to lock the crank at a certain off-tdc position and pressurize there for ring sealing test. It always needs to be done with valves off the cams, obviously.
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Old 07-08-2009, 08:53 PM
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Thomas,

I have an el cheapo as well, but even with that I can get an absolute reading I think by first dialing to zero (with the gauge hooked up to the compressor), attaching the hose to the cylinder and then coupling the gauge and the hose.

I sure hope not to hear any super hissing...otherwise there will be more projects on "the list" but we'll see what happens.

Thanks!
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Old 07-08-2009, 09:11 PM
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dr bob,

That about the percentage makes sense, thanks.

I don't understand what you mean by "It always needs to be done with valves off the cams, obviously."
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Old 07-08-2009, 09:34 PM
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I think he means you need to be sure your timing in in sync, or otherwise your valves may be open at TDC. I can personally verify that this can happen.
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Old 07-08-2009, 10:55 PM
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The cams can not be opening the valves at all, else you'll see the flow through the non-closed valve as leakage.

Folks should remember that a leakdown test is a filter for engines that can't have a normal compression test done. 911 Motor for sale but sitting on the floor? Leakdown test is your friend. Ditto airplane engines. A good compression test is generally a better indicator of condition if you can do it. Of course, both tests together would be better than either alone.
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Old 07-08-2009, 11:15 PM
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Dr Bob,
I think that a leakdown test is a tremendously vital diagnostic tool, probably moreso than a compression test. The compression test will give you information as to the condition of the cylinder, but the leakdown test can help to pinpoint the exact problem. I completely agree that the two together are superior to either alone, however I think that if I had to pick just one test, I would choose the leakdown, especially in aryan's case. Presumably his car runs, and he is trying to diagnose some problem. The compression test would pinpoint a failing cylinder, but so would the leakdown test, and it could help him decide how to procede. The only case that I can think of where the compression test would be superior is if all cylinders were terrible but equally bad. In that case, they would all appear to be within percentage of each other, but if they were that bad, then you would hear the air howling through the valves.

That's how it makes sense in my head, but if there is some reason why that logic is flawed, I'd like to know so I don't walk around spewing crappy advice, and you always seem to give excellent advice.
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Old 07-09-2009, 02:23 AM
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Thomas-
The challenge is that a leakdown test only tells you about the very top of the bore and the way the rings seal there. The assumption is that any scratches in the walls go all the way to the top. Sometimes they do. It's a pre-purchase filter that helps you avoid the engine that has a burned piston or some other major combustion chamber fault.

A compression test is equally tricky, especially if you look at regular testing off the same engine to map wear for predictive maintenance purposes. The engine must be in a recently-run condition so that there is a suitable and normal oil film on the cylinder walls. Consistent test temps promote consistent readings. The throttle MUST BE WIDE OPEN while testing. All the plugs must be removed while testing. Good idea to have a battery charger set up to maintain consistent cranking speeds on all cylinders. The best testers have the smallest internal volume; long or large diameter connecting hoses will affect the readings of small-displacement cylinders under test. Best to use the same gauge throughout the engine's life, since for some reason the actual gauges are seldom calibrated to any serious standard. Cam profiles dramatically affect compression readings, with long overlap or long duration cams allowing air to go in and out of engine valves during the compression stroke. For the average guy, it's just too many things to remember. Plus most average owners will take a few readings, think that whatever they see is OK, and they don't log the readings. So when they come back a year or two later, is the new set of readings better or worse than the last? If you don't track your test methods and ambient conditions and log them, you have other possible causes for inconsistencies. Reality for most street car owners is that if the engine runs, it's fine. They aren't going to do an overhaul because the engine logs 5PSI less than it did a year ago.
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Old 07-09-2009, 02:50 AM
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Thanks for the discussion. Good info.
David
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Old 07-09-2009, 03:52 AM
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Originally Posted by dr bob View Post
Thomas-
The challenge is that a leakdown test only tells you about the very top of the bore and the way the rings seal there.
Ere - gotta disagree with you here my friend. You can check the leakdown from near BDC as long as the intake valve is off the ramp. It may take a long 3/4 ratchet to move the piston up but I've done it. One of the reasons for this is to cause the rings to expand in the bore as the piston moves up.

Leakdown testers are really valve check tools. If you test while warm(just run) the leakdown should be 100% through the rings and into the crankcase. In this situation you will hear slight hissing only from the oil filler neck with the cap off. If you hear hissing from the intake with the throttle open, or from the exhaust, you have a valve leak. The measure of the piston/ring seal quality is usually expressed as output pressure over input pressure - "66/80" would be 80PSI going in and 66PSI on the output gauge. The leakage in this case would be 14PSI or 17.5%.
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Old 07-09-2009, 11:33 AM
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I am gonna put my .02$ in here.

As a A&P mechanic one of the tools we use for testing recip aircraft engines is the 2 gauge type tester.

We use 100 on the first gauge and record the pressure, generally anything below 100/80 is cause for a tear down IMHO.

