Exhaust Valve Leak - What's next?

[Captain Ron]

Edit: Dang - of course I missed the spelling error in the thread title before posting. I guess we can't fix those later. Valve Leak!

Hello All,

I need some input from vast bank of knowledge here!

Experimental PA-18, O-290D2B, 260TTSO. For the past 10 years compressions have been solid at 75 +/- 2 on all four cylinders. Last year (June 2020) number 3 went down to 72. I thought ok, lower but only 1 psi out of normal variation, couldn't hear a leak and borescope looked fine. Fast (slow?) forward to this year (25 hours of flying) and compression on number 3 is down to 70 with noticeable leak through the exhaust. I flew it again today for an hour and re-did the compression test and got 60. Staked it a couple times with no change. Had a much closer borescope look (by the way - make sure you read the temperature restrictions on your scope - nearly cooked it) and still seems normal. Tested compression again after borescope and when cooler and got 64. Otherwise the engine is running well, oil consumption is stable at around 8 hours per quart, doesn't lack power, runs smoothly, isn't making metal and oil analysis is normal (have not received this years results yet though).

I'm certainly no expert in reading borescope images so I'd appreciate some opinions on the photos. The glare from the LEDs is a bit annoying but to my eye the complete valve face looks uniform with no asymmetry. Here's probably the best one:



So what's next? The compression trend has me worried but the borescope doesn't appear to indicate imminent failure. Fly another say 5 hours and check again? Find a local mechanic and get a wobble test done? I've read about some success with lapping with the cylinder in place - is that worth a try? Or just pull the cylinder (*shudder*)?

Appreciate the help!

Cheers,

Jeff

 

[BC12D-4-85]

I believe the light ring around the valve is the valve's seat. The valve appears to be off center (?) = thicker seat on one side. A wobble test might help determine the degree of valve guide or stem wear. Maybe have a look at the other exhaust valves with the scope and compare pics?

Gary

 

[8GCBC]

Nothing ever gets better with a cylinder from my experience. Start a plan to replace it. GREEN means burning.

Every (300) hours on a helicopter I do SB 388C:

https://www.lycoming.com/sites/defa...termine Exhaust Value and Guide Condition.pdf

https://www.aopa.org/-/media/Files/...ety/14FN0000-ASI-Valve-Safey-Poster_Final.pdf

 

[BC12D-4-85]

Great info on valve color! Thanks. Always a learning experience here.

Gary

 

[EdH]

Take the rocker cover off and see how the exhaust valve side looks. I just had one with similar symptoms and it was pretty obvious the valve guide was wearing out with the cover off.

 

[BC12D-4-85]

^^^^ This

Gary

 

[Steve Pierce]

I have never seen a valve green all the way around. That means the valve is leaking all the way around the seat? I would do a wobble test and go from there. Probably have to pull the cylinder and at least dress the valve and seat.

 

[irishfield]

Fix it before you get here... I got lucky 11 years back and almost got my glider endorsement!

 

[Cub Builder]

Your valve isn't showing anything terrible in it's condition. I would do the wobble check, and if that is acceptable, if it was mine, I'd pop the valve springs off and lap the valve in place. That's an easy job that only takes a couple of hours and your done. If the valve seats, it may hold for many years. If the compression starts dropping again in the next year or two, you'll want to pull the cylinder to regrind the seat and either grind or replace the valve depending on condition.

-Cub Builder

 

[8GCBC]

Also, check for induction leaks that may be leaning your mixture. A guy at my field had an engine lose (3) of (6) cylinders simultaneously in flight which was not enough to keep airborne. It was a huge induction leak which burned up the cylinders.

I seen induction leaks on almost every air cooled aircraft engine in service. It’s something commonly overlooked during inspection from my experience.

 

[BC12D-4-85]

Generally a tight engine with minimal induction leakage should show low intake manifold pressure or high vacuum near idle. Note when new or tight then check before shutdown as it ages. Lower MP than teens is good from what I've experienced. If it bounces around some suspect something mechanical is happening like a sticky valve. Read.

Gary

 

[8GCBC]

Generally a tight engine with minimal induction leakage should show low intake manifold pressure or high vacuum near idle. Note when new or tight then check before shutdown as it ages. Lower MP than teens is good from what I've experienced. If it bounces around some suspect something mechanical is happening like a sticky valve. Read.

Gary

Similar to a hover check on the R44. Depending on the W/B and DA after pulling into a stabilized hover, the MP is noted. If MP is higher or lowered than typically needed, time to trouble shoot and remedy the issue.

Generally, my DA is about -300’ and take off weight is at gross on the first flight of the day. I should see about 20” MP IGE hover.

MP pretty much tells it all in an indirect way. Good article.

 

[Captain Ron]

Thanks guys! Lots of good stuff as always.

I believe the light ring around the valve is the valve's seat. The valve appears to be off center (?) = thicker seat on one side. A wobble test might help determine the degree of valve guide or stem wear. Maybe have a look at the other exhaust valves with the scope and compare pics?

