Cylinder check
- Thread starter frcabot
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Oil usage pretty consistent (quart every 8 hours or so?) and no metal. No oil leaks. Mainly concerned about the differential, I think Lycoming says a 10-15 difference between any two cylinders should be "investigated."
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bnt83
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Exhaust or intake valves leaking? Spray soapywater on each cylinder while pressurized with shop air?
Did you hear any air in the exhaust of the low cyl?
tyndall
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LawnDart
How many hours since last annual?
James331
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How often was it flown, when was the last flight? I've seen planes sit and get lower compressions numbers, then come up after getting some exercise.
Bore scope, filter cut, log book audit, etc.
Bore scope, filter cut, log book audit, etc.
Jay Honeck
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We've been working through almost the identical scenario.
Ours is an O-360. One of the exhaust valves can be heard hissing, resulting in compression of 68. Borescoping showed tons of lead build-up on...everything. It's just a mess in there, not unlike what your 9 mm pistol barrel looks like after shooting lead bullets.
So, here's what we've been doing:
1. No more 100LL, a fuel that our engine was never designed to use.
Well, okay, we're running about 25% avgas to 75% unleaded car gas. No more fouled plugs. Engine runs MUCH happier.
2. 4 ounces of Marvel Mystery Oil in every tankful of gas.
I personally find it hard to believe that this will do anything, but every grey-beard on the airport swears by the stuff.
3. I have not added any MMO in the crankcase, which is what those same grey-beards recommend. If there's no improvement, I will try that next.
I will post the results at the next compression test. So far, she's running great.
Tom-D
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which one is 68?
No 4 cylinder. They're in order 1 through 4 are also highest to lowest.
A little over 100, didn't do too much flying this past year unfortunately.
Tom-D
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What do the plugs look like? are they running rich or lean? (4).??
What is the trend, getting worse?
Lean mostly. No clue on the rest as I have to speak to the mechanic.
Tom-D
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What's the trend on #4 ? getting worse?
Like I said they were all 75 or greater on the last annual so # 3 is down a bit and # 4 is down by 8 or 9
Tom-D
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understand a trend is not just two inputs.
Yeah, my impression is for non-radial engines a 68 is perfectly acceptable but wanted to see if anyone had a different view or was alarmed by the drop.
Tom-D
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I certainly wouldn't be alarmed by a 68, specially with no other symptoms.
Where was the air escaping? crank case? exhaust pipe?
Tom-D
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When it is 68/80 there should be a audible at one or the other point.
SbestCFII
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With the basic engine top and bottom components reworked/replaced since the prop strike, the difference between that and a field overhaul are the accessory components (i.e. fuel, oil, pumps, mags, etc.). Those can be dealt with on an as needed basis. If everything else is OK, the compressions are good and a decent pre-buy, it's most likely a pretty good bargain (but not too good to be true).
I'll double check.
Tom-D
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Yep, hide your head in the sand and wait until the engine has a catastrophic failure then make the decision to do some thing about it.
jesse
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Yes, doing what is suggested by Lycoming guidance and often results in a cylinder that will be just fine at the next check is most definitely putting my head in the sand and waiting for the engine to have a catastrophic failure
Per Lycoming:
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MAKG1
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Doesn't anyone do leak down tests on aircraft engines?
If you think a valve is leaking, that will tell you beyond a doubt.
If you think a valve is leaking, that will tell you beyond a doubt.
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jesse
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A leak down test is exactly what was done ?
MAKG1
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jesse
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In the aviation world when people say compression test they mean leak down. I don't know of anyone that does a traditional compression test because well, the prop is dangerous, and the leakdown is super easy to do (because you can hold the crank with the prop) and provides more useful results.
MAKG1
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No kidding?
So those ~70 PSI measurements indicate 30% leak down, and that's considered normal?
I know they are air cooled, but dang.
I prefer to measure leak down over compression in my cars, too. It really is a lot easier to interpret.
So those ~70 PSI measurements indicate 30% leak down, and that's considered normal?
I know they are air cooled, but dang.
I prefer to measure leak down over compression in my cars, too. It really is a lot easier to interpret.
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Huh? It's out of 80 PSI. So even a 68 is just 15%.
jesse
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Well, the input is 80 psi, so 68 would be about 15% which isn't a reason to go tearing things apart in most of these engines. I'd be keeping an eye on it per Lycoming guidance. Often times it'll climb back up anyhow.
Continental leak down guidance is more detailed than Lycoming. With them you come up with a value to reference based on the value you get by first letting the air flow through an orifice of a known size then reference that value instead of the 80. Helps account for gauge and other such errors. Big bore Cont's can go their whole life with some rather low numbers.
Tom-D
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Pray tell us what you believe a leak down test is.
The aviation standard is the pressure differential test using 80 PSI as the standard.
jesse
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What is considered a "leak down test" in basically the entire engine world outside of aviation is what we do in aviation that everyone calls a "compression check".
In the rest of the engine world a "compression check" involves hooking a pressure gauge up and cranking the starter to see what the value is. Not a very precise science.
MD11Pilot
Line Up and Wait
#3 of the Lycoming procedure says a lot.
I have done a compression check today and then do another one the next day and get totally different readings. As advised here, listen for leaks out the oil breather, exhaust stacks etc. Nothing...just check it again in a few hours. Stake the valves and check it again.
Personally, I would do these things, observe indications of engine power, roughness etc. and fly it about ten hours (per Lycoming) and check it again. Borescope it also.
I have done a compression check today and then do another one the next day and get totally different readings. As advised here, listen for leaks out the oil breather, exhaust stacks etc. Nothing...just check it again in a few hours. Stake the valves and check it again.
Personally, I would do these things, observe indications of engine power, roughness etc. and fly it about ten hours (per Lycoming) and check it again. Borescope it also.
JOhnH
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Thanks for that information. I have been meaning to ask that question ever since I bought my Bo (Cont IO520) and the leakdown (compression) test showed one cylinder at 68 (with about 75 hours on the engine). Everyone I asked would say "It's a Continental, dont' worry about it, it's fine), but I never knew why.
Why are you using any leaded gas at all?
use unleaded, get lean, and the engine will stay clean. Since you're experimental, why not switch to EFI and really save money on fuel and maintenance?Skip the MMO in the oil, won't do anything positive. If you want to get rid of the carbon, flood the cylinders with Berryman's for a while then change the oil and go run it hard for 15 minutes.
comanche pilot
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What about doing a hot leak down? I've heard people do it, I have never done it. I would think that cyl. is going to cool down pretty fast anyhow. Anyone with experience doing this and seeing a big difference between a cold and hot cyl? My I/A brought this up, when I had a weak cyl. We determined it was leaking by the rings, and just flew it. This cyl. has always been lower since factory new. We finally pulled the cyl. and found the compression ring gaps in alignment. I know they're supose to float and rotate but after reassembly and flying it seems to be back to the 68 psi it was before. 
Been flying that way for the past 200 hrs. Blow-by and oil use doesn't seem excessive and the oil stays clean looking for the most part.
Been flying that way for the past 200 hrs. Blow-by and oil use doesn't seem excessive and the oil stays clean looking for the most part.