Higher-than-normal compression on one cylinder

[BigBadLou]

Imagine you do a compression check (a real one, not the fake misnomer) on an airplane engine. Your compression numbers are within spec on all cylinders but one. We are talking warm engine, of course.
All readings were within spec but one cylinder showed almost 20% more pressure. That seems like a big inconsistency in numbers.

Of course my mechanic discounted the test completely, waving arms and shouting that performing an automotive engine test on an airplane engine is pointless and that all one needs to know that if all cylinders show 75-79 psi on the leakdown test, the engine is perfectly fine and good and there is no problem.
It did not matter to him that airplane engines used to be automotive engines a few decades ago. *shrug*

I have some thoughts in my head but, against my better judgement, wanted to check with the experts here to see what y'all have to say about those results.

 

[Tom-D]

Imagine you do a compression check (a real one, not the fake misnomer) on an airplane engine. Your compression numbers are within spec on all cylinders but one. We are talking warm engine, of course.
All readings were within spec but one cylinder showed almost 20% more pressure. That seems like a big inconsistency in numbers.

Of course my mechanic discounted the test completely, waving arms and shouting that performing an automotive engine test on an airplane engine is pointless and that all one needs to know that if all cylinders show 75-79 psi on the leakdown test, the engine is perfectly fine and good and there is no problem.
It did not matter to him that airplane engines used to be automotive engines a few decades ago. *shrug*

I have some thoughts in my head but, against my better judgement, wanted to check with the experts here to see what y'all have to say about those results.

Tell us what test you are preforming, then tell us the "spec" that is normal?
and then tell us when the aircraft engine was a auto engine.
curious minds want to know.

 

[coloradobluesky]

I've heard of high compression in old 6volt cars where the starter couldnt turn the engine. It only happened when the engine was hot. It was due to oil blowby or some such odd explanation. I don't really know why, but I owned an old 1951 Jeep pickup that did that. Jump it with a 12V and it would start right up. Started fine cold.

 

[airheadpenguin]

Tell us what test you are preforming, then tell us the "spec" that is normal?
and then tell us when the aircraft engine was a auto engine.
curious minds want to know.

Sounds like the test was the automotive compression test, pull the plugs, insert the gauge, run the starter

 

[Dan Thomas]

Imagine you do a compression check (a real one, not the fake misnomer) on an airplane engine. Your compression numbers are within spec on all cylinders but one. We are talking warm engine, of course.
All readings were within spec but one cylinder showed almost 20% more pressure. That seems like a big inconsistency in numbers.

Of course my mechanic discounted the test completely, waving arms and shouting that performing an automotive engine test on an airplane engine is pointless and that all one needs to know that if all cylinders show 75-79 psi on the leakdown test, the engine is perfectly fine and good and there is no problem.
It did not matter to him that airplane engines used to be automotive engines a few decades ago. *shrug*

I have some thoughts in my head but, against my better judgement, wanted to check with the experts here to see what y'all have to say about those results.

Do the proper differential compression test and find out why the other cylinders are so much lower than that "high" one. Spinning the engine doesn't tell you where the leaks are.

 

[Tom-D]

Sounds like the test was the automotive compression test, pull the plugs, insert the gauge, run the starter

Sounds like ??? WTF.. sounds like some body is doing some thing nonstandard, and expecting standard results.

 

[Maciej]

I have some thoughts in my head but, against my better judgement, wanted to check with the experts here to see what y'all have to say about those results.

Yeah... Find a different mechanic.

 

[unsafervguy]

Where is it written it must be between 75 and 79?

Bob

 

[Fearless Tower]

Where is it written it must be between 75 and 79?

Bob

It's in the top ten rules of the Tire Kicker's Handbook.