Vibration problem

How to keep the Cessna 170 flying and airworthy.

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gobrien
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Vibration problem

Post by gobrien »

Hi All,

I have a problem with vibration in flight. I’ve tried various things to rule out possible problems as described below and I would appreciate any feedback or ideas. I have not flown any 170 but this one, nor any early C172s with the O-300 so I have no feel for what it’s performance should be so again, please feedback how yours compares.

Thanks,

Gareth.
----------------------
1948 Ragwing with O-300-BCC (B converted to a C using a replacement crank) McCauley 1C172/EM7653

Run-up is perfect, the engine sounds and feels great. Appropriate drops on mag checks and carb heat. Oil pressure and temperature consistently green. No vibration felt on the ground.
Static power check yields about 2220RPM on the electronic tach – the needle just passes 2200 on the mechanical tach, so I would call it just shy of the low end of the range. Field is at 300ft, OAT 23C. Range from the C172A TCDS (O-300 C with this prop):

McCauley 1C172/EM 7652, 53, or 55
Static rpm at maximum permissible throttle setting: Not over 2350, not under 2230

Take-off power is smooth and she’s off comfortably in 300m (<1000ft) with 250lbs of overweight pilot and half tanks. At full power in level flight the engine never gets over 2450 RPM – balls to the wall 2500ft MSL and on take-off (70mph IAS) never exceeds 2350.

I ran a rich test – set 800 RPM (on the ground) with the engine nicely warm after a short flight and then pulled the mixture quickly. I would expect at 300ft MSL field height to see about 50RPM increase in RPM before it starts to die which would show the carb to be set rich of peak at full rich. However, there is no such increase, it just dies. Thus the carb could be set a little richer, but I don’t think it’s running lean enough to cause the vibration (which is only perceptible in flight).

The vibration problem did not occur on the first couple of test flights. It did then occur on the third flight when we let the fuel get low while operating on the L tank (we wanted to drain it). Thinking it was related to that, on the next flight I filled both tanks to the brim. Flew 2.5 hours of circuit training in 2 sessions without vibration. I also flew a 100mile trip without problem. Then …

It has happened as follows quite consistently (all performed flying orbits over the field):
From about 300ft AGL (or 1 min into the flight) there is a noticeable vibration in the aircraft – as in shaking the water in the compass and making me uncomfortable! At first I thought it might be a resonance at a specific RPM so changed RPM to max or pulling back to 1900 for slow flight, the vibration remains.

Leaning has no effect on the vibration.

Running at max power – added carb heat: power dropped as expected and the vibration seemed worse. However, repeating this yesterday at 2200RPM the carb heat does not change the vibration.

Mag check – climbed to 2000 ft throttled back. Cylinders 2 gets hotter than the rest (445F) at full power on the climb and inspecting the plugs it is running a bit leaner than the others (Number 3 runs hotter than the rest but not as much as 2). I've read George's description of the induction system's inconsistencies, so this does not appear unusual. Pulling back to cruise power of 2200 and leveling off, all are back to the green in <30 seconds. Max allowed per TCM = 525F. As cylinder 2 is consistently hotter than the rest I replaced the intake gasket and ensured the jubilee clips on the rubber sleeve on the inlet for 2 are snugged up nicely. No change.
At 2000 ft I let all EGT and CHT even out – and they are very even and consistent. Switched off L mag and flew 2 mins on R mag only. Dropped 60ish RPM as expected and no engine roughness. No cylinder readings (EGT CHT) any different from the others. Back to both, then R off, flew on L mag only … same. No change in vibration.

Swapped the plugs from the hot cylinder (2) with the top plugs from 4 and 6. No change.

I’m stuck and hoping someone here can help:
1. I can’t see how it could be an ignition problem – or is it?
2. While the engine is running slightly lean on the ground – it’s nothing to write home about and I can’t see how it could cause the vibration.
3. Fuel flow at take-off is about 11gph and in cruise at 2400RPM is 8.5gph. Seems about right?
4. I have considered a prop imbalance, but would I not feel that on the ground, especially at max (static) power? Also, this vibration didn't happen for about 3.5 hours of operation.
5. There is nothing loose on the airframe. All bolts have been replaced and torque striped and everything remains attached correctly.
1948 170 Project (N4180V) now EI-AEN SN:18513 - Dublin, Ireland
https://www.taildragger.eu/
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c170b53
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Re: Vibration problem

Post by c170b53 »

The engine details ? I’d do a compression check and screen check for peace of mind before more experimentation.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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Richgj3
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Re: Vibration problem

Post by Richgj3 »

You have a sticking exhaust valve. If you can’t catch it in the act you’ll have to do the rope trick and remove the valve springs and check which valve(s) are tight in the guides. I know you think the symptoms should be worse, but I’m here to tell you of a guy who took off in a 170B with two people on board and full tanks running on 5. And got to several thousand feet before deciding to look at the EGT and seeing #4 dead. In my defense it was my very first take off in a 170. I thought the run up should have been smoother, but what the heck did I know.

