Cylinder Leak-down test with cold motor; what are acceptable levels?


11 replies to this topic
  • Arjan van Druten

Posted November 25, 2015 - 04:11 AM

#1

Hi all,

 

Just performed a cylinder leak-down test on a WR400 with piston at TDC and enough input pressure (circa 5 bar) to the measurement device ( a general leak-down tester for all kind of engines)

 

- Engine was cold; as i am worried to damage the aluminium cylinder head when removing a spark plug from a hot engine.

 

1) Set the leakage meter to "set"  (right gauge) by opening the input valve

2) Connected the leakage meter to the engine 

3) Read-out the leakage % (see blue line in the picture) - 37%

 

 

Results are shown in this picture. According this device this is a moderate level of leakage (37 %).

However, all info I have seen -about this on the internet-  tell me that circa 20% is the max...(?)

 

 

- My theory about this is, that the allowed % leakage is fully dependent on the orifice that is used in the tool. So, to mention a allowed leakage rate without specifying the orifice and input pressures etc, doesn't mean much. This orifice may not be standardized, so one type of measurement tool may given 20% and the other 80%. The gauge measures a pressure drop! The smaller the orifice of the tool, the larger the drop (the leakage % ). Furthermore, higher input pressures lead to turbulence and is similar to having a smaller orifice and a larger drop (leakage %).

 

By the way, the picture does not show the tool I used, but looks very similar..[talking about the colors and numbers and text on the gauge].

 

 

* What do you think?

--> Is this piston/rings/cylinder at the end of life? (engine from 2001, unknown miles, starts after 5 kicks usually, has reasonable power did not loose any power - at least I think its ok..., cannot compare with what it was 10 years ago as own it for 2 years now)

--> Would the reading be much better with a warm engine?

--> Do I have to believe the tool its judgement (scale says: leakage is at Yellow level / moderate leakage) or The general 20% (according most info on the internet this 20% is the max leakage allowable -?-)

--> Do Dirt Bikes have more leakage over the cylinder due to the high bore / stroke ratio?

 

 

Thanks!

 

 

 

 

PS I did not check were the leakage sound came from, I may do that later. I guess that leakage from a valve is much worse than leakage over the piston (being cold).

Attached Thumbnails

  • leakdown.png

Edited by Arjan van Druten, November 26, 2015 - 03:56 AM.


  • bkoz

Posted November 25, 2015 - 11:27 AM

#2

At 37% I would suspect an issue (your pics are not showing).  A leak down test is just a differential pressure test.  I am not sure what you are talking about as far as orifice size.  It should not make a difference. 

 

With 37% leakage you will hear air leaking past the problem area.  For example you could hear air coming out of the intake boot (carb off) if the intake valves are bad, air coming out of the exhaust port (header off) for bad exhaust valves, or air coming out of the crankcase if the rings are bad. 

 

What is your bike doing that you believe it needed a leak down test?  And how did you hold the piston @ TDC when you connected the air?



  • nucular

Posted November 25, 2015 - 01:11 PM

#3

Yeah 37% is a lot. If the bike runs, I would question the method of the test. If you are doing a high pressure test, it can be tricky to keep the engine from rotating when you apply the air. As the other poster said, listen for the leak and see where it is.

  • Krannie McKranface

Posted November 25, 2015 - 02:59 PM

#4

You have to lock the piston in place



  • Arjan van Druten

Posted November 26, 2015 - 04:07 AM

#5

Sorry for not putting the picture in.

 

Here are the results again and

 

here is a picture where you can see the orifice in the scematic.

 

The value of this "resistor" determines the pressure drop you will see, this value is not standardized so each tool may have a different reading of leakage.

 

Mine said 37%, which looks a lot, but according the tool it is in the Yellow (moderate) leakage range as determined by the company who made these. I guess (I HOPE) they use a much smaller orifrice than other tools....thats why 37% is called still moderate for their measurement tool. It is all relative to this orifice.  I cant judge of this Yellow level is ok....for a WR400.

 

Anyone ever done a test with this kind of tool?

Actually the real one I used (had to use a m10 - m8 adaptor) looked like attached in the RED BOX. - Guess its Made in China as it is very cheap...and doesnt have any brand name on it...

 

  • I believe I only can use it to see changes over time....

Attached Thumbnails

  • Leak_down_tester.GIF
  • leakdown.png
  • images.jpg


  • Arjan van Druten

Posted November 26, 2015 - 04:10 AM

#6

At 37% I would suspect an issue (your pics are not showing).  A leak down test is just a differential pressure test.  I am not sure what you are talking about as far as orifice size.  It should not make a difference. 

 

With 37% leakage you will hear air leaking past the problem area.  For example you could hear air coming out of the intake boot (carb off) if the intake valves are bad, air coming out of the exhaust port (header off) for bad exhaust valves, or air coming out of the crankcase if the rings are bad. 

 

What is your bike doing that you believe it needed a leak down test?  And how did you hold the piston @ TDC when you connected the air?

 

 

Hi,bkoz, 

 

I did hold in TDC properly.

The bike is rather old and I drove for many hours last 2 years, that is why I was curious if I need to replace pistons and so according maintenance handbook.

Although I do not MX style riding, just a bit of enduro (am not a pro or extreme agressive rider)...

 

See further for extra info (above --> latest new post)


Edited by Arjan van Druten, November 26, 2015 - 04:15 AM.


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  • Krannie McKranface

Posted November 26, 2015 - 06:29 AM

#7

You are WAAAAAAAY over thinking this

Pull the piston and measure it for tolerance. That's the only measurement that has any meaning for top end  wear.

If you have ' many hours the last 2 years' your are going to need a new top end.

A 2001 piston is 14 years old............

 

The leakdown test is more useful for determining head leakage...........and regardless of the system type, you are measuring a loss of pressure over time.

The leakdown test equipment is not measuring it's losses' but the losses in the head.  Orifice size is literally irrelevant.



  • bkoz

Posted November 26, 2015 - 09:13 AM

#8

OP you are pissing in the wind with your cheap tester.  Use a good known tester to confirm.  And as previously stated @ 37% leakage you will hear air leaking from somewhere.  At 37% the bike probably wouldn't even start. 



  • Arjan van Druten

Posted November 26, 2015 - 11:32 AM

#9

You are WAAAAAAAY over thinking this

Pull the piston and measure it for tolerance. That's the only measurement that has any meaning for top end  wear.

If you have ' many hours the last 2 years' your are going to need a new top end.

A 2001 piston is 14 years old............

 

The leakdown test is more useful for determining head leakage...........and regardless of the system type, you are measuring a loss of pressure over time.

The leakdown test equipment is not measuring it's losses' but the losses in the head.  Orifice size is literally irrelevant.

 

Krannie, thanks for your reply, but I was just reading this on wikipedia:

 

A leak-down tester is essentially a miniature flow meter similar in concept to an air flow bench. The measuring element is the restriction orifice and the leakage in the engine is compared to the flow of this orifice. There will be a pressure drop across the orifice and another across any points of leakage in the engine. Since the meter and engine are connected in series, the flow is the same across both. (For example: If the meter was unconnected so that all the air escapes then the reading would be 0 or 100% leakage. Conversely, if there is no leakage there will be no pressure drop across either the orifice nor the leak, giving a reading of 100 or 0% leakage).

Gage meter faces can be numbered 0-100 or 100-0, indicating either 0% at full pressure or 100% at full pressure.

There is no standard regarding the size of the restriction orifice for non-aviation use and that is what leads to differences in readings between leak-down testers generally available from different manufacturers. Most often quoted though is a restriction with a .040in. hole drilled in it. Some poorly designed units do not include a restriction orifice at all, relying on the internal restriction of the regulator, and give much less accurate results. In addition, large engines and small engines will be measured in exactly the same way (compared to the same orifice) but a small leak in a large engine would be a large leak in a small engine. A locomotive engine which gives a leak-down of 10% on a leak-down tester is virtually perfectly sealed while the same tester giving a 10% reading on a model airplane engine indicates a catastrophic leak.

With a non-turbulent .040" orifice, and with a cylinder leakage effective orifice size of .040", leakage would be 50% at any pressure. At higher leakages the orifice can become turbulent, and this makes flow non-linear. Also, leakage paths in cylinders can be turbulent at fairly low flow rates. This makes leakage non-linear with test pressure. Further complicating things, nonstandard restriction orifice sizes will cause different indicated leakage percentages with the same cylinder leakage. Leak down testers are most accurate at low leakage levels, and the exact leakage reading is just a relative indication that can vary significantly between instruments.

 

I agree that disassembly would provide me the answer, but thats a lot of work, that is why I tried to estimate the risk of being wrong by this measurement.

This device says that 37% is moderate... I am confused. Hate it when measure tools do not include proper documentation! :smashpc:


Edited by Arjan van Druten, November 26, 2015 - 12:17 PM.


  • Arjan van Druten

Posted November 26, 2015 - 11:44 AM

#10

 

 

 

 PS another issue may have been that the adapter was leaking. I will in any case do the test again soon!

Also may try to put some engine oil in the chamber to seal the piston ring better and check the leakage difference...


Edited by Arjan van Druten, November 26, 2015 - 12:03 PM.


  • Krannie McKranface

Posted November 26, 2015 - 03:14 PM

#11

You are enjoying the process....but you have an old motor that will need a top end, period...



  • Arjan van Druten

Posted November 26, 2015 - 11:42 PM

#12

You are enjoying the process....but you have an old motor that will need a top end, period...

 

Think you are right...I just try to find excuses...to delay the expenses.  :(






 
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