2 Bad Cylinders

[Kyle Wolfe]

Looking for your input...

Have 2 cylinders that were found to have low compression on annual. Both of these were overhauled by Poplar Grove Airmotive. The first was 2 years ago, the second cylinder was overhauled last year. I've got the yellow tags and (from memory) they installed new guides and ground the valves - they did not install new valves.

I'm checking to see what recourse I might have with Poplar Grove.

I'm leaning towards just installing 2 new cylinders as I'm suspicious about the fact that the 2 bad cylinders were already overhauled. And cost doesn't seem to be much different than overhauling.

What would you do?

What cylinders have we had success with?
I've searched the forums and am aware of the ECI cracking problems.

My mechanic is also advising a carburator overhaul and installing new gaskets on the intake to make sure we eliminate any possibility of leaks or lean conditions. Comments?

[GAHorn]

Bad time to arbitrarily overhaul a carb unless it definitely has problems. (Have you read about the recent Precicion troubles?)
"overhaul" of a cylinder may be generic use of the term.
As for what action to take, I'd be curious as to WHY the compressions are low. Rings? Valves? Cracks?
Superior cylinders and TCM cylinders are my personal favorites.

[robert.p.bowen]

Did your A & P borescope the cylinders? Did he calibrate his compression tester with a master orifice tool? What makes him think the cylinders need to come off?

As you probably know, TCM has a Service Bulletin that describes how to test compression and what to do if compression is low. Basically, a cylinder with compression as low as 42-45 lbs. may be entirely serviceable. Even with compression lower than this, pulling the cylinder isn't the next step. Go online to TCM website and get SB03-3.

[Kyle Wolfe]

Thanks George for the reminder about the Precision problems. I think I'll stall on the carb issues.

I'll pull up the TCM service bulletin.

The most recently OH cylinder is leaking past the exhaust valve - I can hear the air escaping from the muffler.

The 2 year old OH is leaking past the rings through the carb.

[davevramp]

The most recently OH cylinder is leaking past the exhaust valve - I can hear the air escaping from the muffler.

There are a few things an A&P can do to inspect and correct without removing the cylinder.

The 2 year old OH is leaking past the rings through the carb.

I would suggest that it is leaking past the intake valve. Check both valves guides. And have the valve springs tested. The best performance for the dollar is replacing the valve springs. If I can help please pm me with your phone number.

Dave

[jrenwick]

Hi Kyle,

Sorry to hear of the problems! I recently had low compression on three of my cylinders (Superior, with 640 hours on them). Two were leaking through the exhaust, one through the intake. Bolduc replaced the exhaust guides on two, the exhaust valves on all three, reground the valves, and they're all back on the airplane. Seems to run very well now, with 7-1/2 hours since the repairs.

Good luck!

John

[Bruce Fenstermacher]

Kyle,

I agree with Dave's assessment. It is a pinch to early to say that the cylinders were not overhauled correctly. I've had several exhaust valves with nothing more than carbon on the seat holding them open. I've corrected this with a bit of valve lapping without removing the cylinder. I have access to a bore scope so I've also been able to determine there wasn't other issues. The TCM procedure and recommendations for what is a good cylinder are pretty liberal . More than likely your cyclinder meets the TCM criteria and you can just fly a few hours as they are and the carbon will be gone.

As for the leak throught the carb Dave is right that would be an intake valve, not the rings. If it were rings the air would be heard through the crank case breather tube.

I'd also want to use a bore scope and inspect the intake valve and seat. If all looks well then Dave could be right in that the problem may be nothing more than a weak valve spring.

[170C]

Kyle, sorry you are having cylinder problems. You are getting some good suggestions from others on the forum, but I put 6 new ECI steel cylinders on my plane at overhaul in 1999 & now with just over 600 hours, I can't complain. Yea, I have had some valve push rod tube leaks, but that can happen to any of them. I did have to pull one cylinder for replacement of a bad exhaust guide around 450-500 hours, but other than that the ECI's have given me good performance. If I had to replace all of mine again I would most likely go with ECI's again. This past Feb at my annual I had one cylinder that had low compression. It was at the exhaust also. Compression check was done on a cold cylinder and I prefer to check them when the engine is hot, but AI said it was likely a piece of carbon under the value (as Bruce mentioned). That cylinder now seems to be OK, so I hope you will explore some of the suggestions before pulling either of those two cylinders.

