Lycoming Says Aromatics In Fuel, Leaning Techniques May Have Caused Valve Problems


Lycoming says it doesn’t think its engines are to blame for the valve seat recession problems experienced by the University of North Dakota flight training department last year after it switched to Swift Fuels’ UL94 fuel. Rather, it says in a news release (copied in full below) its analysis suggests that “under certain conditions, use of UL94 may impact valve seat recession.”

Lycoming says the analysis indicates that aromatic chemical concentrations in UL94 may cause the erosion of the valve seats under some flight conditions. Aromatics like benzene, toluene and xylene are used to boost octane in fuel and in “elevated” concentrations may result in “particulate abrasiveness to valve seats that may contribute to valve seat recession.”

Lycoming says it’s working with the FAA to further study the effects of aromatics on engines and notes that aromatics concentrations are not specified by the ASTM D7547 that applies to aviation gasoline. If aromatics turn out to be a problem for engines, Lycoming says it will work with the fuel industry to address aromatics in the standard and give guidance to operators on appropriate leaning techniques to prevent the engine damage. We have asked for comment from Swift and from General Aviation Modifications Inc. and will update this story as those responses come in.

Russ Niles
Russ Niles is Editor-in-Chief of AVweb. He has been a pilot for 30 years and joined AVweb 22 years ago. He and his wife Marni live in southern British Columbia where they also operate a small winery.


  1. I can’t comment on the cause, but that recession sure is bad. I’m glad it’s not mine to repair. $$$

  2. As an engineer I have designed and built racing engines for 44 yrs. It’s not a fuel problem, it is the cheap valve seats these engines have. If the would use todays valve seat materials the seats and valves would last indefinite. The use of aluminum bronze valve guides, hardened valve seats on the exhaust and aluminum bronze intake seats fixed this simple problem.

    • Is an AD needed to force owner’s to replace valve seats, valves, valve guides, etc.?

      Good luck with that.

      • It should have been done, by the manufacturers, shortly after the automakers started doing it.

        • Nope, this is supposed to be a DROP IN REPLACEMENT for 100LL. It’s not a replacement if it required all new procedures and all new valves be installed. As said if you just remove the lead in 100LL you have a cheaper and perfectly compatible 94UL.

      • So, what about the entire existing fleet?
        This is an expensive blend that requires expensive retrofits. It’s worse than what it replaces!

    • “Aromatic limits are not directly specified by ASTM D7547, and elevated aromatic concentration may result in slower flame speed”

      Translation: The new SWIFT blend slower burn rate is just enough to have more burn/heat now happening as the exhaust valve is just opening. The exhaust stream is now a hotter blowtorch in the area of the valve seats. (Of course, simply taking the lead out of current standard 100LL will give you 94 octane, lower cost, no lead, and zero new problems; but that’s to easy).

    • Exactly, and old engines can be fixed to use new fuels — there are various artisanal ways of hardening exhaust valve systems, for engines where getting a new head and valve set is not possible.

  3. Just like Lycoming or Continental to blame recession caused by the use of crap materials on the fuel. In 40 years of work in the auto industry I’ve seen maybe 2 valve seats suffering from recession and those were in the early days. I am not even convinced there are enough “old timers” left in those 2 companies to even know how or what goes on inside their engines and I think that is largely why they are so averse to the tiniest change.

    • Do the replacement cylinder assemblies, such as Superior, use better materials? I know there are different flavors of cylinder heads, such as chrome cylinder walls or not. Just wondering. I’d assume most of the older low compression pistons singles no longer have their OE cylinders on them.

        • Probably no more than an STC, and you could give comparative analysis based on tens of millions of cars (or even just pick the air-cooled VWs and Porsches, and a myriad of motorcycles) which had no issues.

    • I saw that at an airplane company I worked at. Stuff designed years ago by long since retired engineers and the youngster engineers of the day were loathe to change stuff they did not understand.

  4. If valve recession wasn’t a problem with 100LL, the fuel for which these engines were designed, wouldn’t it follow that the metallurgy of the valves, seats, and guides was properly suited to their original application? If it happens only with this fuel, wouldn’t that be prima facie evidence the fuel is the cause?

  5. I think I’ll wait on the evidence before deciding either way. AOPA is running a trial on a Baron, one side only 100LL, the other only No Lead. Both engines were overhauled before the test. Mike Busche’s shop at Savvy is periodically inspecting and checking critical measurements.

  6. “Aromatics like benzene, toluene and xylene are used to boost octane in fuel…..”

    I’m not sure we should say that out loud.

    The hand wringers are never satisfied. As soon as the attack on 100LL succeeds, they’ll scamper out from under the dumpster fire they’ve created attacking lead to start gnawing on “aromatics”.

    Over the past 64 years I’ve witnessed many advances in aviation, mostly well thought out resulting in safe positive changes. But this bungled fuel mess takes the cake. This is without a doubt one of the biggest steps backwards we’ve ever experienced in GA. All its doing is creating uncertainty. In my opinion, it’s because our aviation community and those who materially support it are splintered. We’re no longer priority focused. Remember the good ‘ol days of SAFETY FIRST, innovation and circling back to SAFETY in the process. It’s a beautifully simple process and it’s still there. But over time someone has slipped the word appease into the process and buried it somewhere under piles of special interest agenda paperwork where it will never be seen again. We need to wake the heck up, push back harder in SAFETY’s name and return to a process we know works.

    • Yes, this is exactly what will happen. It would be really nice if we could start getting UL94 at airports, so the majority of us who would prefer it, can get it?

      And, in anticipation of the standard “60% (or is it 75%?) of the fuel is used by aircraft that require 100LL” I note that 85% of statistics are made up on the spot, so please provide evidence for the claim – because if it exists, I’ve never seen it.

  7. I wonder if the Rotax guys have had any valve seat issues. They seem to go out of their way to get 94UL.

    • Two of my aircraft use Rotax engines, a 914 and a 915 iS. Both are turbocharged and one is electronically fuel injected. “Going out of their way to get 94UL” is about getting the lead out. When leaded fuel is used in a Rotax engine, lead deposits foul the propeller gearbox (among other things) and the oil change interval is reduced in half. I use 91 octane ethanol free pump gas in one of the aircraft because tank sealants degrade with ethanol. The other aircraft which is about 17 years newer does not care about ethanol.

  8. “aromatics concentrations are not specified by the ASTM D7547 that applies to aviation gasoline.”

    Does this bring into question the quality of the ASTM Standards?
    Is the STC standard more thorough? Less thorough?

  9. For those of us wondering what exactly “ASTM D7547” is, it appears to be the “Standard Specification for Hydrocarbon Unleaded Aviation Gasoline”, i.e. “Grade UL 91 unleaded aviation gasoline”. Swift Fuel’s UL94 product appears to conform to ASTM D7547. By contrast, 100LL appears to be specified by ASTM D910-24, “Standard Specification for Leaded Aviation Gasolines”.

    I am not an aviation fuel expert, but this is what some web searching and cross-checking turned up.

    I look forward to a 20-minute Paul Bertorelli video explaining all about ASTM D7547 and its rules for aromatics.

  10. Seems some shade is being thrown on the engine manufacturers, but can they really justify changing material’s financially? Our engines now cost more than entire cars that come with much nicer engines. I don’t think it’s some capitalist conspiracy waiting to be corrected by a revolution of the proletariat. Last aircraft engine I bought, in the plane, was one of the newest in the fleet, was made in 1995.

    • So what is the difference in price between, say, the valve seats used today (and for seventy years) versus what cars have used for 40+ years? Pennies? Say it was five bucks (it’s not) difference, times 12 (even though the intakes are likely OK). Heck, make it sixty bucks per valve. So under a grand, for engines that cost more than a car.

      • Sorry, I accidentally reported your message.

        Anyways, if GM wants to change the valves in an engine, I suspect they test it to their own satisfaction, and then do so. They then sell tens or hundreds of thousands of that engine a year.

        OTOH, change the valves on a lycosaur, and how much does it cost to test things to the FAA’s satisfaction? And then, how many get sold per year?

        The requirements are now such that it’s a crazy risk to do anything in light aircraft manufacturing.

        • “Sorry, I accidentally reported your message.”

          Glad I’m not the only one who accidentally reported a comment instead of replied to it. Avweb really needs to fix the issue of the two of them being too close together (and the “report comment” in blue, making it look like it should be the one to click on).

  11. First : don’t mix RON + MON /2 = ANI at the gas station in the US . Second never compare a car engine with an aircraft engine . A car engine don’t sustain 75% power very long !! Even racing engines will disintegrate when they run on high power for a longer period ( even when they run on very rich fuel/air mixture) or you overhaul them every few hours ! Under 75% power you cannot over lean or destroy an aircraft engine only through leaning , it’s impossible !
    Look at : several engines are certified for Mogas but pump gas is prohibited !!

    • Any car I’ve owned since my ’69 Merc was happy running for sustained periods of time at full or near full power. The Merc showed 110 for stretches on the New York throughways; ten years ago I took the desert route to Vegas where I maxed out at about 130 but cruised at 110+ for half an hour or so in my base model Eclipse.

      It’s comparatively easy to routinely overbuild automotive engines when the weight penalty isn’t so bad.

      • Naw. Run it at 75% power (not RPM, but power output) for 1,000 hours and get back with us.

    • This “car engines don’t run at 75% power for hundreds of hours!” statement is a common trope when people talk about GA engines, but as someone whose career has been in high performance auto engine and subsequently the aerospace industry, it’s one I shake my head at. Why not compare the other way around? It’s not even debatable that aero engines are technologically a mandatory retirement age behind auto engines. Just because you can fly your 160hp lycoming wide open for hundreds of hours doesn’t mean it wouldn’t be a better engine if it was made as advanced in some ways as a 30 year old auto engine and it’s a falsehood to assume that you can’t run any auto engines like that. There are plenty of impressively durable and long lasting automotive engines, some are even able to be more than doubled in peak power output by tuners and still lead long lives. There are engines used in racing that spend most of the race making as much power as possible and are expected to last whole seasons. Heck, there are ordinary factory car engines used in endurance racing that last multiple seasons being beat on worse than a rental at a flight school.

      Compare two painfully ordinary engines here. An O-320-D2J and a Honda K20A3. Both make 160hp rated power. The O-320 makes this power with 320ci of displacement, 5.2L, the K20 2.0L or 122ci. The O-320 has a compression ratio of 8.5:1, the K20 9.8:1. Specific outputs are 31hp/L vs 80hp/L. Now based on these figures do we think it’s even wise to compare them anymore? If we detuned a K20 until it made 62hp and ran it at 46.5hp it would probably last longer than an O-320. If we were able to squeeze 80hp/L out of an O-320 then run it at 75% power you can bet it wouldn’t last long, its internals may not even hold up to the attempt. It’s a two way street. Aero engines make absolutely laughable power, and it’s because they’re expected to last a couple thousand hours at 50-75% power output while restricted to designs that are older than the oldest airline pilots, and they’re running fuel that is made with additives banned everywhere else decades ago. Auto engines last hundreds of thousands of miles and actually perform well while running cleaner. I’m not getting into whether the lead emissions are significant, it’s just a statement of fact that inertia and unwillingness to change simple parts is the only reason we still use it.

      If we used some of the “advanced” materials in auto engines, such as valve and seat materials, as well as other internals (the pistons aren’t even hypereutectic, we could gain durability from that material change alone), we could have more durable engines that may not bat an eye at running for thousands of hours on 94UL and may even last longer between overhauls. The problem is that these ancient designs we rely on already cost more than the car that aforementioned K20A3 came in, and who knows what they’d cost if they were to make certified changes in parts to bring them up to modern material standards, let alone redesign them for better specific power output, fuel economy and emissions. Just because the old bromide of running GA engines at 75% power for an overhaul interval is technically true, doesn’t mean you *can’t* do it with auto engines, it doesn’t mean we have nothing to learn from auto engines, and it doesn’t mean that a lighter, smaller, gear reduced modern engine wouldn’t do the job better without requiring leaded fuels.

      • I was second owner of a 97 Toyota Land Cruiser now with its third owner at over 350k miles with only minor repairs and maintenance. That engine, and it’s V8 replacement are very well known to last that long.

  12. Gee, “aromatics like benzene, toluene and xylene” that sounds like MoGas and octane enhancement for non-ethanol gas! Which begs the question: why with decades of use by pilots and owners, MoGas doesn’t have the same issues? The whole UL issue is really a mess, and for those of us who have been flying with MoGas since the mid-1980s, it seems way too overcomplicated.

