Sudden Engine Vibration

phantomphixer · Post by **phantomphixer** » Thu Nov 29, 2007 2:36 pm

Here's my sunday morning story:
Took off with a friend(Harrier Driver, who had to eject Tueday night due to engine failure-he's okay) and his dad. Got appx 5 miles north of airport and all of a sudden had an engine vibration, bad one. Went full rich on the mixture, no change-started a turn back to the airport-made an eyeball check of ignition key-on both-started a shallow climb just in case-then I was a little leary about switching mags, but tried it any way and the vibrations didn't make any difference on either mag. Seemed to get better as I got closer to runway and by the time I landed it was almost gone. Annual started immediately after, low compression on #4 but engine was already cool by the time I got to compression testing. I use autogas so plugs looked normal except for #2 and #3. They were sootie(is that a word?). #4 upper plug was tough to remove compared to others. Cleaned up plugs and all bomb tested good. I'll check ignition harness this week. All other compressions good so I do not suspect a stuck valve. I'll pull the valve covers and check for broken valve springs. Any other things I should look for? OAT was 55F. 700smoh

cessna170bdriver · Post by **cessna170bdriver** » Thu Nov 29, 2007 7:20 pm

Classic symptoms of a stuck valve. To identify the offending cylinder, run the engine on the ground for a short while and note afterwards if any cylinders are cool to the touch. If it still runs rough, and the problem is a stuck valve (most likely exhaust) it'll be easy to see with the valve cover removed. The valve spring will be obviously compressed, and the rocker arm will just be along for the ride.

BTW, when I ran mogas, ALL my plugs (and exhaust pipes) ran sooty, ALL the time.

Good Luck! (Been there, done that, know how it feels).

Miles

Indopilot · Post by **Indopilot** » Thu Nov 29, 2007 9:25 pm

I once had a bad vibration that went away briefly. Thought it was carb ice, did numerous mag checks as well as full power runs. Seemed ok so went to take off for the return trip and it came back. This time I found a chunk of ring stuck in one of the spark plugs. A ring had broken then wallowed back and forth till it finally came out and entered the combustion chamber. If I remember correctly it was about 1100 smoh. Just something else to consider.

ak2711c · Post by **ak2711c** » Fri Nov 30, 2007 8:37 am

Did you try carb heat? It is common to stick valves in these motors but usually not while running car gas. I'm sure I'll get chastized for that comment. However these motors are a lot more prone to carb ice while running car gas. Just a thought.
Shawn

Post by **Bruce Fenstermacher** » Fri Nov 30, 2007 1:44 pm

Shawn

I've never had more carb ice with MOGAS in fact and it's just coincidence I've had more now running 100LL. Running MOGAS in my opinion helps tremendously with stuck valves it does not eliminate the problem.

While I wouldn't rule out a stuck valve I think it unlikely as they generally stick with a change in engine temperature such as in a cold engine at start up, a cooled engine after throttle reduction.

Finding no other cause I'd think carb ice but this flight took place in Arizona. In my mind there isn't enough water in the air in Arizona to make carb ice. But of course what do I know about Arizona from my cumulative 2 weeks total time in the desert there.

Post by **GAHorn** » Sat Dec 01, 2007 3:31 pm

Mogas/Avgas...these engines stick valves with either. Incorrect valve-stem-to-guide clearance is usually the problem in new/rebuilt/repaired cylinders, and carbon/coked-oil from excessive heat is common on engines with improperly maintained cooling baffles and cowls, although operator technique can also contribute.
I stuck a valve on an O-200 simply by flying into a rainsquall. (100 ft AGL on a pipeline patrol. It was an attention-getter.) It was on an engine with new cylinders that had very tight valve-guide clearances. And those engines were run exclusively on 80/87 avgas.
Lead in Avgas is possibly a contributing factor, however the stuck valves I've seen after removal did not seem to be caused by lead. They were stuck by improper assembly clearance or by coked-oil/carbon. Use TCP if you want to get rid of lead. (If lead were a primary cause of stuck valves I'd think more engines would experience it as thousands more engines are run on Avgas than Mogas.)
Marvel Mystery Oil has mixed results. Some folks have used it and swear by it. Others have used it and it did no good at all. The downside is that using it in the oil as a solvent to loosen up stuck rings/valves carries risk of breaking free other sludge and sending it through the engine, and it's effect is unknown/unproven with regard to itself being a source of coked valves. In any event it is not an approved additive. (I've also never heard of anyone being slammed by FAA for using it except an old-timer mechanic for an airline who told me Delta got turned in by AA back in the days of DC-6's/7's/Connies. He said Delta believed it kept their dispatch reliability up... but the guy who told me that was also Delta's lead mechanic at the time, and he was a MMO proponent. I suspect he was the one who authorized it's use. What else would he say?) Using it in the gas contributes to fouling. In my view I can't see much difference in that and deliberately contaminating the system with jet-fuel.
Those who are predisposed to believe snake oils have solved their problem without their having to pay the costs of true mechanical repair will continue to promote those beliefs, even if the problem re-occurs time and again over the years. Each time they have the problem they pour in snake oil and swear by it again. My expereince with the pipeline patrol fleet was that the rope-trick or a cooled down engine will solve it for a while also. In fact the rope trick has solved one-time-never-to-occur-again events, in some cases. So has mechanical repair.
There is no controlled experiment that I know of that has proven mogas (or any other scheme other than mechanical repair) is a cure for stuck valves... and even a mechanically sound engine can develop stuck valves again due to coked oil.
Change oil regularly. Keep cowls and cooling baffles in good condition ...and correct. (You might look yours over and be surprised to find they may not comply with the IPC. Home-brew changes to engine cooling baffling, etc. is not a good idea.) Avoid rapid engine temperature excursions and use only approved lubricants and fuel additives. (I'd like to add that practice has resulted in my own engines never having stuck a valve but then it'd probably happen.)

