cyl leakdown test results...?...

[H-K]

I have this SBC 350 that I was told has somewhere around 100K miles on it. I bought a leakdown tester and did my first leakdown test. I had to do it cold because the wiring harness and accessories are removed. I followed instructions carefully, but was surprised by the results and think I must have done something wrong.

1 = 2.5% @ 21psi
8 = 1.5% @ 38psi
4 = 2.5% @ 26psi
3 = 2% @ 29psi
6 = 2.5% @ 50psi
5 = 2% @ 25psi
7 = 3.5% @ 57psi
2 = 2.5 @ 28psi

The numbers seem low, even if it were a new motor, but I'm inexperienced with leakdown testing, so I'm not sure if this is true.

I also don't know what to make of the variation in PSI from one cyl to the next. I'm missing something and having a hard time putting the pieces together.

Can somebody throw me a bone here?

thanks

 

[SLC97SR5]

Were you cranking it by hand to seat the valves or did you use the starter?

It looks like the valves were not closed, you can put a few drops of motor oil to help compensate for the engine being cold and help equalize the cylinders to get a more accurate reading .

 

[H-K]

I rotated the crank by hand, loosened the rocker arms, and tapped the valves with a mallet to help seat the valves.

 

[waynehartwig]

Based on them, I would say you have a good motor as they are all under 5% and they are fairly even.

The PSI might be changing based on your air source. Are you using a regulator? What is the regulator set at? Is it a known good regulator? etc.

Adding oil to the cylinder to do the test will skew the results. Here's why....

If you have bad numbers - a lot of leaking -
...and you add oil and it FIXES it, then you have worn rings.
...and adding the oil doesn't fix it, then your rings are most likely ok.
...and tapping the valves solves it, then your valve isn't seating and could either have build up on it or a worn valve guide/stem
...none of the above helps, then all bets are off. Pull the heads as it could be anything - including head gasket or even a hole in the piston.

The key in all of this is to try to pinpoint where the air is going. Do you hear it in the intake? Exhasut? PCV? Is your radiator empty now?

 

[H-K]

Based on them, I would say you have a good motor as they are all under 5% and they are fairly even.

The PSI might be changing based on your air source. Are you using a regulator? What is the regulator set at? Is it a known good regulator? etc.

I have an upright air compressor with a regulator set at 100PSI. AFAIK it's good and consistent, but all I can base that on is the fact that it always seems to provide good air pressure for my air tools and always seems to take about the same amount of time to re-compress the tank, so I think the regulator is consistent because of that. However, it's not regularly calibrated or anything like that. I made sure not to get any readings while the compressor was re-compressing the tank. Does this answer your question or do you need more info?

Adding oil to the cylinder to do the test will skew the results. Here's why....

If you have bad numbers - a lot of leaking -
...and you add oil and it FIXES it, then you have worn rings.
...and adding the oil doesn't fix it, then your rings are most likely ok.
...and tapping the valves solves it, then your valve isn't seating and could either have build up on it or a worn valve guide/stem
...none of the above helps, then all bets are off. Pull the heads as it could be anything - including head gasket or even a hole in the piston

So you're saying this is applicable if my results showed higher percentages of leakage, but my numbers look okay -- or are you saying that the variability in the pressures at which I got my results suggests a problem even though the percentages were low?

The key in all of this is to try to pinpoint where the air is going. Do you hear it in the intake? Exhasut? PCV? Is your radiator empty now?

I could hear some in the crankcase and the intake. There may have been some coming from the exhaust, but if there was it might have been hard to hear because the manifolds are removed. Any of the sounds from escaping air I heard seemed low, but I don't have a trained/experienced ear when it comes to leakdown tests, so I don't have a frame of reference or benchmark to compare it to. I mean, I know that a healthy motor will leak air past the rings to a small degree, but I don't know what "normal" sounds like for comparison with my motor.

