Cylinder Issues

How to keep the Cessna 170 flying and airworthy.

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Cylinder Issues

Postby darhymes » Mon Sep 20, 2021 4:13 pm

After completing the exhaust reinstallation, I started up the engine and immediately noticed engine roughness. Cylinder #5 (on my engine monitor) was not making heat (EGT and CHT). I shut down and pulled the spark plug from #5 and stuck my borescope in there. Both valves appeared to moved freely and seat properly. Headed home.

The next day I ran the engine again. Same thing…rough running and no power on #5. The monitor is a recent installation so I checked the #5 probes to make sure they were installed correctly and were indicating the correct cylinder. They are. The #5 exhaust pipe was also cooler to the touch than the other pipes (which were very hot). Checked upper and lower spark plugs. Gap was good. Very minor lead in lower plug. Cleaned them both anyway and put them back in.

Taxied out and did a runup. Mag checks were good (for a 5 cylinder motor). Engine appeared to run smoother at around 2000 RPM and the engine monitor indicated a rise in EGT and CHT on #5. As soon as I throttled back, the engine roughness became more evident and #5 cylinder indicated an immediate EGT drop. The CHT stayed relatively low since I didn't run at the higher power for very long.

Next day, my mechanic inspected the exhaust installation with me. We were both sure there was an intake gasket that didn’t have the middle portion of the gasket material removed. Wrong again. Everything looked good. Checked torques on all connections to be sure. Took the plane outside and started it up…smooth as butter and all temps on #5 were normal. Yay! and Ugh all at the same time…

I am hesitant to fly without further investigation. I have also spent hours perusing this and other O-300 related forums as well as the available technical material to better understand the possible nature of this problem and possible solutions. The most likely problem still seems to be valve-related. I think the "simplest" and most effective path would be to "rope trick" the valve into the cylinder and measure the valve and guide to determine clearance. if out of or close to limits, the valve stem can be cleaned and, if necessary, the guide reamed. I like Bruce's copper pipe idea. McFarlane also sells a purpose-built cleaning reamer-it's expensive but looks to be a useful tool. If anyone has used this tool, let me know what you think. I also ordered a set of springs from Fresno Air Parts. If the engine runs smoothly after all this, I will fly it. Still not sure if I've found a "smoking gun", though...

Are we missing anything? Any other ideas from the forum?
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Re: Cylinder Issues

Postby Bruce Fenstermacher » Mon Sep 20, 2021 5:21 pm

So what you are thinking is that #5 exhaust stuck open on your last shut down before this exhaust maintenance. And that it unstuck itself. :| Could have happened.

While #5 was not making heat did you happen to notice if it had compression? It you did and it did, couldn't be a stuck open valve.

But it is fairly easy to remove the exhaust valve spring on #5 and slide the valve in and out looking for any drag. Then wiggle the valve looking for clearance. Any stiffness or if there is not wiggle between the valve and the guide, clean the valve and guide before reassembly.
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Re: Cylinder Issues

Postby voorheesh » Mon Sep 20, 2021 10:08 pm

Is it possible that during exhaust installation, a small piece of FOD worked it’s way into #5? A rivet or small chunk of metal could hang up a valve while it bounces around in that cylinder. Later it could beat itself up and basically disappear. It would probably leave some marks on the cylinder or valve face. Just a possibility maybe worth considering. Hope it gets resolved.
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Re: Cylinder Issues

Postby Richgj3 » Tue Sep 21, 2021 12:12 am

Most likely an intermittent stuck exhaust valve. Just bad luck that it showed up now. I had two like that when we woke my engine up from a 13 year sleep. One stuck hard but the other would stick and release. Finally it stuck long enough to show no egt.

As an aside, we did the rope trick on all cylinders in order to change all the valve springs. In addition to the stuck one, we found another that was very very tight in the guide. We changed springs because we originally changed one cylinder due to a crack during the pre buy/annual. When we started having the stuck valves we tested the springs off the original cylinder and the were about 50% of the spec. I got the idea about the springs from a 172 forum or Swift forum regarding stuck continental valves.