Larry Holtz · Post by **Larry Holtz** » Sat Dec 01, 2007 4:02 pm

Its annual time and #4 and 5 have air hissing thourgh. I pulled #4 off yesterday and the rings are stuck. I hadnt realized a problem with it. #5 started acting up on the way back from Alaska last summer. On startup it wouldnt fire until it warmed up. It got better after I was home for a while and was back on mogas, but still wasnt fireing for about 15 or 30 seconds after startup. Valve isnt stuck so suspecting burnt or guide problem. This engine always runs near the upper limits on the temp gage. I'll pull it off next week.

Larry

ginbug92b · Post by **ginbug92b** » Sat Dec 01, 2007 8:30 pm

I once had a similar problem in my Cub with a Cont. C-90,(same cylinder assemblys as 0-300). The engine would run rough and lose about 250RPM. I went nuts checking EVERYTHING, gas, plugs,leads,mags, even tried a carb from another plane that was running fine. Long story short the problem turned out to be weak valve springs. The offending spring was most likely only on 1 cylinder. Instead of testing all the springs I just bought all new springs for the whole engine. Springs can lose their tension over time and inactivity and what I was getting was valve float, hence the roughness and RPM loss. Anyway, new springs compleatly cured the problem. ginbug92b

ak2711c · Post by **ak2711c** » Sun Dec 02, 2007 9:49 am

I started suspecting a connection to carb ice and mogas after a few years of almost exclusive use. I would usually get carb ice 2-3 times per flight hour and in extreme conditions it was almost constant, as soon as I would turn the carb heat off it would start forming again. I started noticing that on the occasion that I would run 100LL I would rarely get carb ice. I started paying close attention to it over the next couple hundred hours to note what fuel I was running when I got carb ice. There was a dramatic difference between the two. I started asking around to see what other local pilots had experienced running mogas and they had similar results too. The biggest eye opener for me was the last year I ran my stock motor I ran only 100LL and what a treat it was to not have to constantly baby sit the carb heat. However if you are not in an environment where carb ice is very prone you probably will not notice much difference.
Shawn

mrpibb · Post by **mrpibb** » Mon Dec 03, 2007 2:26 pm

I kinda concur on the stuck valve because prior to my ordeal as posted earlier http://cessna170.org/phpBB2/viewtopic.p ... highlight= my precursor was that a month or two earlier I had the same symptoms but not as bad. mine was when I started my airplane on a cool summer morning and proceeded to run up I got the bad vibration, I knew it wasn't carb ice, ignition or fouling, the kicker is I KNEW it was a stuck valve but after a few couple of seconds it cleared up and being an A&P I felt nigh invulnerable so onward I went. But the problem remained, somewhere there's a tight valve guide in one of them cylinders. Well no more than a month later I had my Ordeal, got a lot of crud and carbon from that guide I reamed. Now on the next annual when I pulled plugs to check compression that cylinder that I had issues with were a little moist, mind you I just flew almost three hours up to NH to do the annual so everything was toasty. So I yanked of the cylinder to find all the rings stucker than stuck, couldn't even break them lose. The conclusion was that I had a oil pumper contributing to carbon build up. By the way that cylinder was giving me 78 on the compression check.

AR Dave · Post by **AR Dave** » Thu Dec 20, 2007 11:15 am

Does a stuck valve vibration, sound / feel like a car tire out of balance? When the prop is rotated by hand, one cylinder has lost compression. I was going to do a compression check this afternoon to find which cylinder, didn't think to feel for a cool one. Need to fly more often!

Post by **Bruce Fenstermacher** » Thu Dec 20, 2007 1:55 pm

Dave

If you have no compression in one cylinder the valve is likely stuck open.
It could also be sticking and not really sealing till RPM is increased so looking at them you may not know which one is sticking.

The heat trick is probably the fastest way to narrow down to the cylinder which one is sticking.

An engine with very stuck valve shakes and runs like when you start a very cold engine and it's only running on a few cylinders. Actually might be smoother because it's only missing the one cylinder but that is the idea.

blueldr · Post by **blueldr** » Thu Dec 20, 2007 8:15 pm

Pull the top spark plugs and "THUMB" check it. If that doesn't find it, you're ready to use the compression checker.

phantomphixer · Post by **phantomphixer** » Thu Dec 20, 2007 9:09 pm

Sunday morning vibration problem solved. Had low compression on the #4 cyl during compression checks. I could hear air leaking thru the intake filter. Pulled valve cover and staked the intake valve. Also gave the exhaust a shot or two. Compression checks good per Continentalâ€™s Service bulletin and ran up just fine. Flew for 2 hours. No vibration problems. Appeared to be trash under intake valve. Thanks for all the advice. I was expecting it to be a stuck exhaust valve.

AR Dave · Post by **AR Dave** » Fri Dec 21, 2007 4:19 am

Like blueldr suggested, we pulled the top spark plugs and did the thumb check which showed cylinder #3 to be the culprit. Upon pulling the cover we found the exhaust valve stuck. It was stuck hard! I got to feed a rope through the spark plug hole, push the valve through, and clean the valve guide. Ended up lapping the valve face. We used a close hanger to pick the valve up and hand it to a magnet, on a stick, and then pulled it back into place. Getting the springs compressed and locked into place was the hardest part.

Well I know what a stuck valve feels like now. I was about 300 ft and climbing out from takeoff when it became a 5 cylinder engine.

So what's the Marvel Mystery Oil recipe?

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