Had a second one stick intermittently later which is when I checked them all and replaced all the valve springs. One of the old ones tested a 50% of the spec. (The only one I bothered to test). This was on a C145-2 with 250 SMOH that had been sitting for over ten years. Using TCP and MMO since we cleaned the guides and replaced springs. no problems for over 100 hours.
Rich Giannotti CFI-A. CFI-I SE.
1952 C170B
N2444D s/n 20596
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c170b53
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Re: Vibration problem

Post by c170b53 »

Again engine history will fill in the picture as in has it had a top O/H, times and recent work. I won’t disagree Rich likely you’re on the right track as in valve train but I would do the basics first for safety.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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gobrien
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Re: Vibration problem

Post by gobrien »

When I bought it the engine time state was:

Total time 3204
SMOH 702
STOH 568

However it had been on a C172 that rolled over in a storm when not tied down properly. It caught the prop on a kerb which bent the crank out of spec.

The engine has been rebuilt with a yellow-tagged crank ground 10 thou under with appropriate bearings, seals, gaskets all replaced so the core has zero time SMOH.

The cylinders were left as they were and reinstalled with 568 hours. It has run only 6(ish) hours since then.

Plugs are new, harness and mags are as they were. Compressions are between 75 and 78. EGTs remain similar on all six cylinders throughout.
Last edited by gobrien on Mon Aug 08, 2022 7:42 pm, edited 1 time in total.
1948 170 Project (N4180V) now EI-AEN SN:18513 - Dublin, Ireland
https://www.taildragger.eu/
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Bruce Fenstermacher
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Re: Vibration problem

Post by Bruce Fenstermacher »

First, maybe this is a typo
At full power in level flight the engine never gets over 2450 RPM
You should be able to get at least 2600 rpm in level cruise, maybe more. And with that prop angle of 53 you should be in the middle fo the static RPM range. Kind of like a cylinder isn't making full power.

Stuck or more accurately sticking valve? A stuck valve never moves and will be open. It is obvious, there will be no compression in a cylinder with a stuck valve. A sticking valve might momentarily stick then release to stick again. This happening very quickly.

Stuck and sticking valves are caused by two things. First thing is the exhaust valve to valve guide being to tight. Second the exhaust valve to valve guide clearance getting too tight because of carbon and lead deposits on the valve stem and in the guide. Running 100LL can cause stuck/sticking valves. But it doesn't happen overnight. We need a bit more info to diagnose a sticking valve. How much time on the cylinders since new or overhaul (when the parts would have been last clean) running 100LL. Just yesterday I cleaned a very tight valve on a A-65 with only 100hrs since last time we cleaned the valves. So it doesn't take long but usually a little longer.

A sticking valve can sometimes be found by starting a cold engine and running it only 15-30 seconds then feeling each cylinder for uniform heat. The cold cylinder won't be firing as much and be colder. Of course it seams you have a 6 cyl CHT and EGT. Looking at them real quick on a cold start could point out a sticky valve.

True you should get a rise at engine cut with the mixture. At that rpm you only running on the idle circuit which is not in effect over 1200 or so rpm. You should adjust it but it is not likely the issue here.

Seems weight in the tank makes a difference if I've read your statement correctly. 170 tanks are different than A and B tanks which most of us have experience with. You have to decide whether it is a natural phenomena of oil caning or if the vibration is being brought on by the engine. I won't say a sticking valve is it, but I wouldn't rule it out.

I just read the history of your engine and a sticking valve and a sticky valve at 586 hrs is a real possibility. Where the cylinders new 586hrs ago? What were the exhaust valve clearances set at?
CAUTION - My forum posts may be worth what you paid for them!

Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
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gobrien
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Re: Vibration problem

Post by gobrien »

I don't have the old logs here, they're at the office so can't check if the pots were new or just refurbished at 568. I'm assuming they were new at the major overhaul now 720ish hours ago.

I don't have the overhaul paperwork from that overhaul, just the old log book so no idea what the clearances were set at.

The sticking valve suggestion is very helpful.

Plan so far for next weekend:
1. Fill the tanks and see if it stops again.
2. Watch temps carefully on cold start.
2. Assuming 1. doesn't fix it I start learning more about exhaust valves!

I may switch to Mogas. Several hangar mates with O-200, 0-300 and A65s swearing they run better without lead.
Also saves about 3 EUR per gallon.

Thanks gents,

Gareth
1948 170 Project (N4180V) now EI-AEN SN:18513 - Dublin, Ireland
https://www.taildragger.eu/
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GAHorn
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Re: Vibration problem

Post by GAHorn »

The TCM Ovhl Manual specifies valve spring tensions. I believe I’d test those before further troubleshooting.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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darhymes
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Re: Vibration problem

Post by darhymes »

I had a couple instances of stuck valve symptoms at different times. Each time, the engine sounded rough at startup and my engine monitor confirmed which cylinder wasn’t producing heat. Upon inspection, the valves were in good condition and the guides were clean and right at spec. I had a new set of valve springs on hand so I replaced them (the first time I only replaced the ones on the offending cylinder and the second time I replaced the rest). The problem has not returned since the springs were replaced (nearly 100 hours later on the first cylinder).

