School me on compression tests (wet and dry) and leakdown

Jreng87 · Mar 29, 2017

Long story short... down on RPMs, obvious things have been done (clutch, belts, plugs, EV valves, tps, etc..) and tackled to no improvement. Dealer mention next step would be to do a leak down test and scope the cylinder...

I have a sled mechanic where I work and was talking to him and we decided to check a couple quick things...

Ok, so a quality pro gauge used, I did a dry compression test and got about expected results for my elevation. Then added a bit of oil to each cyclinder and performed same test. One cylinder had an ever so slight increase (1-2psi), the other cylinder increased by 34psi!

From my reading on the internet a very slight increase is "normal" as it is the change in volume due to the oil in cylinder, where as a large increase tells there is a seal problem somewhere. Correct?

Tomorrow we are doing a proper leakdown test to try and find the culprit, but if I was a betting man I'm guessing ring seal...

I'm not an expert so just trying to get educated on things as we go so I have the knowledge for future reference.

chevota84 · Mar 29, 2017

Were they even when it was dry?

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Jreng87 · Mar 29, 2017

chevota84 said:
Were they even when it was dry?

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Within 1-2 psi

chevota84 · Mar 29, 2017

Wait on the leakdown test before you get too worried. If it was low to begin with it'd make sense but the compression being reasonable and then only one cylinder going up 32psi seems pretty odd.

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Jreng87 · Mar 29, 2017

chevota84 said:
Wait on the leakdown test before you get too worried. If it was low to begin with it'd make sense but the compression being reasonable and then only one cylinder going up 32psi seems pretty odd.

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Ya the dry test numbers were right in the expected PSi.

Jreng87 · Mar 29, 2017

chevota84 said:
Wait on the leakdown test before you get too worried. If it was low to begin with it'd make sense but the compression being reasonable and then only one cylinder going up 32psi seems pretty odd.

Sent from my SM-G930V using Tapatalk

Ya the dry test numbers were right in the expected PSi.

Merlin · Mar 30, 2017

The leakdown test will definitely confirm but with all due respect it seems very unlikely that there would be a 34 PSI gain in a wet VS dry compression test when the dry test yielded results that were within spec for that machine / altitude.

With those type of results you have to wonder about things like accuracy / repeatability of the gauge readings, cranking speed, how much oil was poured into the cylinder, etc.

No disrespect intended, just trying to give some honest feedback.

Jreng87 said:
Ok, so a quality pro gauge used, I did a dry compression test and got about expected results for my elevation. Then added a bit of oil to each cyclinder and performed same test. One cylinder had an ever so slight increase (1-2psi), the other cylinder increased by 34psi!

Jreng87 said:
Ya the dry test numbers were right in the expected PSi.

Jreng87 · Mar 30, 2017

For sure, def agree. The gauge is definitely accurate. The amount of oil we put in was not measured and I've def thought of that as a potential issue in the reading since it takes up volume.

With that said and for what it's worth, the PTO side was the cylinder which was off by 1-2psi, which also showed going lean ever so slightly on the plug, and also jumped the large psi with the wet test.

chevota84 · Mar 30, 2017

1-2 is within the margin of error, I think polaris says 5% but it's been a while since I looked.

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rab · Mar 30, 2017

Merlin said:
The leakdown test will definitely confirm but with all due respect it seems very unlikely that there would be a 34 PSI gain in a wet VS dry compression test when the dry test yielded results that were within spec for that machine / altitude.

With those type of results you have to wonder about things like accuracy / repeatability of the gauge readings, cranking speed, how much oil was poured into the cylinder, etc.

No disrespect intended, just trying to give some honest feedback.

I would repeat the test to asetain if my results are valid !!

But after reading your last post ref pto lean , I'm thinking your ring is fused on the piston , probably gonna have to pull the top off

Jreng87 · Mar 30, 2017

Leakdown showed 22% in PTO and 11% in the MAG

adamg · Mar 30, 2017

rab said:
I would repeat the test to asetain if my results are valid !!

But after reading your last post ref pto lean , I'm thinking your ring is fused on the piston , probably gonna have to pull the top off

rab, can you talk through this process for someone that has never taken apart a sled motor?

Here's my guess, I'm sure it is wrong.
- drain some of the coolant
- remove overstructure?
- unbolt head bolts, lift off head
- then what? Do the cylinders lift off?

Jreng87 · Mar 31, 2017

Don't need to remove overstructure, just stearing post.

I have a service manual and it walks you through disassembly. If it was something you were going to do you'd want the manual for procedures and all specs/torques/etc...

Sage Crusher · Mar 31, 2017

At your elevation in Colorado what was your acceptable compression?
We are seeing 105 psi on Axys with 300 and 1400 miles and in between

Jreng87 · Mar 31, 2017

Sage Crusher said:
At your elevation in Colorado what was your acceptable compression?
We are seeing 105 psi on Axys with 300 and 1400 miles and in between

I'm at about 9,500ft and was told 95-100psi depending on actual location is expected.

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School me on compression tests (wet and dry) and leakdown

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