Rough engine

[tshort]

So we finished the annual on 3949V this weekend, and I took advantage of tonight's beautiful weather to make a test flight post annual.

No issues or squawks during the annual, we didn't make any changes or modifications.

Engine starts and runs normally, runup and mag check was uneventful.

On takeoff about 3-400AGL the engine starts running rough. Not tear the wings off rough, but very noticeable and there was loss of power. Carb heat made no difference. It was intermittent, but lasted 5-6 seconds. I flew a tight pattern and landed, called and talked to my friend the A&P/IA that I do owner assisted annuals with.

During the annual, we cleaned the plugs on the tailgate of the truck with one of the small portable pneumatic cleaners with the little bag on the side. We were careful to clean them thoroughly, but our thought was maybe one of the plugs was intermittently fouled. So I started up again, did a full power static runup, mag check, etc - all normal. Took off again and orbited the airport for 20 minutes, no repeat of the issue.

Here's the kicker. A good friend who I fly with frequently (CFI, long time old airplane guy, wright bro's master pilot award winner - he's been around these things forever) made a comment a couple weeks ago when we were flying:

"Hey, do you feel that?"
"What?"
"I don't know, in the bottoms of my feet, the engine just doesn't feel quite as smooth as I remember it feeling."
"I dunno, feels fine to me"

I really didn't notice anything. But today doing the test flight I was wearing a different headset than I normally wear (couldn't find the other) - a Bose A20. While orbiting the airport I noticed a rattle. It was the instrument panel cover; if I put my hand on it the rattle stopped. I don't know if that was there before, or if the different headset made it noticeable. If you asked me, I'd say the engine felt maybe sorta kinda a little rougher ... but that's kinda like the single engine night or over water or mountains rough engine, probably more due to my experience on the first takeoff. But Stu's comment makes me wonder.

So the question (finally!) is what to do? Plan is to fly it and monitor (don't worry, I will be very in tune for the next few takeoffs!), but I am wondering if you all think any further diagnostics are in order.

Sure makes me appreciate the 6 cylinder engine monitor in the RV10; I had a similar episode on takeoff and with the data easily identified and replaced a failed spark plug.

But I also like the simplicity of the 170...

Thanks in advance

Thomas

[DaveF]

How long since magneto IRAN or overhaul?

[n2582d]

Thomas,
My guess is a sticking valve. You might want to refer to the Engine Troubleshooting Guide or SE 76-24 found in the Maintenance Library.

[GAHorn]

If a thorough annual was performed the magneto points were checked, timed, etc. and an oily feeler gauge or other contamination may have occurred until it burned away. An oily finger on a spark plug lead can do similar.
I had a similar incident 20 years ago and it turned out to be a leaking mag oil seal but after a few hours it went away until the airplane sat for a week...then it returned, repeat, etc.
Replacing both mag oil seals made the fix permanent (at least as far as such things may. Mags should be seriously removed and completely reworked at least every 500 hrs, IMO.)

A stuck valve doesn't run rough. It scares the fecal matter out of you and shakes so bad you can't read some instruments. Didn't sound like that to me from your description.

[n2582d]

George, I think there are degrees of valve sticking. Ten years ago there was this thread in which Drew had similar symptoms to what is being discussed now. After ten pages here is his conclusion:

This thread has a happy ending.

Short version. All along, it was a sticking valve, and getting stickier. Just what many of you posited.

While waiting for the analyzer, I kept working on things that might be a contributor to the issue. Things I could see. One of those things was a welded intake elbow on #2. The weld didn’t look that great, so I bought a replacement. The day the analyzer arrived and we began prep to install same, I began to replace the elbow. I discovered the flange nuts were only finger tight. Once off, I saw that there was nothing wrong with the weld, visually, but that the gasket had visible tracks of fluid; it had been leaking. Equals xtra lean mixture. You see where this is going.

It was overheating the cylinder and had created a 'coking' cover of oil residue on the exhaust valve and in the valve guide. Heat buildup was causing the valve to periodically stick open. The repeatedly over-heated valve springs probably had lost some of their strength, and could not overcome the sticking valve, leaving the valve open until the cylinder cooled as a result of there being no combustion in that cylinder with the valve stuck open.

