Jump to content

Teach me about leak down tests


wvumtnbkr
 Share

Recommended Posts

So I am trying to determine if I have a weak engine....

 

Got a leakdown tester from horrible fraught.  It works (1st one had a leak!).

 

After following the instructions, it was set to test with about 20psi of pressure.  That's just where the regulator wanted to be to zero out the % gauge.

 

It is showing about 25 to 30% leakdown across all cylinders.  

 

I have not run this engine in a few months.  I did not warm it up (for a multitude of reasons including it won't run at this moment...  non mechanical related). 

 

The gauge is showing this as in the middle of the good range.

 

What do y'all think?  Would warming it up make a drastic difference?  Is the number bad?  Am I throwing away a lot of horse puppies?

 

Thanks for your help!

Link to comment
Share on other sites

We did a leak down early in the offseason after compression was slightly low. We showed 30% leakdown cold on all but one cylinder. I warmed it up and saw differences in the reading. I did notice that we had leakdown at top dead center on 2 of the 4 hot or cold, and one of the ones that leaked did not leak if the cylinder was not at TDC on compression stroke.

 

If you can easily make it run and warm up id probably try that just to be sure, if not id think its time for a tear down or junkyard engine. I know guys drop some oil down the cylinders to help with a compression check i wonder if you could do the same with a leak down?

Link to comment
Share on other sites

Not sure if yours reads the same as mine, but anything less than 10% on any one cylinder and I question running the motor at the next event.  My motors tend to be 98 over 100 on the two gauges, or better.

 

A few things I've learned

- the higher the pressure you can put to it, the more differential you'll get on the guages, thus the easier to read (5% on 20 is a smaller number than 5% on 100), I try to max the high pressure guage at 100psi

- at 100psi on the cylinder, if you're not at TDC, that sucker is going to spin, use a LONG handled breaker bar and pay attention

- hot leak downs are difficult at best because "how hot is hot" and how do you replicate it on a motor that is on the re-build stand?
- Do it in a quiet environment (no heaters, air compressors, radios etc).  Put your ear to the tail pipe.  Then to the throttle and then open the throttle.  Pull the PCV and listen.  Then you'll find out where its leaking

- Don't get discouraged when the numbers are low but you can hear it through either the intake or exhaust.  Pull the valve cover and give the valves a tap while the pressure is on.  Not uncommon to have a bit of carbon make a valve hang up a bit.  With 100psi on the cylinder, tap the top of the valve/rocker/lifter stack and you'll get a "WHOOOP" noise and usually the second gauge comes up some.

- if it is a valve issue start checking your valve adjustment before you take it apart

- If it is out the PCV, use a tube as a pipette and dump some light oil down the bore.  Spin it over a few turns and try again.  If it gets better, you have a ring issue.

 

I'd do it again, or maybe twice and see how you get different variation before you start pulling things apart.

  • Like 1
Link to comment
Share on other sites

Sooo, no valve adjustments.  When I push on the valves, I hear air going out the valves.

 

Pretty sure the air is leaking down the piston rings.  I can hear it best out of the open valve cover....

 

Maybe time for a 3500 instead of a 3400.  It won't cost me points because I need to use the s5 rx7 value and the corresponding heavier weight which means the swap is points free for another dozen horsepower....

Link to comment
Share on other sites

The leak down tester has an orifice between the two gauges that largely determines where the needle on the one gauge is for a specific amount of leak. I questioned one of my leakdown testers that I had a long time ago because the "green" area seemed too high. The one I have now is the 100 psi one and it reads the way I'm used to.

 

Generally speaking, 12-15% is getting high enough to affect power and will probably have a limited number of hours left. One of the bigger issues is that it can blow oil out because of the pressure build-up in the crankcase so best not to let it go further than that.

 

One thing to be careful of is to make sure the rings are sitting on the land and not lifted up. To do that, try to stop rotating just before the piston goes over TDC. On a leaky cylinder, I carefully use a long breaking bar to rock the piston at TDC to get the rings to sit on the land. Usually best to have two people for this: one to man the breaking bar and the other to plug the air coupler in case the pressure and resultant torque overpowers the breaker bar person. As you can imagine, it's a bit dicey so be very careful.

 

 

Edited by mender
  • Like 1
  • Thanks 1
Link to comment
Share on other sites

8 hours ago, wvumtnbkr said:

So I am trying to determine if I have a weak engine....

 

Got a leakdown tester from horrible fraught.  It works (1st one had a leak!).

