One cylinder low on compression
 

I have a Superior IO-360, 340 hours on it. First run in 2006. Previous annuals the cylinders were : 77, 78, 78, 75. Pretty good. Today my guy told me that all cylinders were mid 70's but #3 was 50 !!! Yea 50.

He said the plugs were in good shape and he didn't suspect the rings, but that perhaps a chunk of carbon maybe stuck on the valve preventing it from seating.

I lean on idle and taxing, lean during flight, run 65% power pretty much all the time. Change oil every 30-45 hours. He said that #4 bottom plug has some good sized carbon chunks.

So..... do any of you engine guru's have an idea to remove the potential carbon chunk from #3 ?

The compression check was done cold not after engine warm up. I asked him to check it again tomorrow after the engine is warm. I am praying there is a difference. Am I looking at having someone take the head off/overhaul ?

This sucks as I fly regularly and have done all I can to prevent a top overhaul.

I'm kind of bummed. I treat the engine as best I can and then this happens.

Thanks in advance for any suggestions.

Darren

It is not uncommon for this to happen. Run the engine 3-4 hours and retest the compression. This has happened twice to me over the yesrs. Both times were false alarms and the second reading was fine.

Like Larry mentioned, run it.

I did my condition inspection and compression check last month; I did a short ground run to warm it up before pulling the plugs and hooking up the air hose... 3 jugs were fine, but #3 read roughly 5 psi and I could hear/feel air coming out the exhaust pipe. Oops. I took it back out, ran it for 10 minutes or so including a mag check and prop cycles, then checked #3 again. All was well, 75/80. Top overhaul averted.

You guys have given me hope !!!!!!!!!!!! Thanks for the advice. What do you guys think was the reason for the low readings ?

The engine was cold so things weren't sealed up as well ? You weren't at TDC at first ? Was it a carbon chunk preventing the seating of the valve ?

Either way I am surely going to warm it up good and get my guy to check it again.

Fingers crossed, prayers said :)

Darren

what about lead.........
 

issues instead of carbon? when the plugs are out look inside just past the threads and see if there is any lead buildup. how about a look inside with a borescope? if it is lead you can use TCP to help out. hopefully is just a bit of carbon but i would want to know. here is a pic of what i was talking about. good luck.

Crack
 

Ok this sounds like what I just went through. I had no idea as it was running perfect so I assumed it was carbon too, NOPE. Cracked cylinder, I had to buy a new one and then have it bored to match the other cylinders.

Mike

Something similar happened to me recently. Fairly new O-360 from Bart, about 150 hours on it, Lycoming cylinders. Last condition inspection, #2 cylinder was 60/80. Ran the engine, re-checked it, same. A couple of months later, checked it again, same. I spoke to a local engine guy who suggested boroscoping to check for carbon under a valve and possibly "staking" the valve (basically banging on the valve to try to loosen the carbon).

I spoke to Bart and he said "send me all four cylinders and I'll check them." Seemed like overkill to me, but I did take him up on his offer and sent him the #2 cylinder while I was doing this year's condition inspection. He turned it around in a couple of days, so total down time was just under 2 weeks. Bart re-ringed the piston, ground the valve/seats, honed the cylinder, etc., no charge of course, and he paid the shipping back. Bart is great about this! Bart said he could find nothing wrong, but we suspect a bad ring (I had pulled the rings before I shipped the cylinder to him). So I'm running mineral oil again for the next 25 hours, so far so good! :)

Not sure I'll ever know what happened for sure.

[Oh yeah, Paul's post below reminded me and I forgot to mention that air was escaping out the breather, so this, too, would indicate a ring problem.]

I am afraid I have never understood mechanics who want to pronounce a problem after doing a COLD compression check. Yeah, it's easier to do the compression check when the engine isn't going to burn your hands, but the purpose is to get a good compression check, not make it easier to do the test. I agree with everyone here - run it, then check again - most likely, the "problem" goes away. The other thing your mechanic should have done was tell you where the air was escaping - listen to three places - the inlet, exhaust pipe, and oil dipstick tube - this will tell you if you have air coming out the Intake valve, Exhaust valve, or past the rings.

If it's a valve, stake the valve then try again. Lots to try before you pull that jug!

Paul

Could be any of the things you mentioned, all it takes is one.

I am going thru something similar, and just want to point out one more thing to check...I had #4 come up 62/80 warm a couple months ago. Freaked me out...I had our guys look at it (I work at an FBO), and after scoping it, finding no carbon, we decided to pull the hydraulic lifters. Found one of them had some "junk" in it, preventing the exhaust valve from fully seating. We did another compression (yes, cold this time as the airplane and engine were no where near ready to run at this point), and it came up to 68/80 cold. I haven't checked it warm yet, but I have flown it 7 hours since then, and will check it soon...have to pull the cowl to finish my smoke system. ;)

So...I can suggest checking the lifters...:cool: