Hot v. Cold Compression Test

Dennis M Carleton

Dennis M Carleton

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Looking at a plane with a Continental IO-470-J that was overhauled in 1999 and has about 400 hours since then. Logbook consistently shows compressions in the 60's for all cylinders since 2002. Most recent annual shows 5 in the 60's and one at 45. Prebuy inspection showed all in the 70s and owner claims its because prebuy inspection was done with hot engine. Is this possible? Should I be worried?

Dennis
 
Dennis M Carleton said:
Looking at a plane with a Continental IO-470-J that was overhauled in 1999 and has about 400 hours since then. Logbook consistently shows compressions in the 60's for all cylinders since 2002. Most recent annual shows 5 in the 60's and one at 45. Prebuy inspection showed all in the 70s and owner claims its because prebuy inspection was done with hot engine. Is this possible? Should I be worried?

Dennis
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Others will be along soon, but a couple thoughts…
-as far as I know, compression tests should always be done hot.
-I would only use compression (actually differential compression/leakdown) as one part of many when looking at an aircraft to purchase. Borescope, oil filter inspection, etc etc should all be done in addition.
-low compression test numbers followed by better ones after some use would possibly correlate with a period of non-use …if so, what else may be suspect?

I’ve said this in other threads, but per my own experience, I wouldn’t buy an airplane with anything but a high-time engine unless the rest of the package was too good to pass up. The reason is that you will almost certainly pay a hefty premium for a low- to mid-time engine, even if OH was 20+ years ago. Then, you may get 1 hour or 1200 hours before it makes metal. Better in my opinion to buy at a discount with a runout engine and OH yourself.
 
Dennis M Carleton said:
Is this possible?
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Yes. Most guidance recommends checking with a hot engine. Or if not, to run run the engine and retest prior to making any determinations. I've done compression tests both cold and hot.
Dennis M Carleton said:
Should I be worried?
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Don't know. But with all 70s it passes the check. Given you seem to have ongoing doubts (worries) about this aircraft you're looking at based on your previous posts, I would highly recommend you ask the APIA that will maintain your new purchase and have him give you that input. After all he will be the person that will tell you it's not good on your first annual.;)
 
The compression test is a method used by a mechanic to determine the general health of the cylinders. These are large bore engines and with just 80 psi, whether hot or cold, there is going to be some leakage. The A&P is going to listen to determine where that leakage is occurring, it's not just a numbers thing. Think of it more like a canary in the coal mine type of test. When something starts to go south, such as a valve or rings, this will be the first indication long before you will see anything visible with a borescope or mini camera. As to "should I be worried?" - not based on the numbers you are citing but you are looking at an engine that was rebuilt 22 years ago so you have to consider that. One of the metrics looked at with compression is consistency. One reading of 45 is not a reason to rip the thing apart but an indication that you want to monitor that cylinder. If you never get another low reading then disregard it. The manufacturer's guidance is pretty clear on how to deal with this and the first step is to run it in cruise for an hour and recheck.
 
Where the air is escaping from is as important as the actual reading. It could be something as a chunk of carbon under a valve seat which a bit more running will clear out... or it could be something more serious like a burned valve.

My engine (O-290) had three cylinders at 35-40 recently, and you could hear the air escaping past the valves. We (local A&P and I) ground the valves in place, two hours of work, now all cylinders are around 70 again.
 
My aircraft is experimental so my experience may not be as relevant. I've done differential compression test both hot & cold and the numbers did not vary greatly. I do think a difference would be found if the engine has sat for any period of time. My plane likes to get out and at least go around the neighborhood once or twice a week and get out of town as often as possible.
 
Just taking a top sparkplug out can dislodge a bit of carbon that can fall onto a valve and inhibit its seal when it closes for the compression test. Figure out which valve, run the engine again and see if it clears up. Or take the rocker cover off and bonk the rocker over the offending valve with a soft hammer while there's tester pressure in the cylinder; often the leak will stop as the carbon is blown out.
 
Clip4 said:
I suppose cylinder salesmen like the cold test.
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I've done them hot and cold and didn't find a large difference. Hot, the piston is a bit bigger and will close up the ring gaps a little, but after being run there's more oil in the ring grooves, and that tends to help them seal a little better.
 
Dan Thomas said:
... after being run there's more oil in the ring grooves, and that tends to help them seal a little better.
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I mentioned earlier that I believed that if the engine was run frequently the numbers might be better. My thinking is that over a period of time (not sure how long it might take) the oil would run out of the ring grooves and allow more leakage to pass.

Sometimes mechanics will add a little oil to a cylinder during the test to see if the numbers improve ...
 
Daleandee said:
Sometimes mechanics will add a little oil to a cylinder during the test to see if the numbers improve ...
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FYI: you add oil to troubleshoot the leaks not "improve the numbers.";)
 
Clip4 said:
I suppose cylinder salesmen like the cold test.
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Not really. Its more the owner likes the hot test to squeeze out all the cylinders life. 99% of the time if a cylinder is bad cold it wont magically become good hot. But its something that needs to done to stay in line with the existing guidance.
 
Daleandee said:
True ... but the changing numbers prove the leak. I think you got my drift ...
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As long as you're not trying to put lipstick on a pig which unfortunately some people do to make their numbers better. If you catch my drift...
 
Bell206 said:
As long as you're not trying to put lipstick on a pig which unfortunately some people do to make their numbers better. If you catch my drift...
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I do ... and no I don't want any lipstick on a pig that I gotta fly or ride in. ;)
 
You add a bit of oil and retest when doing a cranking compression test to see if it was the rings. With a leak-down test you can just listen and determine the source of the leak, no need to oil the cylinder. Leak-down test is easy to do on an airplane because you have a big propeller to turn and hang on to but they are kind of a pain on an automobile.
 
My take is compression is mostly significant at operating temperatures.

Cold checks may improve after flying but I’ve never seen readings drop.

Brief ground runs prior to the test often provide odd readings that improve
after flight.

Could be abnormal heating/cooling?
 
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