 

After following the instructions, it was set to test with about 20psi of pressure.  That's just where the regulator wanted to be to zero out the % gauge.

 

It is showing about 25 to 30% leakdown across all cylinders.  

Is this the one?

https://www.harborfreight.com/cylinder-leak-down-tester-62595.html

 

It looks like a regular tester that uses 100 psi of air (left gauge). How did you end up at 20 psi?

Link to comment
Share on other sites

5 hours ago, mender said:

Is this the one?

https://www.harborfreight.com/cylinder-leak-down-tester-62595.html

 

It looks like a regular tester that uses 100 psi of air (left gauge). How did you end up at 20 psi?

Because the instructions say to adjust the regulator until the gauge on the right is at zero with air moving through the tester.  Then, you hook up the tester and see what value it is reading.

 

It is totally possible that I am doing this wrong....

Link to comment
Share on other sites

1 hour ago, wvumtnbkr said:

Because the instructions say to adjust the regulator until the gauge on the right is at zero with air moving through the tester.  Then, you hook up the tester and see what value it is reading.

 

It is totally possible that I am doing this wrong....

 

I try to test a 100, then at 50 psi.  If it fails at 100, you have a big problem.  On a street car, anything above 15% leakage is a failure to me.

Link to comment
Share on other sites

7 hours ago, mender said:

The leak down tester has an orifice between the two gauges that largely determines where the needle on the one gauge is for a specific amount of leak. I questioned one of my leakdown testers that I had a long time ago because the "green" area seemed too high. The one I have now is the 100 psi one and it reads the way I'm used to.

 

Generally speaking, 12-15% is getting high enough to affect power and will probably have a limited number of hours left. One of the bigger issues is that it can blow oil out because of the pressure build-up in the crankcase so best not to let it go further than that.

 

One thing to be careful of is to make sure the rings are sitting on the land and not lifted up. To do that, try to stop rotating just before the piston goes over TDC. On a leaky cylinder, I carefully use a long breaking bar to rock the piston at TDC to get the rings to sit on the land. Usually best to have two people for this: one to man the breaking bar and the other to plug the air coupler in case the pressure and resultant torque overpowers the breaker bar person. As you can imagine, it's a bit dicey so be very careful.

 

 

100% what I did great info here.

 

We tested ours @ 100psi, long breaker bar is a must, i was able to wedge it to the ground so it wouldn't move. There is a lot of force there so be careful. I bought a stethoscope to hear air leaking, witch i did not need as you pulled the dipstick and it sounded like a hookah bar down there.

Link to comment
Share on other sites

A leak down kit is a must for any racer I would not use the harbor  one they are  junk  I use a  mor-89600_w_m.jpg

moroso one you want the biggest gauge you can get much easier to read I try to keep my engines in the 5% range  but have had to run them at 20% cooked a piston at a track day not fun.

it is best to keep it a TDC when testing with a big bar you can rock the piston and see how the rings seal. www.DRVOLKS.com 

Link to comment
Share on other sites

  • Technical Advisory Committee
17 minutes ago, wvumtnbkr said:

I am very confused by the comments about doing the test at 2 different pressures.  How did you use different pressures?  Don't you set the pressure to whatever it is needed to get the leakdown gauge to start at zero?

I've never seen instructions like that Rob. Not saying your tester instructions are wrong, maybe there is a method I'm not aware of, but having worked as a mechanic for many years, the method I was taught and always used was to:

1) put the cylinder being tested at TDC (Make sure it's perfectly at TDC, or it will rotate the crank. We've also done this with someone sitting in the car in 4th gear and holding the brakes)

2) screw the tester into the spark plug hole

3) connect air hose to tester.

4) adjust regulator to a set value - usually 100 psi

5) read leakdown percentage on 2nd gauge. ex: If 100 psi is reading on the first gauge (input), and the gauge reads 85psi, you have 15% leakdown.

 

Anyone I've ever seen or heard of using a leakdown tester has done it this way.

Edited by mcoppola
  • Like 1
Link to comment
Share on other sites

1 hour ago, mcoppola said:

I've never seen instructions like that Rob. Not saying your tester instructions are wrong, maybe there is a method I'm not aware of, but having worked as a mechanic for many years, the method I was taught and always used was to:

1) put the cylinder being tested at TDC (Make sure it's perfectly at TDC, or it will rotate the crank. We've also done this with someone sitting in the car in 4th gear and holding the brakes)

2) screw the tester into the spark plug hole

3) connect air hose to tester.

