Looking for help with low compression.

J

Josh Rasmussen

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Razz2o4
I am getting ready to get my aircraft listed for sale(1975 Warrior W/160hp STC). I had a potential buyer question my compressions as they are working their way down. As of 2019 My compressions were Cyl 1-70 2-72 3-75 4-70. Those were not horrible. Fast forward to my 2020 Annual, and we had 1-69 2-64 3-65-4-74. For me, this is a concerning range, but Mechanic was not super concerned as It did not get flown that much the previous year, and wanted to see more hours on it. Between January 20th and August 30th it only saw 11 hours of use. The Engine has 689.4 SMOH currently. When the buy came to check the plane out he was in love with it and how well it climbed. We came back and did a comp check at his request, and the numbers were 1-67 2-62 3-69 4-63. So at this point Mechanic decided its time to investigate. We pulled all 4 CYL off and looked at ring positions and found that some of them were rather close, we thought that might be the issue. So we adjusted the rings, and installed jugs. Did a runup today and we were at 1-63 2-65 3-55 4-63. I am baffled as to where my compression is and why its so low. Cylinders were good, had some lead buildup on the heads, but all the air is heard coming from the dipstick tube which makes is believe its the rings.
He is suggesting we pull jugs and take them to be redone. I am wondering if we should hone the cylinders and just put new rings on the pistons. I had two oil analysis come back with slightly elevated ferrous metal and had me thinking that we had some corrosion on the cylinder walls. Not sure if that would have done enough damage to the rings to wear them down to where they would be getting a lot of blow by.
Not sure if anyone has been in this boat or not. Any suggestions would be great.
 
Was the check done hot or cold in each case?

Same equipment and mechanic each time?

Those numbers are fine for a Continental. Not sure about Lycoming.

I would not have pulled a jug for a buyer, but that’s personal preference. Depends on how badly one wants to sell, or the price, or other factors that are none of POA’s business.

Iron rings, steel cylinders, slight corrosion, may show up in oil. As @kyleb suggests, running should stabilize or improve it over time.
 
Was the same measurement instrument used for all of the checks that were lower than expected?
 
I’ve never been a big believer in compression checks meaning very much at all. Hot vs. cold will make a difference, but more importantly what has the oil analysis trends shown? Have the filters been clean or did you find metal? I would scope the cylinders and inspect the camshaft for actual indications of the engine’s condition, along with more subjective factors like has it been producing smooth, consistent power, how have the temps been, how quickly is it consuming oil, etc.
 
455 Bravo Uniform said:
Was the check done hot or cold in each case?

Same equipment and mechanic each time?

Those numbers are fine for a Continental. Not sure about Lycoming.

I would not have pulled a jug for a buyer, but that’s personal preference. Depends on how badly one wants to sell, or the price, or other factors that are none of POA’s business.

Iron rings, steel cylinders, slight corrosion, may show up in oil. As @kyleb suggests, running should stabilize or improve it over time.
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Every compression check has been done hot. Within 4 minutes of a run up. All of the compression checks have been done using the same tester as well. You can definitely hear air in the Dipstick tube. As far as same Mechanic, yes same mechanic. I did not pull jugs for a buyer, he backed out when his mechanic told him to walk away. My biggest issue is time at the moment. I have two partners that wanted to get their license. I knew I would fly between 2-5 hours a month and them flying would help keep this plane healthy. 3 years later, neither of them fly, and its just me. I am lucky this year to get in it for an hour a month which is why I have talked them into selling it. I have a flight instructor that fly's it about 2-3 hours a month when he has time as well to help me keep hours on it.
 
dwalt said:
I’ve never been a big believer in compression checks meaning very much at all. Hot vs. cold will make a difference, but more importantly what has the oil analysis trends shown? Have the filters been clean or did you find metal? I would scope the cylinders and inspect the camshaft for actual indications of the engine’s condition, along with more subjective factors like has it been producing smooth, consistent power, how have the temps been, how quickly is it consuming oil, etc.
Click to expand...
.
Aluminum is from pistons and chrome shows ring wear. Iron is from steel parts and copper shows brass/bronze wear. At least the copper can be chalked up to the oil type and that's harmless, but the other metals are still more than twice universal averages. If any work has been done recently, the metal could be wear-in and silicon could be harmless sealer. Silicon could also be dirt, so check air filtration and the intake for leaks/cracks. Has the engine been sitting? Watch for oil filter metal & check back in 10-15 hrs This was the first Oil analysis.
and this is the 2nd one: It's nice to see metals reading lower this time around, especially since this wasn't all that much shorter of an oil run. There's still room for improvement considering most of the metals are higher than averages that are taken from oil with 35 hours on it, but there's plenty of progress evident already, and that could mean we're just watching lingering wear-in and/or corrosion washing out. If so, metals should hopefully improve more next time. No fuel or excess silicon was found. If the oil filter is clean and temps are normal, just follow up in 15-20 hours for an update.