The only reason I had springs on hand was I had read a few accounts of apparent valve issues in 170s and early 172s that were solved by replacing the springs…figured I might as well keep a set handy.

I enjoyed reading through your troubleshooting process. As an enthusiastic and involved owner I can appreciate your concern. Best of luck solving the problem.
Dustin Rhymes
'55 170B N4410B S/N 26754
'06 G550 “The Silver Bullet”
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gobrien
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Re: Vibration problem

Post by gobrien »

Update - yes the saga continues!

I replaced all the valve springs. The previous springs were in good condition and while I have not measured them, I have no reason to believe there was anything wrong with them.

As the weather turned cold and wet here the problem started happening on the ground and even right on startup, this made it easier to diagnose. During run-up I now got intermittently no EGT on cylinder 2 on the left mag along with rough running. The mags have had a cursory inspection and are going for overhaul. I think the reason for the original behaviour was the this problem in the mag is intermittent and initially was causing intermittent reduced burn in cylinder 2 due to arcing in the magneto (there is some carbon visible and the contact spring for 2 was corroded). This would cause an imbalance in the engine which transmitted a vibration throughout the airframe. Initially subtle, this got worse over time. Also, lower pressure and higher humidity apparently encourage arcing and the weather turning wintery made the problem more consistent.

The sad part is I could have had this overhaul done a year ago while the aircraft was being reassembled. The intention then was to replace both mags with P-Mags and have automotive spark plugs and brand new harnesses => absolutely no ignition problems! I only discovered when the order form went in that while they have a Continental 6-cylinder product they don't have the interface for the O-300, nor will they as there is no market demand. Because that was the plan, the mags went back on the engine "on condition". That said, I'm a fan of "on condition": it requires taking any "message" from the airplane seriously (which I did) and the inclusion of full engine monitoring in my rebuild helped in the diagnosis.

Anyway, with any luck we'll be sorted out in a couple of weeks - just in time for winter :D

Gareth.
1948 170 Project (N4180V) now EI-AEN SN:18513 - Dublin, Ireland
https://www.taildragger.eu/
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n2582d
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Re: Vibration problem

Post by n2582d »

Gareth,
It’s been a year since you wrote about the issue you were having with vibration. Any luck (of the Irish) finding the source? I’ve filed Cessna’s Service Letter SE 76-24 here. Item #2 there is a “Vibration Checklist”.
Gary
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ghostflyer
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Re: Vibration problem

Post by ghostflyer »

you also stated that the “Water” in the compass was.. Compasses have alcohol in them or a special compass oil. I worked in a shop where they used “Johnny Walker “ scotch whisky or “JimBeam”. The bottle on the shelf was for instrument purposes only but it evaporated by itself . Strange …
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gobrien
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Re: Vibration problem

Post by gobrien »

Hi All,

Finally fixed ... Yes only last weekend did I finally figure out what had caused this.

This was a post viewtopic.php?p=145364&hilit=engine+mou ... ll#p145364 about replacing the firewall-engine frame bolts. The spacers that Jim showed in that thread were missing and a previous A&P had butchered the bolts by hand cutting thread extensions, so they got filed in the trash.

I made replacement spacers and used new bolts as specified in the IPC and torqued everything up nicely. For 2 hours of operation no problem and then gradually the vibration problem started and got worse.

Chasing down this shivering I discovered the valve guides were worn way out of spec and replaced all the cylinders. The mags were in bad shape and I replaced those. Each time hoping, but not necessarily expecting the vibration would go away. The last old accessory on the engine was the carb, so I replaced that. Balanced the prop and still she shivered.

Well it turns out that the spacers were about 0.8mm too short (call it 1/32in) and the nuts were thread-locked. The bolts remained up to torque but were allowing that 0.8mm of movement of the engine-mount frame relative to the airframe which gave me the vibration. The fix: 1 more washer and retorque for each of 4 bolts.

18 months of pain and suffering and I now have a zero-timed engine (well 5 hours on it now) and no more vibration; and a lot of new knowledge ... although that may just be rationalization!

Just need some decent summer weather so we can put some hours on her and it'll be all worthwhile!

Gareth.
1948 170 Project (N4180V) now EI-AEN SN:18513 - Dublin, Ireland
https://www.taildragger.eu/
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GAHorn
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Re: Vibration problem

Post by GAHorn »

I’m GLAD you tossed the “cut threads” fasteners. Aircraft fastener threads are not “cut” because cutting creates stress-risers and the fasteners can later fail. Aircraft fasteners have “rolled threads” when they are mfr’d. Good choice on your part.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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