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

saltshaker80

saltshaker80

Filing Flight Plan
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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.
 
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.
www.aopa.org

Anatomy of a Valve Failure

www.aopa.org
saltshaker80 said:
But it didn’t turn the prop like the other 3 and air was leaking out of the exhaust pipe.
Click to expand...
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.
 
Silvaire said:
huh?
Click to expand...
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.
 
AV8R_87 said:
You could be lucky and it's only some carbon trapped under the exhaust valve. Fly it for a few hours and re-check.
Click to expand...
:yeahthat:

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.
saltshaker80 said:
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.
Click to expand...
More likely you were at exactly TDC.
 
Checkout_my_Six said:
I'd rather "see" what they look like....and then I'd make an assessment. ;)
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735785fd-4af6-4dad-810c-a4cd78701e83.jpg

9b797d3a-8092-4b17-a1b8-9c3d06794026.jpg
 
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.
 
saltshaker80 said:
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.
Click to expand...
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?
 
saltshaker80 said:
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.
Click to expand...

Just out of interest, when was the last time you had a wobble test done on your engine?
 
saltshaker80 said:
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.
Click to expand...
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.
Steamflyer said:
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.
Click to expand...
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.
 
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. :eek:
 
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.
 
Dan Thomas said:
Even when I was young it took way more than 15 minutes. How many have you done?
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Some people are slow. Young, old, just s-l-o-w. I can't count the number done as I have an engine which is known for exh valve leaks. I guess, if it's really that important to you, I can go time it. So, what if it takes 20 min? Are you arguing the point that it's a waste of time, of that I'm doing it wrong, or are you just being a curmudgeon?
 
Another thing that's commonly done is to "stake" the valve: With the pressure applied (and somebody else holding the prop!) you rap the back of the rocker arm with a soft hammer. If it's a bit of carbon holding the valve from sealing that will sometimes dislodge it.
 
robertb said:
Hmmm. What's the "rope trick"?
Click to expand...
When you have a stuck [open] valve, you push a bunch of rope into the cylinder through the spark plug hole, then (carefully!) rotate the prop until the piston packs the rope into the top of the cylinder and pushes the valve close, hopefully unsticking it. If a valve is sticking it'll usually stick again so it's a temporary (or diagnostic) solution, but it can get you home.

It's also used to keep valves closed so the keepers and springs can be removed if compressed air isn't available.
 
Capt. Geoffrey Thorpe said:
Click to expand...
That's the process that takes a lot more than 15 minutes. I lapped a couple of the exhaust valves in my A-65 that way, spinning the valve with a drill using a 3/8" bolt with the head cut off and short chunk of hose between it and the valve stem. One has to be really fussy about cleaning all the lapping compound out afterward. This takes lots of time.
 
Dana said:
When you have a stuck [open] valve, you push a bunch of rope into the cylinder through the spark plug hole, then (carefully!) rotate the prop until the piston packs the rope into the top of the cylinder and pushes the valve close, hopefully unsticking it. If a valve is sticking it'll usually stick again so it's a temporary (or diagnostic) solution, but it can get you home.

It's also used to keep valves closed so the keepers and springs can be removed if compressed air isn't available.
Click to expand...
We weren't talking about stuck valves. We were talking a leaking exhaust valve. A stuck valve would never give 75/80. It would give 0/80.
 
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