The manual that came with the leakdown tester didn't mention anything about observing the PSI readings, but instead just focused on the leakage percentages. On the few webpages I looked at, they focused mainly on the percentages too. Is it correct to assume that pressure readings are not very telling and instead the leakage percentages are the important part?

thanks for the help
JP

 

[BlackSheep]

I'd tend to agree with Wayne that your engine is good. For a cold engine to leak less than 5% I don't think you're going to have any trouble with it.

For future reference his comments about where the air is leaking from is important. I did a leak down test on my Wrangler I-6 last year when I was questioning my compression test readings. I didn't have the actual leak down tester, but removed the schraeder valve from my compression tester fitting and hooked my up air line directly. I used only the air escaping to determine I had 2 leaking exhaust valves.
When I pulled the head, sure enough, the seating surfaces on most of the valves (and especially those two) were in pretty poor condition. A fresh valve job and I was on my way.
Even though I don't know what my percentages were, back in my motorcycle repair days, even as much as 10% leak was a judgment call whether anything should be done with the motor or not.

 

[SLC97SR5]

Adding oil to the cylinder to do the test will skew the results. Here's why....
...and you add oil and it FIXES it, then you have worn rings.

Yes and yes, I should have been more specific...this is done to help eliminate possible causes and identify the failing component.

Thanks Wayne

 

[78mitsu]

I usually do a soapy water test on the valves while doing a leakdown test, spray a little soapy water while you are doing the test around the exhaust valves and to the intake valves (assuming you have the inake off). They will bubble (a lot) if you have a leak, then follow with WD-40. Basically you eliminate the valves leaking, then it pretty well has to be rings or worse.

 

[waynehartwig]

Correct. I think you're on the right track and I think you have a good enought motor to run. How well, who knows. But it will at least run and less than 5% should be telling you that you have a good, tight motor.

My whole point on the variable PSI's, is say you had your regulator set at 60 PSI and you got all of those numbers... But if you have 100 PSI on the line and are still varying between 50-60 psi, it's most likely not your air supply causing it.

Does your leakdown tester have it's own regulator built into it?

Look at the tops of all of the valves and see if there is a swirl pattern or a line. If you have a line, that means the valve hasn't been rotating and the rocker has been wearing just that spot down. Sometimes you can pop the keepers and rotate the valve 180*, put it back together, and run it. Most of the time it will fix itself and start rotating again. If it's rotating on it's own, it won't build up in the seat area and you'll have a tight seal.

 

[H-K]

Correct. I think you're on the right track and I think you have a good enought motor to run. How well, who knows. But it will at least run and less than 5% should be telling you that you have a good, tight motor.

My whole point on the variable PSI's, is say you had your regulator set at 60 PSI and you got all of those numbers... But if you have 100 PSI on the line and are still varying between 50-60 psi, it's most likely not your air supply causing it.

Does your leakdown tester have it's own regulator built into it?

Look at the tops of all of the valves and see if there is a swirl pattern or a line. If you have a line, that means the valve hasn't been rotating and the rocker has been wearing just that spot down. Sometimes you can pop the keepers and rotate the valve 180*, put it back together, and run it. Most of the time it will fix itself and start rotating again. If it's rotating on it's own, it won't build up in the seat area and you'll have a tight seal.

this is good information. Thanks -- I appreciate it. I'm probably looking at installing performance heads and it sounds like the rings are probably in good shape, so I have a good small block to work with and that was my biggest concern.

 

[waynehartwig]

this is good information. Thanks -- I appreciate it. I'm probably looking at installing performance heads and it sounds like the rings are probably in good shape, so I have a good small block to work with and that was my biggest concern.

I would think so. If your numbers were like 20%, then I'd be worried. But 2-3%? go have fun

Since you are going to swap out the top end, you might add some oil to each cylinder to see if the numbers change. Or when you have it apart, measure the ring gap to double check....You can also look at the cylinder walls and see a lot, too. ie do they have scratches from the piston moving up and down or do they still have the boring scratches. and etc...