It didn’t take more than two or three hours to do this.
Last edited by Richgj3 on Sat Sep 25, 2021 8:02 pm, edited 1 time in total.
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Re: Cylinder Issues

Postby Bruce Fenstermacher » Tue Sep 21, 2021 10:08 am

My post may have made it sound like a stuck valve couldn't be the issue. It could. What are the odds it stuck just before maintenance? I don't know but it did happen to me once so it could happen again. Checking valve clearance is the only way to know.
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Re: Cylinder Issues

Postby darhymes » Tue Sep 21, 2021 2:09 pm

Thanks for the replies and for backing up my troubleshooting. I did a very thorough inspection of the cylinder with my borescope a few days ago. Wanted to be sure the valve and piston hadn't connected. Everything looked good in there. Didn't see any evidence of scoring or other damage.

My mechanic and I are going to inspect the valve and replace springs on Thursday. If cleaning of the valve stem and reaming the guide is necessary, we will do that as well. I I will check all 6 cylinders and replace valve springs along the way as mentioned by Rich.

I agree with you, Bruce. It's unlikely to have such a coincidence like this before unrelated maintenance. I was sure we would find something in the exhaust installation that caused the issue. It's always been my assumption that the worst problems with airplanes are intermittent ones. I'm used to experiencing intermittent problems with avionics or electronic systems but rarely mechanical. Anyway, I'm hoping that the upcoming work solves the issue and gives me added insight and understanding of the plane.
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Re: Cylinder Issues

Postby brian.olson » Tue Sep 21, 2021 3:01 pm

This likely has nothing to do with your issue, and I'm almost hesitant to mention it as I risk being flamed (as my mechanical capabilities and depth of knowledge limited, at best), but I had something similar show up as a "symptom".

During an inspection a mechanic found a crack on one of the exhaust manifolds two years ago and a loaner exhaust was installed on one side. For the next week-ish I had a rough-running engine and, at times, some "pinging" coming out of the exhaust system (or at least it sounded like it). A different mechanic investigated. Fast forward: found two nearly-stuck exhaust valves. Based on that and a number of other factors did a top overhaul with brand-new cylinders. Fixed the value problem. However - the pinging and a little roughness still existed.

Fast-forward again: what we actually discovered was causing the roughness was that after installing the exhaust (first), and then the intake manifolds (second), the different gaskets caused a slightly different alignment configuration between two of the exhaust flanges and the intake manifold flanges, and that cause a slight - not visible to the human eye - gap to appear between two of the intake manifolds, the intake gaskets, and the intake faces on the cylinders, even when everything was torqued down. It took a smoke test to find them. The mechanic was able to do a little creative grinding to get achieve proper clearance again and that solved the problem of the gaps and the roughness/pinging. In hindsight it looks like the problem was first introduced when the replacement exhaust was installed on one side, and then carried through post- top overhaul.

Two completely different issues - the stuck valves had not yet caused a problem, but would have in an hour or ten hours or a hundred hours, but it was the gap on the intake manifolds that was causing the roughness.

Again - nearly certain this has nothing to do with your issue, but it caused my mechanic and I to scratch holes in our collective heads trying to figure it out. And I've been noodling on this a bit since I first saw your post about replacing exhaust gaskets.

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Re: Cylinder Issues

Postby hilltop170 » Tue Sep 21, 2021 8:42 pm

A repeatable problem with a cylinder not firing at idle and starting to fire at run-up sounds like an intake leak of some sort to me.

An easy way to check for an intake leak is to disconnect the carb heat duct from the muffler side and connect it (with duct tape) to a good leaf blower. Pull the carb heat control and turn on the blower. Using a hand spray bottle filled with soapy-rich water, spray it all over the intake system. Any leak will immediately be evident.

That is a quick and easy way to determine if there is an intake leak.
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Re: Cylinder Issues

Postby Bruce Fenstermacher » Tue Sep 21, 2021 9:08 pm

hilltop170 wrote:A repeatable problem with a cylinder not firing at idle and starting to fire at run-up sounds like an intake leak of some sort to me.

An easy way to check for an intake leak is to disconnect the carb heat duct from the muffler side and connect it (with duct tape) to a good leaf blower. Pull the carb heat control and turn on the blower. Using a hand spray bottle filled with soapy-rich water, spray it all over the intake system. Any leak will immediately be evident.