As far as wear, the Aluminum went from 25 from one analysis to 15 over a industry average of 6, Chromium went from 14 to 10, iron went from 65 to 59. When checking the filter screen, there was no metal, as well as when we broke the oil filter open, we found no metal in there as well.
 
Probably should have been using Marvel Mystery Oil. :goofy:

(It's a joke ... settle down!)

Seriously ... a little oil in the cylinders during testing may bring the readings up and confirm that the problem is with the rings not sealing.
 
Josh Rasmussen said:
Every compression check has been done hot. Within 4 minutes of a run up. All of the compression checks have been done using the same tester as well. You can definitely hear air in the Dipstick tube. As far as same Mechanic, yes same mechanic.
Click to expand...

Before doing anything else I’d try a different instrument. The mechanic may be part of the problem as well, if the trouble started at the same time that he did.

There will always be some leakage past the rings. It is just a matter of how much.
 
Josh Rasmussen said:
I am getting ready to get my aircraft listed for sale(1975 Warrior W/160hp STC). I had a potential buyer question my compressions as they are working their way down. As of 2019 My compressions were Cyl 1-70 2-72 3-75 4-70. Those were not horrible. Fast forward to my 2020 Annual, and we had 1-69 2-64 3-65-4-74. For me, this is a concerning range, but Mechanic was not super concerned as It did not get flown that much the previous year, and wanted to see more hours on it. Between January 20th and August 30th it only saw 11 hours of use. The Engine has 689.4 SMOH currently. When the buy came to check the plane out he was in love with it and how well it climbed. We came back and did a comp check at his request, and the numbers were 1-67 2-62 3-69 4-63. So at this point Mechanic decided its time to investigate. We pulled all 4 CYL off and looked at ring positions and found that some of them were rather close, we thought that might be the issue. So we adjusted the rings, and installed jugs. Did a runup today and we were at 1-63 2-65 3-55 4-63. I am baffled as to where my compression is and why its so low. Cylinders were good, had some lead buildup on the heads, but all the air is heard coming from the dipstick tube which makes is believe its the rings.
He is suggesting we pull jugs and take them to be redone. I am wondering if we should hone the cylinders and just put new rings on the pistons. I had two oil analysis come back with slightly elevated ferrous metal and had me thinking that we had some corrosion on the cylinder walls. Not sure if that would have done enough damage to the rings to wear them down to where they would be getting a lot of blow by.
Not sure if anyone has been in this boat or not. Any suggestions would be great.
Click to expand...
Has anyone determined why the compression is low? rings are not the only reason compression maybe low?
What is your oil consumption?
 
Read Mike Busch’s book called “Engines.” In it, he argues that low compression is not a problem if the engine is developing full power. Interesting and eye opening couple of chapters.
 
Yesterday my 182's 0470 with 500 hours on it since major had jug #5 at 44/80. The engine was cold & my airplane hadn't moved in two months so we decided to test fly it today & take a warm engine reading. BTW, we were performing the annual.

I took it up today & the air was horrible. Only 10 knots when I took off but 28 knots with 35 knot gusts when I landed 40 minutes later. I intended to stay up several hours but that's all of the turbulence I could handle.

When we re-checked the cylinder after my flight it was 70/80. The rough flight was well worth it.
 
Kynadog said:
Read Mike Busch’s book called “Engines.” In it, he argues that low compression is not a problem if the engine is developing full power. Interesting and eye opening couple of chapters.
Click to expand...
the word is "Dynamic compression" understand that, you'll understand why an engine will run properly with low compression.
 
Did the mechanic wiggle prop a little while bringing the pressure up? That often gets those rings seated much better and you get an accurate, better number.
 
Dan Thomas said:
Did the mechanic wiggle prop a little while bringing the pressure up? That often gets those rings seated much better and you get an accurate, better number.
Click to expand...
How are you do a compression check with out doing that?
 
Josh Rasmussen said:
Every compression check has been done hot. Within 4 minutes of a run up. ...
Click to expand...
What does "run up" mean?
You ran the engine for a couple minutes at 1000 rpm?
You flew for an hour heat soaking the engine?
 
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