I think the off center is a bit of an optical illusion caused by not being able to get square with the scope. I noted that when I rotated the scope to view the intake valve it was the opposite side that appeared thin and the same thing appeared in all cylinders.

GREEN means burning.

I need to research this more. Previously when reading the various valve health articles I could find it stressed the issue was asymmetry. Having the whole valve head green is unusual it seems. I note that all the cylinders have uniform green on the exhaust valves. I wonder what causes the green specifically. I believe O-290 valves are solid stemmed which the Sky Ranch manual tells me means they run several hundred degrees hotter than sodium cooled valves and I wonder if that contributes? FWIW I also run 100% MOGAS.

Take the rocker cover off and see how the exhaust valve side looks. I just had one with similar symptoms and it was pretty obvious the valve guide was wearing out with the cover off.

Had the rocker cover off to stake it and nothing seemed out of place. But I will remove the springs and take a deeper look.

I have never seen a valve green all the way around. That means the valve is leaking all the way around the seat?

Yeah, weird eh? I guess leaking around the whole seat could be possible if the valve wasn't allowed to close all the way? But then I'd expect no compression at all. I did check to make sure the rocker clearance was on spec and not holding the valve open. I'll hunt down a local mechanic for a wobble test.

And thanks for fixing the thread title!

Also, check for induction leaks that may be leaning your mixture

Oh, ahead of the curve on that one I think. I tighten the clamps and nuts each annual. Also replaced the gaskets and hose connectors at last annual (based on calendar age). But I will double check that - cheap insurance.

Generally a tight engine with minimal induction leakage should show low intake manifold pressure or high vacuum near idle. Note when new or tight then check before shutdown as it ages. Lower MP than teens is good from what I've experienced. If it bounces around some suspect something mechanical is happening like a sticky valve. Read.

This is interesting and I'll have a read. I of course have no MP gauge but I could slave something-up for a test run each annual. Would it need to be done on each cylinder to be representative? I don't think there is common place to tap a pressure from but I could pull the primer lines on each cylinder.

I've read something similar about trending crankcase pressure (Continentals?) for monitoring ring blow-by. Time to experiment!

So it looks like a wobble test is next. Have to hunt down a local mechanic.

Thinking ahead, if the cylinder needs to be pulled, what is the latest opinion on honing & replacing rings if the jug comes straight off and straight back on leaving the piston & rings in place? It seems like it was 50/50 between "absolutely essential" and "waste of time". As you know I'd have to sell an organ to pay for O-290 rings...On the bright side I guess I'll get a free look at the cam shaft.

Cheers,

Jeff

 

[BC12D-4-85]

Thanks guys! Lots of good stuff as always......

This is interesting and I'll have a read. I of course have no MP gauge but I could slave something-up for a test run each annual. Would it need to be done on each cylinder to be representative? I don't think there is common place to tap a pressure from but I could pull the primer lines on each cylinder.

I've read something similar about trending crankcase pressure (Continentals?) for monitoring ring blow-by. Time to experiment!

Cheers,

Jeff

The cylinders share a common plenum for an induction system, so one tap on a cylinder (primer hole for example) should reflect the rest. Maybe being closer to or on the offending cylinder changes that somewhat tho. MP gauges and primer fittings are usually dampened via a small orifice so can be less sensitive that a common automotive vacuum gauge. What would be nice would be an annual trend in vacuum (or the reverse - MP) info as the engine ages, similar to the leak down or optionally direct compression tests that are performed. The crankcase pressure can be read via a vacuum gauge if it has a reverse or pressure scale as well....similar to Continental's suggested use of an airspeed indicator. All of this is just another way to assess potential causes of sickness in an older engine.

Gary

 

[Captain Ron]

Update...

Built a wobble tester and valve spring compressor, bought some rope and...exhaust valve stem movement is approximately 0.045" vs. the maximum of 0.030" in SB388C. For fun I played around with the valve with the compression test air applied and could get readings between 55 and 70psi just by rotating it and letting the air pressure close the valve (i.e. turn on the air with the valve open and then pull on the stem to get it moving). I also tried, for fun, lapping it in place...which no surprise made no difference.

So I guess that's it, the cylinder comes off. Any words of advice? I assume I can have a shop replace the guide and seat easy enough, just need to find one.



[just in case anybody with sharp eyes is looking - that is a rocker shaft from the junk pile installed to hook on the spring compressor...the actual shaft is pristine!]

Cheers,

Jeff

 

[Steve Pierce]

Sal's Aircraft Cylinders
https://g.co/kgs/FoTNbq

 

[frequent_flyer]

Update...

Built a wobble tester and valve spring compressor, bought some rope and...exhaust valve stem movement is approximately 0.045" vs. the maximum of 0.030" in SB388C. For fun I played around with the valve with the compression test air applied and could get readings between 55 and 70psi just by rotating it and letting the air pressure close the valve (i.e. turn on the air with the valve open and then pull on the stem to get it moving). I also tried, for fun, lapping it in place...which no surprise made no difference.