That is a quick and easy way to determine if there is an intake leak.


Yes a leaf blower would be enough power. 8O I just got done doing the exact same thing to a 0-470 in a 182 using a worn out shop vac which work just fine.
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Re: Cylinder Issues

Postby darhymes » Tue Sep 21, 2021 9:49 pm

hilltop170 wrote:A repeatable problem with a cylinder not firing at idle and starting to fire at run-up sounds like an intake leak of some sort to me.



I was sure the problem was intake related after watching the valves move and seat properly. My mechanic was pretty sure of it as well. After removing the intake attached to the cylinder and having a look around (all looked good), he reattached everything and torqued the nuts. The engine start that followed ran perfectly. And subsequent starts and run ups as well. Could have been a loosely attached gasket…?

I appreciate your insight on this issue. It is very helpful. When I finish my diagnostic work, I’ll let y’all know if there was anything definitive.

Thanks for all the great input. This is why I posted the problem on this forum. Everyone is very helpful and knowledgeable as well.
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Re: Cylinder Issues

Postby darhymes » Fri Sep 24, 2021 4:05 am

My mechanic and I checked the intake and exhaust valves on cylinder #5 today. They are in excellent shape. The exhaust valve guide had almost no carbon and the valve stem looked good as well. In guide clearance was well within limits. Both seated well. I replaced the springs and put it all back together. Engine run was good. Smooth as usual.

When I got back from running the motor, I noticed the two old springs that were removed from the cylinder. One set was noticeably shorter than the other. The taller springs matched the new springs perfectly, which I verified part numbers from the O300 Parts Manual. I can’t find info anywhere that there should be two different sized springs in these cylinders. The shorter springs were on the intake side…might be related to my initial problem?

Any of you know if two different spring sets should be installed? These are Continental cylinders.
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Re: Cylinder Issues

Postby Richgj3 » Fri Sep 24, 2021 10:40 am

As I recall when we did my springs they were the same for both intake and exhaust, but I’m not 100% sure on that.

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Re: Cylinder Issues

Postby GAHorn » Fri Sep 24, 2021 11:08 am

<edit for bran-pharrt, Thanks Gary for correcting me. It’s the smaller engines…or those earlier C145 that still (rarely) have the iron cam that use 2 springs.). :roll:

7A7570FD-D134-4169-803F-B698F738A1FB.png


Weak/Old valve springs are often overlooked as causes for rough running. I’ve got a zip lock bag full of them (red-tagged) which came from cylinders I’ve sent out for overhaul.
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Re: Cylinder Issues

Postby n2582d » Fri Sep 24, 2021 1:09 pm

Figure 8 of the Continental IPC shows three springs: an inner - p/n 24031, an intermediate - p/n 24029, and an outer - p/n 625958. The springs are the same for the intake and exhaust valves. Here’s a picture from Fresno Airparts:
9BBE8EDA-D4B8-4B0E-A731-9E3947502DF9.jpeg
If you order a set of valve springs for the O-300 from Fresno Airparts you get 36 springs. There are other Continental engines that require just two springs per valve.

ECI Service Instruction 98-2 also shows that the O-300 requires three springs per valve. It includes the following chart which gives spring pressures which match Continental’s Table of Limits but also gives additional data points of the approximate free length and the spring force of the outer spring at 1.875”.
80B4B001-BB78-4D38-BAE3-BF55FDB0E5EC.jpeg
Click to Enlarge

See also Continental Service Bulletin 18-03. It lists a different p/n for the outer spring on the C-145.
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Re: Cylinder Issues

Postby darhymes » Fri Sep 24, 2021 11:59 pm

Thanks again, guys. Your responses alleviated my concerns that I was missing something with the springs. I ordered a set of springs from Fresno Air Parts and confirmed the part numbers are correct. The “shorter” spring set that I removed from the cylinder may just be compressed from time in service. When I separated the three individual springs, the shorter set appears to have a different inner spring. The photos show the difference between the two sets.

At any rate, the new ones are installed and the plane is purring along just like normal. Will be replacing the remaining springs in the future.
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