[Wasn’t sure whether this should go into “Maintenance & Avionics” or “Non-Certified” as this relates to a Permit aircraft. Peter – feel free to move of course if appropriate.]
Our Continental 90 is exhibiting two problematic and currently unresolved behaviour. We are exploring with our engineer, but I thought there would be no harm in soliciting input from the global engine cognoscenti here.
The engine emits a mild popping sound at idle which tends to manifest itself at landing when pulling the throttle back. It occasionally appears at pre-flight. All the above is despite a careful warm-up. FWIW, the engine appears “solid”, she starts first time, every time irrespective of temperature (if done correctly), full power available etc..
The port exhaust stack periodically demonstrates a slight movement or play so it is carefully checked at preflight. During the recent annual, both exhaust stacks were completely removed, inspected and reinstalled with new gaskets. Nice and tight at pre-flight, not checked after flight, movement again in port exhaust at next pre-flight, so grounded the a/c pending a proper resolution.
These are two separate problems. We are trying to understand/identify the popping sound. There may be a correlation with the slight movement in the exhaust (vibration).
Any thoughts?
Every time I’ve had a popping sound when the engine is at idle, it has always been a leaky exhaust gasket. The popping is in the exhaust not the engine.
The Cessna 140 (with a C85) I had in the USA took some time to sort out, and we tried all sorts of different exhaust gaskets to no avail. In the end measuring the mating faces of the exhaust showed them to be slightly warped – they looked perfectly flat, but there was indeed a very slight warp, and we had them skimmed, new gaskets put in, and the problem went away. Your exhaust flexing may have warped the mating faces, check for that – not just by eyeballing it, the warp might be too small to be seen that way.
I had a popping noise start in the Auster’s (O-320-B3B) exhaust at idle last year when I went on a trip. Again, it turned out to be a leaking exhaust gasket. (Fortunately the trip just happened to be a fly-in at our LAA inspector’s home base so fixing it there and then was easy).
The problems might not be separate, your port exhaust stack moving may be breaking the seal at the gasket in very short order or may have warped the mating faces on that side.
If it’s not the exhaust, then what….
Does leaning the engine change the popping in any way ? If it goes away when leaning you might have a stuck valve where unburnt fuel ignites.
Do you have an EDM that gives you a readout of EGT and CHT ?
Do you have a clue which cylinder it is ?
What does the mag check say ?
Did you check the plugs ?
And you might have a look at the intake manifold bolts and then the rubber couplings. The problem can be on the way in, not only the one out
One day a friend at the aeroclub who knew I was supposed to fly a M20C from Hamar ENHA to Helsinki EFHM in the next few days called me to warn me that he thought there was a popping sound from the engine. So I decided to take the plane to the shop before my trip. I drove to ENHA, very carefully pre-flighted the airplane, made a long run-up, even to high power and did not find anything. So I took off for the flight to ENKJ where the shop was located. The flight was completely uneventful. Once on the ground I did another run-up and could hear a very distinct popping sound.
I walked in to the shop. The mechanics had heard the run-up and told me the engine sounded perfectly normal, but they would take a look. Then I went to work.
In the late afternoon I was back. The mechanic gave me a strange look and told me to follow him to a corner of the hangar where there was a pile of tubing. He said that when they removed the cowling, the pieces of the exhaust that were not bolted to the engine fell out. The cowling had been keeping it all together, but metal expansion when hot, would open up gaps between pieces and that was where the popping sound came from.
No trip to EFHM for me.
Don’t take potential exhaust issues lightly.
I have heard the same on a C90 running oxygenated auto fuel. On Avgas the issue isn’t present.
I thought oxygenation is what happens when you turn fuel into exhaust?
Silvaire wrote:
I have heard the same on a C90 running oxygenated auto fuel. On Avgas the issue isn’t present.
On our Cub (C90) I often fill up with auto fuel that has MTBE (or at least had, when Statoil still sold fuel). I can’t remember hearing a popping sound, but I think the engine sounded a bit “harder” on autofuel (could be imagination of course).
Cobalt wrote:
I thought oxygenation is what happens when you turn fuel into exhaust?
You’re thinking of oxidization.
Thank you all for your responses. I really like how one answer built on another in terms of a progressive checklist!
We are still working through the problem, but strongly suspect that @alioth has identified the issue. There may be another issue related to this, but bottom line is the gap which is intermittently causing this popping, despite new gaskets. Thanks for your experience on gaskets as we might otherwise have worked through every size in the book! We are also checking the screw threads…
@Euroflyer – EuroFlyer wrote:
Does leaning the engine change the popping in any way ? If it goes away when leaning you might have a stuck valve where unburnt fuel ignites.
Do you have an EDM that gives you a readout of EGT and CHT ?
Do you have a clue which cylinder it is ?
What does the mag check say ?
Did you check the plugs ?
And you might have a look at the intake manifold bolts and then the rubber couplings. The problem can be on the way in, not only the one out
@Silvaire – operating on avgas now. I am told that the engine could take mogas, but AFAIK, has been flying with avgas for a number of years.
@aviathor – we grounded the aircraft as a precaution against the type of incident you describe.
Frustrating these problems, but on the upside, having taken part in the annual, I have learnt a few things (and got my hands dirty). Still so much to learn… but good one to learn ones aircraft.