So I guess that's it, the cylinder comes off. Any words of advice? I assume I can have a shop replace the guide and seat easy enough, just need to find one.

I went through a similar exercise on my PA28's O-360 about 5 years ago. Pulled the cylinder expecting worst case to pay for seat, guide, valve, and labor. Cylinder shop found the cylinder cracked and told me they could not fix it. They were closing shop in a few months and sold me a used/reconditioned cylinder at a very fair price. Now, several years, later it's 79/80. Pulling the cylinder was easy. Getting the wrist pin out not so much. Had to fabricate a custom puller and use some heat but eventually it decided to come out.

Cost for cylinder with piston and rings, gaskets, labor, and sign off well under $1,000 but I had a few hours of sweat invested. The good thing was that it gave me a chance to look at the exposed cam lobes and followers. They all looked like new.

 

[Captain Ron]

Sal's Aircraft Cylinders

Thanks Steve. Looks my kinda place but I'm going to have to settle for something a little more local to me. Too much hassle shipping back and forth across the border. Cylinder came off the this morning without any trouble and will be shipped tomorrow to AeroTec Engines. Had a look at the cam and tappets. Cam looks good but one tappet is certainly starting to show wear. I'd better start planning for more fun in the next couple years.

 

[BC12D-4-85]

Make sure the wearing tappet isn't trying to open a tight valve.

Gary

 

[skywagon8a]

Had a look at the cam and tappets. Cam looks good but one tappet is certainly starting to show wear. I'd better start planning for more fun in the next couple years.

Best to think twice about that tappet. I had the mushroom of an intake tappet break off in an 0-540. The first clue was the entire engine went silent. While flying low over hills in a light snow flurry. :evil: After what seemed like a long time, the engine started running again telling me "Who ME?". Then after the hills, an airport (Danbury CT) popped up under the nose and it did it again. To make a long story short after an in depth investigation, I found the broken mushroom had jammed the tappet into the case causing the intake valve to stick open, snuffing out all the fuel/air mixture to the other cylinders. The engine was trashed. The lucky part was the second time there was an airport to land on, preventing damage to the airplane and possibly me.

I shudder to think of that happening while over some of the places in Canada where I've flown.

The same thing happened to another 0-540 in the same type of airplane where I worked at the time, with equally good (airport under the nose) results.

 

[Captain Ron]

Make sure the wearing tappet isn't trying to open a tight valve.

Yup, I plan to do the wobble test on all the remaining valves.

Best to think twice about that tappet.

It has been gnawing at me. The wheels are certainly in motion for a complete teardown and overhaul. I'm in the O-290 purgatory...I've got enough new and serviceable parts saved up to make building up an engine feasible but depending on my mood it is either a waste of money or a great idea. As I type this I'm leaning towards to the rebuild rather than switch to an O-320 to avoid having to change out accessories and propeller...but I do fancy one of those Sensenich ground adjustable props and a lightweight starter and...!

I should get the report back on the cylinder Tuesday - that might make some decisions for me.

Cheers,

Jeff

 

[AKaaron]

An example of a bad exhaust valve in an O320. Borescoped a few days ago, cylinder will come off on Tuesday for a new exhaust valve.

 

[Captain Ron]

Cylinder came back from Aerotec Engines Ltd. with new guides, studs and paint job and looks great. Valves were cleaned and inspected and were serviceable. Installed it on the airplane this morning, did a quick circuit and then a compression test and it is back up to 74psi. Plan for full overhaul is in motion also - decided to stick with the O-290 and invest the "savings" in fuel. Thanks for all the help along the way.

Cheers,

Jeff

 

[jimboflying]

In regards to MP what’s a fluttering gauge mean?

 

[skywagon8a]

In regards to MP what’s a fluttering gauge mean?

Did this just start or has it been happening all along? Where is the manifold line connected to the engine? If this is a new happening and the pick up is on one cylinder, check for a leaky intake valve on that cylinder.

 

[jimboflying]

Duration unknown, new plane to me. Lycoming IO 360. MP line goes into right rear cylinder intake fitting. Vacuum test of line and gauge doesn’t show a leak. Compression test was fine.

 

[BC12D-4-85]

When does it flutter the most? Idle or cruise?

Gary

 

[skywagon8a]

fluttering gauge.....?

More questions, is the needle just fluttering or is it moving at a high rate? Does the frequency change with an rpm change or airplane speed change? Is there an audible variation noted? This may not be associated with the manifold pressure system at all. Its could be just a demonstration of a particular resonance in the airframe, such as a prop imbalance. The MP needle is just the visual evidence. What type of airplane is this? Maybe the MP needle is loose on it's shaft?

 

[jimboflying]

It fluctuates in a fluttering motion about 2” at cruise and not at idle. I can not hear any noise because it is an open cockpit Great Lakes.

 

[BC12D-4-85]

One way to test would be to remove the tubing at the cylinder and plug the cylinder hole. Go fly and see if it vibrates from what Pete suggests. If not then there may be an engine issue.

Gary