My IA and I proceeded to do the 'rope trick.’ With the exhaust valve off we found the valve stuck. We could have driven it into the barrel, but decided to pull the cylinder completely off and clean the goo off the valve stem and out of the guide. Which we did. I felt I was assisting an old midwife with my first-borne. He just pulled that jug off like it was no big deal. Scared me half to death. It took all day. I got to look inside the cylinder barrel and actually inside the crankcase through the cylinder opening. An extremely awe-inspiring experience for me. IA took a cursory glance, like it was an every-day experience, and casually mentioned, "sure is clean in there!" I puffed up like my son had just hit a homer. A proud dad. On your advice, I had bought a set of valve springs, so we replaced them on the workbench, me nervous as a cat about every little thing, every piece of dirt, and the IA just keeping on, getting the job done. The cylinder cleaned up and reassembled, we set about sticking it back on the engine. Hey, I'm thinking, I gotta fly this thing back home TONIGHT, before sunset. And, right now I'm holding Danielle's open heart in my hands. Absolutely sobering experience. IA just keeps moving. Getting the cylinder off was the easy part. Getting the muffler, the baffling and all the rest off, and keeping track of all the nuts, springs, etc, etc, was the hard part. Then getting that stuff back on. Cylinder was the easy part. About 5pm, we got her cowling back on, weather turning cold, we were done. I was tired. IA was exhausted. He left for home and I cranked Danielle, relieved by her familiar healthy roar. She sounded fine. I'm in disbelief. I take extra time. A lot of extra time. I'm looking at the engine instruments like I never had. Finally, no excuse to delay, and the sun about to sink out of view, Danielle and I launch over the cliff that is the airport’s approach, into the space 1000' above the valley below. My eyes were glued to the analyzer. #2's temps perfectly normal. Power better than ever. And faster than ever. 2 days to go until the weather window opens for the long trip home. Just in time, as they say.

She flew straight, true, and fast all the way home. 30 knot tailwinds all the way. The third day we landed after a 475nm final leg, with, I would determine after gassing up, 6 useable gallons remaining.

Along the way, not only did the analyzer help with leaning, it was most useful keeping me aware of CHT’s that became borderline a couple of times. My belief is this instrument, though a little pricey, will enable us to enjoy flying Danielle longer before her next TBO. And that will pay for the instrument. So, we don’t feel it was in any way a waste of time and money, even though, ultimately, it wasn’t the analyzer that focused us on #2 as the culprit.

Thanks for all your help along the way on this thing. Especially yours, Bruce.

[GAHorn]

I feel it should be noted that the intake leak Drew found during disassembly … alone.... could have caused the complaint of which he made. That was documented in the ECI/Troubleshooting-charts referenced previously. (It actually could have been the sole-cause... and the valve problem discovered in the repair fortuitous and contributory, but not the root cause. We'll never know but the combination of correcting both faults certainly gives the happy results.)

[c170b53]

I think we can all agree; When you go looking, you’re going to find”. Often it’s hard to tell what initiated the events first but it’s why we look.

[Bruce Fenstermacher]

I would say that after an engine is warm, it would be tough to tell the difference between several issues and a valve that is hanging a micro second on departure.

Most times a sticking valve, not one stuck hard open, shows itself when the engine is cold and tends to go away as the engine warms and the valve guide expands. This is called morning sickness. It's a warning to what's likely to happen soon. And that is one day you will shut down your plane and when it cools and contracts it will seize the valve in an open position. Or if closed the valve will be pushed open and won't close. You won't run your engine long with a valve stuck open. It will shake like you've likely never experienced before. And should you try you won't get much more than 1800 rpm from idle on the ground.

It is possible to stick and seize a valve while in the air. I've done it. This was on my Cub which has practically the same valve system. I was running the engine hard and had just cleared a ridge. I chopped the throttle and descended rapidly about 2000 ft to land. Good think I didn't have to go around cause 3 cylinders just doesn't make enough power in a Cub. Oh and I know that for a fact cause once a spark plug backed out and I was running on 3 cylinders and looking for anyplace to set her down quick.

If this where my engine I wouldn't rule out a stiff valve. It would be on a list with lots of other possibilities.

[170C]

Wonder if you checked the resistance on the plugs. Its possible one has gone bad.

[eskflyer]

That's ironic, just last night I gave a class on checking plug resistance at our easy meeting. Google champion sparkplug resistance. Very good info also tempest gives recommendations. I had had to overhaul a set of slicks because of bad plugs that fired fine in pressure test but we're way put of speck on resistance.

Jp