Good luck!

[Kyle Wolfe]

My AP says he's bore scoped the cylinders and cannot find anything that looks suspicious.

Dave what are some things that can be done without removing the cylinder? I'm thinking replace the valve spring. I'm guessing maybe lap the valve? What else?

I'm just disappointed that with 65 and 130 hours on an OH from a reputable shop they're giving me trouble.

I'll chat with the AP tomorrow and find out what he found from the OH shop.

Appreciate your input guys! It's stuff like this that make this group so valuable.

[GAHorn]

Kyle, 65/80 is not "bad". Run that engine 5 or 10 hours and take the compressions again. You may find it has changed completely. Individual readings may vary every few hours or so. That's OK.
65 psi is NOT disqualifying!
The exhaust valve leak is possibly carbon, or weak springs, etc. RUN it a few hours and check it again (provided the reading wasn't less than 45/80.)

Don't worry about it if you don't have a calibrated "master" compression tester. You are not trying to certify the engine. You are merely trying to get it to run reliably and safely. A standard, everyday compression tester is just fine for this type of troubleshooting, re-flying, and re-testing.

(But watch that leaky exhaust valve. Leaking exhaust valves can be very dangerous. They can fail catastrophically and drop down into the cylinder.
Such failures are extremely rare with INtake valves because Intake valves are constantly cooled by the in-rush of fuel/air mixture. Exhaust valves on the other hand, when they leak, are like cutting torches. It doesn't take long for something serious to fail.

[Kyle Wolfe]

I should have written "65 hours" - it wasn't the pressure reading.

The tester was calibrated.

Readings were both in the mid 40's for PSI.

[Bruce Fenstermacher]

Kyle,

Lapping the valve and springs are all that can be done without removing the cylinder. It would seem by your comment that your IA has already inspected the valve and seat with a bore scope. Perhaps he just did a cursory inspection.

To use a bore scope and see the seat and the valve sufficiently he would have had to drop the valve into the cylinder in my experience. He should have also checked the valve guide for excessive wear. A worn guide would allow the valve to be misaligned and not set correctly. Having found nothing out of kilter the exhaust valve could have been lapped when it was reinstalled.

When I find a cylinder like yours with low compression and a leak at the valves here is the order that I would perform the inspection/repair work.

1. I might run the engine for a bit or alternately with the piston low in the cylinder and the valve cover removed, rap on the offending valve with rubber mallet a few times to “stake” the valve and hopefully break up any carbon deposits. If that doesn’t work then:
2. Do a cursory inspection of the valve with a bore scope to without disassembly to see if any abnormality can be detected. It nothing is seen then:
3. Remove the valve spring and wiggle the valve in the guide to get an idea of the guide wear. Of course wiggling the valve isn’t as precise as measuring the wear which should be done but I wiggle first to determine if I’m going to measure. Assuming the guide seems OK then:
4. I drop the valve into the cylinder and inspect the valve seat with the bore scope. I fish the valve stem through a spark plug hole and inspect the seat and the stem. Assuming I find nothing burnt or out of aligned then:
5. I clean the valve stem and guide of carbon and lap the valve into the seat then:
6. Clean and reassemble.

Chances are you will fix the problem after following the procedure. Time per cylinder should be about an hour or may be less depending on baffling.

You may be right to suspect the cylinder work but to be honest specially with the exhaust leak if it is carbon deposits, it could have been any of your cylinders, just happened to be those.

[GAHorn]

... Go online to TCM website and get SB03-3.

Or from the MX Library:
http://www.cessna170.org/forums/viewtop ... f=6&t=5739

[GAHorn]

... Go online to TCM website and get SB03-3.

Or from the MX Library (This superceded M84-15):
http://www.cessna170.org/forums/viewtop ... f=6&t=5739

[c170b53]

Like many and like Bruce I've had carbon build up that lead to a cylinder removal. I boro'ed the cylinder and I could see the valve was not seating and that the valve seat didn't look right. Carbon is hard to see with the boroscope as it is near impossible to see the far side of the seat (looking across from the spark plug hole). Since it was the same valve that gave me trouble two annuals in a row, I removed the cylinder only to find carbon. I believe when under full pressure (running) the valve(s) will seat.