Sticky valve?

[Brantel]

For the engine experts out there....

Preamble:

Leaving Osh just about passing the edge of the D, bam! Sounded like something smacked the bottom of the plane with a dead blow hammer! Scared the pooooo out of my wife! I felt nothing in the controls, no shudders, nobody around me visually, nobody around me on the ads-b, quick glance at the tail and it is all there, quick shake of the stick and it all still works, quick look at the wings for dents or blood and nothing. I thought we must have hit a bird with the gear or something. Nerves calmed down and we keep going. Due to weather had to spend the night in Danville Il. Checked the plane out real good. No bird strikes. I removed the cowl and checked everything firewall forward and found nothing. Fast forward to departure morning and we blasted off in severe clear with 1:49 to go. On the climb up to 9500 and I got another bang! Just like the one coming out of Osh. Then a few more bumps from the engine a few minutes apart but then they went away. I realized that these might be after-fire bangs from a sticky valve. Normally my engine runs like a sewing machine. Never has had morning sickness. Stayed high till we had our home field made, in the rapid decent from 9500ft, got a couple more bumps from the engine. These were more than the normal downwind pop you sometimes get from closing the throttle too fast. Nothing will prepare you for how a sticky valve will scare you when it sticks while in cruise! I am certain that my wife will need new underwear!

So now I am ready to wobble test/ream the guides on all 4 before any additional flights! I am not fooling around with this kind of stuff. A miss or after-fire bang every now and then seems like a warning sign of an imminent bent pushrod or worse!

To clarify, the after-fire events I have seen have been one cycle events. Bang and then smooth again. No rough running for consecutive cycles. When it is running smooth, it is really smooth. No time to see any changes in EGT's.

Questions:

Can a sticky exhaust (open) valve cause an after-fire thru the exhaust for one cycle? If so, how does this happen if the valve is stuck open? One would think that if it is stuck open, it would not draw in any fuel on the intake stroke or be able to make any compression on the compression stroke?

What else could cause such a thing? I am already committed to the wobble test/reaming procedure but want to ensure I am on the right track. Since I have one Pmag and one Mag, I would thing the chances of late ignition timing to be slim to none?? I checked the intake and exhaust for leaks, blown out gaskets, cracks etc. None found. I still got an RPM rise at shutdown when I pulled the mixture.

Thanks for any comments or advice. I am rounding up the stuff I need to do the wobble test/reaming. The price Lycoming wants for the official tools is ridiculous so I will be making my own. Already have the reamer required. I will be using my friend Mike Bullock's awesome thread as guidance on how to perform the reaming operation as needed.

[Ironflight]

Brian,

Just one thing to check - I was getting some "backfire bangs" on Tsam a few months back when I leaned. This is on an engine that has always been smooth as glass, so I was concerned. It was attributable to a bit rougher running on one P-Mag, and since I had about 600 hours on the engine and P-Mags, I decided to pull them and send them to Brad for a checkup and software verification. WHen I took them off, I did a resistance check on the ignition leads, and sure enough, two were way out of the spec - the same two that, it turned out, had significant corrosion on the end that plugged in to the P-Mag. I got some new leads from Brad, and once again, the engine is smooth.

So lesson - high resistance on the P-Mag plug wires can lead to an after-fire. They should be checked every year at annual - the spec is given as Ohms per inch, so you have to measure the length of each lead and do the math, but it doesn't take long.

Much less invasive than taking the engine apart!

Paul

[Robin8er]

I attended the lycoming class at osh on smooth valve operations and what you described is exactly what they said the symptoms of a sticky valve are. They said if it happens make sure your push rods are straight, if the valve sticks bad enough it can bend the rods/tubes.

[Brantel]

So I have started the troubleshooting session. At this point I have the cylinder exhaust valves stripped down to just the valve in the guide.

I can feel no evidence of stickiness. Slick as hot butter. I am still going run the reamer thru all 4 next trip to the hanger just to ensure a nice baseline.

Paul, I am going to check the wires per your post above for both cross talk and high resistance.

Other than ignition, what the heck else could it be if there are no exhaust gasket leaks or cracks and no intake leaks that I can find? (I have one impulse coupler mag regular aircraft plugs and wires and one Pmag with car plugs and wires)

Is it possible for the stickiness to be on the cam side of things with the lifter/tappet?

[bullojm1]

Brian,

Have you checked the intake valves? While the exhaust valves are typically the culprit, I wouldn't leave it out of the realm of possibility for the intakes to be sticking.

When I had a sticky valve issue, it was much more pronounced than just a blip here and there....it was a major event where I lost EGT in #2 and CHT was rapidly decreasing over the time of the occurrence. Did your engine monitor report anything abnormally other than Shantel squeezing on to your hand for dear life?

Strange enough, on taxi out of SnF and Osh this year, my #1 cylinder exhibited morning sickness right after startup during taxi. EGT's drop off and the engine runs rough as heck. After cycling the throttle, everything smooths out and continues to run smooth for the remainder of my journeys. Something to do with recent storms me thinks? These engines are more complicated than the little man in the canoe!

[turbo]

how many hours on the engine? what was the wobble measurement value?
a new engine is set up with about .010 play. a worn out guide measures about .035. a mid time engine should be about .022.

[DanH]

Originally Posted by Brantel

I can feel no evidence of stickiness. Slick as hot butter. I am still going run the reamer thru all 4 next trip to the hanger just to ensure a nice baseline.

Forget feel. Measure. And don't ream unless you need it.

...what the heck else could it be....?

A little water.

Is it possible for the stickiness to be on the cam side of things with the lifter/tappet?

No.

[Brantel]

Originally Posted by turbo

how many hours on the engine? what was the wobble measurement value?
a new engine is set up with about .010 play. a worn out guide measures about .035. a mid time engine should be about .022.

~400, still waiting on the fixture to do the proper wobble test. Even though the valves feel great, I want to get a baseline reading on them for future reference.

Eyeball and finger engineering looks and feels like they are OK but I want to test them properly. I already have the dial indicator, just waiting on the fixture and extension.

[Brantel]

Originally Posted by DanH

Forget feel. Measure. And don't ream unless you need it.



A little water.



No.

Thanks Dan, plan to follow your advice. Waiting on the fixture to do the proper test. Already have the dial indicator.

As for water, will it cause an after fire pop?

Good to know to forget worrying about the cam side of things!

[Brantel]

Originally Posted by bullojm1

Brian,

Have you checked the intake valves? While the exhaust valves are typically the culprit, I wouldn't leave it out of the realm of possibility for the intakes to be sticking.

When I had a sticky valve issue, it was much more pronounced than just a blip here and there....it was a major event where I lost EGT in #2 and CHT was rapidly decreasing over the time of the occurrence. Did your engine monitor report anything abnormally other than Shantel squeezing on to your hand for dear life?

Mike, will check intake as well.

No indications from the EMS. I have looked at the trends and nothing to report. It was too fast for anything to respond. Like 1 cycle fast each time it happened.