ColoRv

ColoRv

Well Known Member
Good Morning VAF'rs

I've been chasing an issue for a few months now, thought I found the smoking gun a couple of times but alas it still happens.

All was well for first 60 hours, then this issue showed up. If I let the plane sit for 12 hours or more I get an odd occurrence on start up. Crack throttle, mixture rich, boost pump 5 seconds... It fires up normally, runs great for 30 seconds or so then a cylinder drops off. Idle goes up a couple hundred RPM, EGT on that cylinder plummets, CHT starts falling and the engine shakes like a dog after a swim. 30 seconds or so and the cylinder comes back and she runs great from there on out. Normal run up, perfect in flight. Hot starts are normal. Switching Pmags off one at a time has no affect, EICommander says Pmags running normally. It only does this after sitting overnight or more.

The cylinder that drops out isn't always the same cylinder. Has been #1, #3 or currently #4. This leads us to believe it's fuel related and I'm getting air in the fuel line somewhere and that slug of air kills a cylinder until fuel arrives. What I am struggling to wrap my head around is why only one cylinder drops out when this happens, why it changes cylinders and why I'm not seeing any fuel leaks anywhere.

Here is what I've done so far:


Pulled all the injector nozzles and thoroughly cleaned them - no affect
Flow tested spider - seems fine
Pulled spider, checked diaphragm - looks fine, no holes
Pulled every fuel line from spider to tanks and checked the flares - no issues I can find.
Replaced lines from selector to firewall from my lines to professional built - desperation - no affect
Changed boost pump (had air leak when pressurized) - no affect
Capped the tank lines at the selector, pressurized the entire fuel system from spider to selector (10psi) and soapy water tested everything...no leaks I can see.

Spider and Servo (rsa5) were given an experimental overhaul by Airflow Performance prior to build and all was perfect for 60 hours.

Has anyone seen an issue like this before? A neighbor had a plane that would randomly drop a cylinder but only when hot and it turned out to be a bad fuel flow divider. I'm typically resistant to throwing new parts and a prayer at a problem....but this one has led me on quite the merry chase. I'm a heartbeat from replacing the entire fuel system. I want to fly my beautiful new plane without this annoying issue.

Any words of experience from the VAF masses?

Signed Desperate in Denver
 
My first guess would be a stuck valve, sometimes called morning sickness. Try changing your oil. Lots of A&P s here to help you properly ID and resolve the issue if fresh oil does not cure it.
 
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Recently I also had a similar problem. However I didn´t have idle RPM increase. On three startups, always when cold, cyl. #1 didn´t fire up. Easy to see as there was no EGT on #1 and after about 30 sec #1 came online each time. After the third occurence I waited for the engine to cool down and pulled the prop through and had no compression on that #1 cylinder. Pulled the valve cover and manipulated the exhaust valve with a scewdriver and it closed with a noticable klick. Each time when I pulled the prop through this valve stuck open slightly.
What I did was remove the valve springs, push (in fact tap) the valve into the cylinder and reamed the valve guide and cleaned the valve stem with sotchbrite. There was carbon deposit on the reamer when it came out and during the reaming process I could feel the breaking carbon.

Anyhow I would suggest to pull the prop through and look for missing compression when this happens again. This would at least point to a possible valve problem. As far as I know this happens mostly on the exhaust valves.
Be careful turning the prop !

BTW will be in Denver coming Tuesday.
 
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No expert

but one will chime in before too long.
I am very interested in these types of threads and eventually the solution to the stated problem.

The symptoms you describe are classic stuck valve problems.
A bit confusing is the rise in RPMs that you describe and the fact that it happens to different cylinders at different times. That in itself would still indicate a stuck valve problem across all four cylinders.
Seeing what you have already done and pretty much eliminated a fuel delivery problem as well as an ignition problem I would think you are dealing with a valve problem of some sort.
If it was me, I would change the oil and add a quart or 2 of Avblend (formerly Lenkite) to your oil.
Since you are not the type to just replace all the parts to finally get to your problem, spending a few dollars on an old stuck valve remedy, it will not hurt to try. Whether you believe in the "snake oil" remedies or not, As I said,
if it was me I would give it a try.

In case you do want to read up on Lenkite check the write up in this link.
Skip the to the bottom and read only the lenkite or avblend story.
http://www.aopa.org/News-and-Video/All-News/1994/November/1/Airframe-and-Powerplant-(8)
Good luck!
 
Another data point that I forgot to mention. Once the cylinder drops , on occasion....another cylinder will drop out. That leads my mind to a fuel issue which is why I've been focusing on that area. I could be completely wrong, but I can't think of how anything but a fuel issue could switch cylinders so quickly, and leave the EGT's and CHT's dropping.

What is the failure mode of a fuel divider problem? Can a servo put air in the fuel line? How about a mechanical fuel pump? Or, am I chasing fuel when the problem is elsewhere?

I admit I struggle to understand how a 100 hour overhaul would have exhaust valve issues across several cylinders. I am far from certain though, as at this point I'm ready to believe just about anything...but wouldn't EGT's go up or at least produce some heat if an exhaust valve stuck open? Mike Busch's Savvy Analysis swears it is not sticking valves. Here is what the EGT's look like when this occurs:

EGTcopy_zps815a3bbf.jpg
 
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Had that problem on my Cessna 180 for a while, but only on one cylinder. Thought it was a stuck valve. Turned out to be bad Champion plugs. It would warm up and work in a bit.

I replaced than and all was fine.

So if you're running Champions try that.

Dave
 
Thanks David. As it turns out, I have Pmags and use auto plugs. Since the auto plugs are cheap, I did change them out, unfortunately with no affect.

Thanks to all who have suggested things to check. I'll update here if I find anything.
 
Exxon Elite - sticking valves

After all the sticking valve suggestions, I went to the airport to rule it out. I started the plane.....confident it was not a cylinder issue because it wasn't always the same cylinder that dropped. When a cylinder dropped, I shut down and climbed out. Rotating the prop through...one of the cylinders had no compression.

"You gotta be ****'n kidding me...."

A few rotations later, heard a click and the compression was back. Repeated the next day...same result.

A sticking valve.

Prior to the problem showing up I had changed from break in oil to Exxon Elite. A quick google search for Exxon Elite sticking valves found a few pilots had experienced the same issue when they changed to Exxon which went away when they switched to something else...so I put Aeroshell back in. The next start, the cylinder dropped but only for a couple seconds. The following start and every start since then......smooth as silk. I dont know why, but my engine hates Exxon.

Brown Avaition who overhauled the cylinders had previous to my posting here said there was no way it was a sticking valve. Mike Busch's Savvy Aviator analyzed my G3X data and said it wasn't a sticking valve. I was sure I was wasting my time when I went out to start the engine to check for a sticking valve....and we were all wrong.

My heartfelt thanks for those who helped. I was down a rabbit hole chasing what I thought was the problem (fuel)...and would probably still be there if I had not posted here. It's been two weeks of cold starts and flights nearly every day with Aeroshell....and not a single hiccup. With Exxon it dropped a cylinder on every cold start without fail. I don't understand why, but this gift horse's mouth is staying closed.
 
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