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Old 06-08-2022, 06:06 PM
RV6-KPTW RV6-KPTW is offline
Join Date: Oct 2014
Location: Sebastian Fl X26
Posts: 207
Default Sticky valve on a Thunderbolt IO390

Hi all,

I have been dealing with a EGT sag on #2 with a little engine vibration just after startup for a couple of months. Never had an issue beyond a minute or so while first starting the engine until it finally occurred on departure. Under 200 hours on the engine so it was difficult to be aggressive initally.

Lycoming recommended approach is SB388 followed by SI1425A. I opted to skip the wobble test and proceed with reaming the valve guide and cleaning the valve stem.

See the photo of the shop towel. Some crud was evident after reaming and pulling the material off the reamer. It is black and does not look like lead. I looked thru and did not find anything solid or metallic valve guide material.

See the black on the valve photo. Is this common finding for sticky valves?

I am a little surprised at how little material caused the sticky valve. I have not yet tested so this conclusion might be premature. So is it me or just stuff happens?

Is this cylinder too hot? Cylinder #2 CHT is the hottest by 30 or 40 degrees. All cylinders are at 320 or so and #2 might be at 350 or 360. I had reduced the size of the original air dam thinking it was causing higher CHT issues. Cylinder 1 and 2 are hottest. Counterintuitively, would adding the original air dam back help reduce temperatures?

Are we having fun?
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Dennis Callaghan (X26)
- RV14 sn:140193 - Awesome airplane!
- donation 11/27/2021 * 2
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Old 06-08-2022, 08:21 PM
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czechsix czechsix is offline
Join Date: Mar 2006
Location: Spring Hill, KS
Posts: 399

With the parallel valve Lycomings, stuck valves and worn valve guides are some of the most common issues you'll encounter. The O-360 in my RV-8A started having valve issues around 800 hrs TSMOH (wasn't a new engine but the valves and guides were new at overhaul). I have not seen many reports of stuck valves on the angle valve engines...not sure if that's because they are less likely to have the problem, of if it's just because there are a lot fewer of them at least in Experimentals. This is the first I've heard of it on an IO-390, bum deal to have it happen after only a couple hundred hours.

There are all kinds of expert opinions out there about what causes stuck valves and how to prevent it. Some say you're running it too hot, others say you're running it too cool. Too lean or too rich. Some are convinced that certain oils are better or worse. Unleaded fuel might help but I'm not sure there's enough objective data out there to conclusively demonstrate a reduction in valve issues. I did my best to manage the engine temps and mixture carefully and ended up with valve problems anyway, and still don't know why or what I could have done differently.

With 390's priced like a new SUV, you'd think Lycoming could offer an engine that doesn't leak oil and valves that don't stick. But, no competition = no motivation to address these issues...
Mark Navratil
Spring Hill, KS
RV-8A #80583 - built/flew/sold
RV-14A #140017 - wings/empennage/tailcone complete, fuselage in progress...

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Old 06-09-2022, 07:17 AM
mountainride mountainride is offline
Join Date: Jan 2018
Location: Golden, Colorado
Posts: 76

My 1 and 2 run 10-20 degree hotter as well in the -14. The fiberglass inserts in the upper cowl do a really good job of pulling air aft. I bought replacement air dams for trimming experiments with the goal of getting them all perfectly even but am not sure its worth the effort.
RV-14 N303XC : Flying
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Old 06-09-2022, 07:34 AM
lr172 lr172 is online now
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 7,313

In most engine applications, valves get supplemental cooling with oil flow. Lycoming has VERY low oil flow through the pushrod relative to most all other engines. The oil comes from the hydraulic portion of the lifter. In most every other engine, the liter has a small bypass passage to send pressurized oil past the lifter and up the pushrod. For some reason, Lyc chose not to use this and rely upon bleeding oil past the clearance. The only oil that goes to the pushrod is that which leaks past that close tolerance fit of the two parts in the plunger assembly and they are hand lapped for a proper fit. Therefore, a fraction loose and you get more flow. A fraction tight and you get less flow.

If I were to do anything in your case, I would replace the plunger assembly in #2 thinking your may be a few .0001's tight. Heat causes the oil to coke in the valve guide, but if you have proper oil flow, that reduces the heat in the valve stem and reduce coking. Can't realistically expect to get a lot better on CHTs and lots of folks run 350 without sticking valves. My money is on a tight plunger assembly as the cause here.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 06-09-2022 at 07:40 AM.
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Old 06-09-2022, 07:53 AM
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riseric riseric is offline
Join Date: Oct 2011
Location: Québec, CNV9
Posts: 564
Default SaavyAviation articles

Here's an interesting article by Mike Busch on why valve sticking.
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Old 06-09-2022, 11:35 AM
lr172 lr172 is online now
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 7,313