Always hot and I always move the prop to TDC or close enough that the prop does not try to turn.
And yes, it can be used at any point as long as the valves are closed, if you can hold the engine from turning.

That being said, I friggen hate the 2 gauge type of testers, a single gauge leak down % gauge in combination with a compression test will give a good idea of the condition of the chambers.

A good quality bore scope is also needed IMHO.

On automotive, high performance, street engines I like to see no more than 10% difference in the readings on the compression test.
Engine hot, all plugs out, battery charger on boost, throttle held wide open.

No more than 8% (less than 5% is better) leak down on any cylinder, again with the engine hot.

Any big differences between the two types of testing means you better start looking at valve timing. (IE worn cam lobes)

And nothing wrong that I can see with a bore scope.

For a regular, mundane, DD, 20% leak down and difference in compression tests.

Doc, I know you do not mean you should ever get 100% leak down past the rings do you?
I think what Doc is saying that any leakage that happens should be past the rings, not the valves.

PS, what were the last readings on that special Bonanza.

(Docs old Bonanza was owned at one time by one of the best acro pilots to ever yell "Clear")




Originally Posted by docmirror View Post
Ere - gotta disagree with you here my friend. You can check the leakdown from near BDC as long as the intake valve is off the ramp. It may take a long 3/4 ratchet to move the piston up but I've done it. One of the reasons for this is to cause the rings to expand in the bore as the piston moves up.

Leakdown testers are really valve check tools. If you test while warm(just run) the leakdown should be 100% through the rings and into the crankcase. In this situation you will hear slight hissing only from the oil filler neck with the cap off. If you hear hissing from the intake with the throttle open, or from the exhaust, you have a valve leak. The measure of the piston/ring seal quality is usually expressed as output pressure over input pressure - "66/80" would be 80PSI going in and 66PSI on the output gauge. The leakage in this case would be 14PSI or 17.5%.
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Old 07-09-2009, 12:32 PM
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Valves should seal 100% on most engines. Exhaust valves will sometimes have a slight leak, but that's indication of burning coming up, as you know.

My last reading were 71/80, 64/80, 67/80, 61/80, 68/80, 62/80. Jugs 2, 4 and 6 run the hottest. I took a short flight, came back and we checked 4 and 6 again and they came up a few pounds, but the numbers were already in the logbook so we left it alone. I have the Conti E-225(O-470) so we also have to use the calibrated orifice on my plane(SB3-03). It's common where I'm at to rock the prop back and forth some before noting the final result as the rings don't always expand well without pressure. Note to self; don't let go of the prop!
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Old 07-09-2009, 02:03 PM
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Originally Posted by dr bob View Post
The cams can not be opening the valves at all, else you'll see the flow through the non-closed valve as leakage.
Oh ok I get now what you mean.

Originally Posted by dr bob View Post
Of course, both tests together would be better than either alone.
I have a compression test as well, so I'll do both.

Originally Posted by tveltman View Post
Presumably his car runs, and he is trying to diagnose some problem. The compression test would pinpoint a failing cylinder, but so would the leakdown test, and it could help him decide how to procede.
Reason for doing a leakdown is threefold. One is to eliminate (or determine ) possible piston slap. I have had a noise in my drivetrain for some while now, I had a thread on this last year. I haven't been able to determine the exact source and am trying to eliminate the possible causes. Second is to get a baseline. I bought my S4 without any history, so it might have 125K km or 500K km on the engine. Third I am curious and it is a cool new toy


Originally Posted by dr bob View Post
For the average guy, it's just too many things to remember.
Remember what?


Originally Posted by dr bob View Post
If you don't track your test methods and ambient conditions and log them, you have other possible causes for inconsistencies.
Log everything, gotcha.

Originally Posted by docmirror View Post
You can check the leakdown from near BDC as long as the intake valve is off the ramp.
How do I know when that is?

Originally Posted by blown 87 View Post
And yes, it can be used at any point as long as the valves are closed, if you can hold the engine from turning.
With a breaker bar on the crank that should work right?

Originally Posted by blown 87 View Post
A good quality bore scope is also needed IMHO.
Heh, an idea for a new tool

Originally Posted by blown 87 View Post
For a regular, mundane, DD, 20% leak down and difference in compression tests.
I have on those! Now I am curious to see my test results..

Originally Posted by blown 87 View Post
(Docs old Bonanza was owned at one time by one of the best acro pilots to ever yell "Clear")
Must be a flyboy joke

Originally Posted by docmirror View Post
It's common where I'm at to rock the prop back and forth some before noting the final result as the rings don't always expand well without pressure.
That principle should also work on a car engine by rocking the piston back and forth around TDC.

Ok now to make a checklist...
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Old 07-09-2009, 03:34 PM
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Originally Posted by Aryan View Post


That principle should also work on a car engine by rocking the piston back and forth around TDC.

Ok now to make a checklist...
Whatever you need to do to get the rings to expand into the cylinder. As Dr bob mentioned, write the results down, and be able to redo the test under the same conditions each time so you can get meaningful trend info. If you change the test parameters (hot engine vs cold engine) you will get different info, not good.
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