Low engine compressions - what would you do?

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MountainDude

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MountainDude
Looking at a C182 with an O-470 with 1100 hrs.
Four cylinders are in the 70s, but one is at 65 and one is at 60.
1. How bad are the compressions in the 60s?
2. Would I likely need to replace the two cylinders? How much would that cost (all in)?
Thank you
 
Was it checked hot or cold? Might be that if it gets run for a bit and the compressions get checked again they’re much higher.

Compressions will be higher in a hot engine vs cold.
 
It was checked cold. I asked them to recheck it hot.
 
Fly it and check again while hot. If still not to your liking have a borescope done. Use the results in price negotiations.
 
How much has the airplane been flown in the last year (or two)?

If it has been sitting or rarely flown, I would fly it through one oil change cycle. Track the compressions through those hours and see if they come up before spending money on it unnecessarily.
 
You won't find this anywhere, but it worked very well for me.
I had one cylinder beginning to drop compression and the oil was getting dirty quickly. I dumped a quart of MMO into the sump and ran the engine hard (75%) for about 15 hours. Also, run high MP and low rpm if you can. In other words, build some cylinder pressure.
Also, you may want to borscope before you do anything like this. Check to see that your exhaust valves are in good shape. If they are try the above action and it may very well work for you. It did for me.

Sent from my Pixel 2 XL using Tapatalk
 
More important where the leak is, than how big it is (in the confines of the op)
 
Compression numbers shouldn't even be recorded. The mechanic should do the test then say either it passed or it failed. This is a classic example of why. You saw the number 60 and now you are asking complete strangers who have never seen the airplane if it's likely you'll have to replace a cylinder. :rolleyes:
 
What you do after the compression check is dependent upon where the leak is.
got a burned valve? running it more ain't going to help.
 
I’ve had one that was 47 hot in 2014, next year was 48, then it was down to 44. And that was after flying it hard for a hour and rechecking. 44 was the MO reading so he said I was good to go. The valves looked good and it was leaking past the rings.

I bought a spare cylinder figuring it was getting worse. The next couple of years it was in the 60’s. This year it was low the first check, flew it a bit and it was 58. I’ve put around 500 hrs since 2014, it still burns about a quart every 15-20 hrs and the oil looks clean. I’ve had three different shops do the annuals. I recently bought a guage to check it myself. And I have a borescope. The spare cylinder is still sitting on the shelf.

I have a IO 470 J , which is the low compression, 225 hp continental.

The engine was overhauled in 2000, I purchased the plane in 2012. It had 50 SMOH. It now has around 650 SMOH now. Yes it basically sat for 12 years, a few hours between annuals.
 
A single compression reading is not necessarily very helpful information. If you have a series of measurements over time, then you can surmise something from the trend, if any. Often the readings just bounce around an average value. Tom D is right on about WHERE the leak is if compressions are slowly headed south. Ring leakage with acceptable compression and no oil fouling of plugs is one thing. Valve leakage should get your antennae up.
 
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