4) adjust regulator to a set value - usually 100 psi

5) read leakdown percentage on 2nd gauge. ex: If 100 psi is reading on the first gauge (input), and the gauge reads 85psi, you have 15% leakdown.

 

Anyone I've ever seen or heard of using a leakdown tester has done it this way.

I use the regulator to set the % needle at 0 before hooking up the tester for each cylinder. I don't connect the spark plug hose until after I set the tester.

 

The line pressure in my shop varies from about 120 psi down to 100 psi just before the compressor cuts back in again so I get more accurate readings this way.

Link to comment
Share on other sites

43 minutes ago, wvumtnbkr said:

Oh, that different than my tester.  My tester actually spits out a %.  All of the YouTube videos I was watching all show a % as well.

 

Hmmm.  Maybe I need to borrow a better tester from somebody. 

Like Mike said, the "percentage" is actually the difference in pressure between the two gauges (15 psi less than the 100 psi setting = "15%").

 

It's possible that some testers have a different orifice size and therefore would read differently for the same leak, which is why I prefer the tester that I'm using now. I know what it's supposed to read. :)

Edited by mender
Link to comment
Share on other sites

  • Technical Advisory Committee
1 minute ago, mender said:

I use the regulator to set the % needle at 0 before hooking up the tester for each cylinder. I don't connect the spark plug hose until after I set the tester.

 

The line pressure in my shop varies from about 120 psi down to 100 psi just before the compressor cuts back in again so I get more accurate readings this way.

That sounds more along the lines of what Rob described. Please explain further. Is air escaping through the spark plug hose until it's connected? What is the input pressure reading? After connecting, do you then crank up the input pressure?

Link to comment
Share on other sites

5 minutes ago, mcoppola said:

That sounds more along the lines of what Rob described. Please explain further. Is air escaping through the spark plug hose until it's connected? What is the input pressure reading? After connecting, do you then crank up the input pressure?

No. I have a quick coupler on the spark plug hose so no connection and no leak until I attach the spark plug hose. That way, even if my tester has a leak somewhere (other than the spark plug hose) I zero that out beforehand and get an accurate reading once the spark plug hose is connected. No adjustment after the hose is connected, just read the second gauge.

 

The tester in the link I posted also has the coupler after the tester.

Edited by mender
Link to comment
Share on other sites

  • Technical Advisory Committee
1 minute ago, mender said:

No. I have a quick coupler on the spark plug hose so no connection and no leak until I attach the spark plug hose. 

 

That way, even if my tester has a leak somewhere (other than the spark plug hose) I zero that out and get an accurate reading.

I see, your tester seems similar to the Harbor Freight one you linked. Mine is an old Mac Tools one that merely has 2 100psi gauges. I was thinking opposite when you and Rob said to zero it out. The way mine is read is 100 psi input gauge = 90 psi on the 2nd gauge if there is 10% leakdown . It also has a quick connect on the spark plug side, so I set it at 100 psi in, and read 100 psi on the 2nd gauge before connecting the spark plug hose.

Link to comment
Share on other sites

6 minutes ago, mcoppola said:

I see, your tester seems similar to the Harbor Freight one you linked. Mine is an old Mac Tools one that merely has 2 100psi gauges. I was thinking opposite when you and Rob said to zero it out. The way mine is read is 100 psi input gauge = 90 psi on the 2nd gauge if there is 10% leakdown . It also has a quick connect on the spark plug side, so I set it at 100 psi in, and read 100 psi on the 2nd gauge before connecting the spark plug hose.

Yup, same thing.

 

Your 100 psi = 0% leakage on mine; the difference is only in how the gauge is marked 'cause it does the math for you. :)

Edited by mender
  • Thanks 1
Link to comment
Share on other sites

I honestly haven't had a leak down tester before, well not a real one. 

 

It would have saved me once, i but a car on track with terrible ring seal (bent a ring) and the blowby at WOT was unreal. 

 

Still was surprisingly quick, but Rob your catch can is sort of your first indicator, if you have that much pcv flow you will get noticable increase oil in the can (does vary by track and valve cover baffle design) over what is normal for that event. Mine was a quart in 20 mins, and the crackcase pressure pushed the cam seals and valve cover seals out.....

 

Is your oil consumption bad? Just more sanity checks to help process your readings. If nothing else you have a new baseline of "good" using your gauge.

  • Thanks 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

 Share

×
×
  • Create New...