Originally Posted by riseric View Post
Here's an interesting article by Mike Busch on why valve sticking.
Mikes logic here is that lead is found with carbon in the build up, therefore it must be coming from the fuel. Unfounded supposition IMHO. Your oil has disspersnts that hold debris in suspension. Much of that debris is lead particles, along with carbon. Therefore the oili is filled with lead particles and is then pumped into the rocker box and coats the valve stems and guides. When oil gets too hot, it oxidizes and becomes a hard substance called Coke, with the lead particles that were held in suspension now encapsulated in the Coke. This is an equally reasonable theory for the presence of lead in the guide build up, but Mike chooses to ignore it and go with a different theory. That's his perogotive, but doesn't mean everyone should accept it at face value. If mike were correct that it is lead that is the problem, how does he explain the presence of carbon in the build up?

Unfortunately no one really knows why it is such a problem with Lyc compared to all other engines. At least my theory has a variable that is unique to Lyc engines. Not sure we will ever know the true reason. Clearly lyc thinks it is too much heat, which is why they now use exotic filled valves to keep the valve stem heat down.

N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019

Last edited by lr172 : 06-09-2022 at 11:47 AM.
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Old 06-09-2022, 12:09 PM
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turbo turbo is offline
Join Date: Jan 2007
Location: Stuart, FL /Hartford, CT/Virgin Gorda,BVI
Posts: 3,264

to get some good info I would of wobble tested all of them or reamed all of them. keep us posted when you get back running again. do you have someone to do the wobble test?
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Old 06-09-2022, 01:46 PM
kiwipete kiwipete is offline
Join Date: Oct 2007
Location: Birmingham United Kingdom
Posts: 476
Default Why NO. 2

One observation I’ve made over the years is that it’s nearly always no.2 cylinder that has exhaust valve sticking problems. Mine did it around 350 hours.
For my Rv-10 I’ve made a duct to direct air onto the front of no.2 as per Dan Hortons baffle ramp ideas. I don’t know if it will make a difference but hoping that it gives a more uniform cooling.


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Old 06-12-2022, 11:40 AM
YoVuelo YoVuelo is offline
Join Date: Sep 2015
Location: Tucson, AZ
Posts: 21
Default Sticky #2 Valve

Join the club. I just had a similar problem with my Thunderbolt starting at about 185 hours. Start up was fine and about a minute or so after start, #2 would start dropping out and go cold and with a "bit of coaxing" would come back on line after about a minute. I was "15 hours" from home when this problem developed, but once running and warm, I never had a problem in flight. I made it back to base with nothing more than my morning "scare".

After doing all the normal trouble shooting of injector review, cleaning, and swapping. I even resorted to to swapping spark plugs and wires to ensure there were no spark issues. Wishful thinking makes one do a lot before deciding to ream the valves. Thus, after this journey, reaming was the next step.

While investigating the job, I was advised by Lycoming to ream them all, not just the #2 that was sticking. As suspected, #2 was quite coked up and after reaming, traveled freely. #1 and #4 had no evidence of any coking and #3 did show some evidence of build up, but was still moving freely before removal and reaming.

As far as temps go, my CHT's at 65% power (23 X 23, 50* rich of peak) for #1 & #2 are generally about 335*F and #3 & #4 are a tad cooler at around 320*F.

BTW, for those who do have to go this route, McFarland has the reamers needed for about $165 vs. Lycoming's $1300. Or you can navigate the administrative hassle between Lycoming and ACS to rent the reamer for $100 plus the cost of shipping to and from. After finding this out, I elected to buy the reamer from McFarland.
YoVuelo-Tucson, AZ
RV-14A Kit 294
Flying 6/30/20
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Old 06-12-2022, 04:49 PM
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KeithB KeithB is offline
Join Date: Mar 2014
Location: Granbury, TX
Posts: 339

I am another case - mine was at 270 hours, also on cylinder #2. Like the original poster, cyl #2 is my hottest by about 20-30° (depending on conditions). However, I was typically running close to 300° (above on #1 & 2, below on #3 & 4). I also chose to skip the wobble on such a young engine - the valve was stuck enough that I had to tap the valve out with a wooden dowel. I also purchased a McMaster reamer, and the reamed material was similarly black. I reamed only the problem valve.

I followed the Mike Busch logic and I’ve attempted to increase cylinder temps since - I installed the tunnel flap (have Vetterman exhausts so it was easy) and run close to peak EGT or slightly lean in cruise, increasing my cylinder temps by 30-40° over max lean (still only 300 -350° In cruise). I have another 80 hours since the stuck valve - fingers crossed.
Keith Brown

RV-14A Builder - kit #136 (complete) and #770 (in process)
N314KC - First flight Mar 8, 2017, 24th in the air, >850 hrs
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