Compression 75/80 with leaking sound out of exhaust pipe.

[saltshaker80]

I just did a compression test for good measure after a suspected carb ice incident. I wanted to check my bases and reassure myself it was carb ice that I experienced. Among other things I did a differential compression test.
77-78 over 80 on 3 out of 4 cylinders. Cylinder #3 which always runs hotter than the rest on my O-360 was 75/80. But it didn’t turn the prop like the other 3 and air was leaking out of the exhaust pipe.
My IA told me that’s fine (over the phone) but I can’t get the obvious thought out of my head that it’s a bad exhaust valve on #3.

Any input on this? Seems off to me.

 

[Silvaire]

...But it didn’t turn the prop like the other 3 ...

huh?

 

[AV8R_87]

You could be lucky and it's only some carbon trapped under the exhaust valve. Fly it for a few hours and re-check.
If it still leaks, probably bad valve. Borescope the engine, if you can, and check for the telltale signs of a bad valve.

Anatomy of a Valve Failure

www.aopa.org

But it didn’t turn the prop like the other 3 and air was leaking out of the exhaust pipe.

75/80 should still turn the prop, though. I had a cylinder at 60-something (really bad exhaust valve) that still turned it, weaker than the rest but it did turn.
Easy check before/after flight for the next 10 hours, carefully rotate the prop once/twice, you can feel if one cylinder leaks more than the rest. That will tell you if the valve situation improves.

 

[saltshaker80]

huh?

Opening the valve on the compression tester to 80psi pressurized the other 3 cylinders enough to push the piston and rotate the engine, I had to hold the prop still to prevent that. On the #3 cylinder the air seemed to leak out of the exhaust valve and didn’t push the piston, I didn’t even need to hold the prop, it wasnt moving even although I was getting 75/80.

 

[Dana]

You could be lucky and it's only some carbon trapped under the exhaust valve. Fly it for a few hours and re-check.

If it's still doing it you might be able to lap the valve without removing the cylinder. I did that on my O-290, brought compression on one jug up from around 40 to the upper 70s.

Opening the valve on the compression tester to 80psi pressurized the other 3 cylinders enough to push the piston and rotate the engine, I had to hold the prop still to prevent that. On the #3 cylinder the air seemed to leak out of the exhaust valve and didn’t push the piston, I didn’t even need to hold the prop, it wasnt moving even although I was getting 75/80.

More likely you were at exactly TDC.

 

[Bell206]

Any input on this? Seems off to me.

Was your check done on a warm or cold engine?
Why does it seem off to you?

 

[Checkout_my_Six]

I'd rather "see" what they look like....and then I'd make an assessment.

 

[Gary Ward]

I'd rather "see" what they look like....and then I'd make an assessment.

 

[Checkout_my_Six]

It's sealing and heated evenly....it's fine.

One other thing....if you can watch it seat that will give an indication of guide wear. If it seats squarely and doesn't have an off set as it seats the guides are good. And you can test the rotocoil....if it has one....tap on the valve stem with a plastic hammer....the valve should rotate.

 

[Steamflyer]

Opening the valve on the compression tester to 80psi pressurized the other 3 cylinders enough to push the piston and rotate the engine, I had to hold the prop still to prevent that. On the #3 cylinder the air seemed to leak out of the exhaust valve and didn’t push the piston, I didn’t even need to hold the prop, it wasnt moving even although I was getting 75/80.

At top dead center the connecting rod is inline with the bore and in theory the piston will not go down with pressure. You can use a pencil to find it, or there is a gizmo that you can screw in to the spark plug hole to do it.

Any exhaust valve leak regardless of held pressure warrants investigation. Several suggestions have been proffered here as to next steps.

Not following the carb ice issue that motivated the compression check - did carb heat in flight remedy the situation or not?

 

[Stewartb]

Did you rock the prop back and forth to find the sweet spot?

Unsure? Have your mechanic test it. The internet won’t fix it.

https://www.lycoming.com/sites/default/files/attachments/Cylinder%2520Compression.pdf

 

[Clip4]

I just did a compression test for good measure after a suspected carb ice incident. I wanted to check my bases and reassure myself it was carb ice that I experienced. Among other things I did a differential compression test.
77-78 over 80 on 3 out of 4 cylinders. Cylinder #3 which always runs hotter than the rest on my O-360 was 75/80. But it didn’t turn the prop like the other 3 and air was leaking out of the exhaust pipe.
My IA told me that’s fine (over the phone) but I can’t get the obvious thought out of my head that it’s a bad exhaust valve on #3.

Any input on this? Seems off to me.

Just out of interest, when was the last time you had a wobble test done on your engine?

 

[Dan Thomas]

Opening the valve on the compression tester to 80psi pressurized the other 3 cylinders enough to push the piston and rotate the engine, I had to hold the prop still to prevent that. On the #3 cylinder the air seemed to leak out of the exhaust valve and didn’t push the piston, I didn’t even need to hold the prop, it wasnt moving even although I was getting 75/80.

I do hope you are placing the piston at TDC. Are you just applying pressure without turning to TDC first? If so, that cylinder might have been far down its stroke and the exhaust valve just opening.

At top dead center the connecting rod is inline with the bore and in theory the piston will not go down with pressure. You can use a pencil to find it, or there is a gizmo that you can screw in to the spark plug hole to do it.

Or you can turn the regulator up just a bit, like 20 psi, and move the prop to find TDC. It's obvious from the resistance, which mostly disappears at TDC. Then go to 80 psi.

 

[Barrett50]

Maybe do the rope trick? Cheap, fast, easy. May do nothing, may change it significantly. Takes 15 min to do. Just don't go hog wild with the prop slamming.

 

[Magman]

An old guy like me would likely need at least an hour.

 

[Dan Thomas]

Maybe do the rope trick? Cheap, fast, easy. May do nothing, may change it significantly. Takes 15 min to do. Just don't go hog wild with the prop slamming.

Even when I was young it took way more than 15 minutes. How many have you done?

 

[Domenick]

Here is a tip for taking differential compression, learned the hard way with 7 staples in my scalp.
For each cylinder do the test in two steps.
1. Pressurize the cylinder to no more than 40 psi and find the sweet spot by gently rocking the prop and finding the highest reading.
2. Hands, feet, and head, out of the prop arc, raise input pressure to 80 psi.