    Like anything else, what could possibly go wrong when a manufacturer does it’s own investigation and shrugs its shoulders and says not us! Obviously, the valve seats are an issue. We’ll never know if they had a bad batch that didn’t get through QC. And as usual, non-sport pilots who never use MoGas will pile on. But I’m thinking that the gas and the engine might not be a good combination, but we will never know what percentage of the problem is Lycoming and what is the gas.

  13. “Aromatics like benzene, toluene and xylene are used to boost octane in fuel and in “elevated” concentrations may result in “particulate abrasiveness to valve seats that may contribute to valve seat recession.””

    Automobile fuel is widely formulated with benzene, toluene, and xylene. These three aromatics are made of carbon and hydrogen and nothing else. They are liquid at room temperature and will burn in the presence of oxygen. When they burn, the combustion products are water, carbon dioxide, and a bit of soot. Hard to see where the so-called “particulate abrasiveness” comes from.

    • “…water, carbon dioxide, and a bit of soot.” That bit of soot is most likely the “particulate abrasiveness”.

    • One quick note to add here. Benzene is a proven carcinogen, and as such, has been prohibited in motor gasoline for over 25 years. A short history: When the final elimination of lead in auto gas was taking place, the refining industry went looking for ways to increase octane performance of their fuels. One method was to replace straight chain hydrocarbons with “ring” compounds that naturally have a higher-octane number. The preferred method was to use a catalytic reformer unit that made the conversion (i.e. pentane to cyclopentane, hexane to cyclohexane, etc.). The process also produced the aromatic ring compounds – benzene, toluene and xylene. That was okay, because the BTX’s also have high octane ratings. But the EPA objected to benzene due to its cancer-causing properties, so it has been removed from autogas, and I would assume also from any aviation fuel products. So, toluene and xylene are okay, but benzene is not.

  14. Lycoming’s press release (04.12.2024) on the root-cause analysis of UND’s issue with exhaust valve recession is at best – inconclusive. High aromatic hydrocarbon concentrations in UL94 are not the culprit in this analysis.

    ASTM specification D7547 for UL94 describes the use of aromatics as follows — “Although Specification D7547 does not include an explicit maximum aromatic limit, other specification limits effectively restrict the aromatic content of unleaded aviation gasolines. Benzene is virtually excluded by the maximum freezing point of –58 °C, while other aromatics are limited by the minimum heating value and the maximum distillation end point. Thus, the heating value limits toluene to about 24 %. Xylenes have a slightly higher heating value and, therefore, would permit somewhat higher aromatic concentrations; however, their boiling points (above 138 °C) limit their inclusion at levels not higher than 10 %. Total aromatic levels above 25 % in unleaded aviation gasoline are, therefore, extremely unlikely.”

    So, in the case of UL94, moderate use of toluene (typically 10 – 20%) in the fuel is entirely reasonable and not likely to contribute to valve seat recession. These are not “elevated” levels of aromatics, they are entirely normal having been used across the US for almost 9 years. Swift Fuels stands by our UL94 Unleaded Avgas product. The UL94 fuel used at UND complied with ASTM International D7547 as reported on Lycoming’s Service Instruction SI-1070. Aircraft operators should also observe Lycoming’s mandatory Service Bulletin SB 388C.

    Conjecture about the use of unleaded avgas at high operating temperatures (particularly operating at peak EGT’s) remains an open question for these “conforming” engine components. Lycoming has recently suggested that operating an engine at lean conditions for extended periods could starve the engine of fuel needed for cooling, thus contributing hotter temperatures in the cylinder, resulting in more rapid wear of the exhaust valve guide and valve assemblies. On this basis, Swift Fuels would advise pilots to limit extended flight operations at peak EGT or lean of peak when flying UL94 unleaded avgas until the completion of a comprehensive analysis of the exhaust valve issue is documented and confirmed by industry.

    NOTE: Swift Fuels’ new 100R unleaded avgas contains an anti-valve seat recession additive to prevent this type of valve wear in our 100-octane unleaded avgas product. 100R avgas recently passed over 400-hours of FAA-conforming engine tests, operating with sustained periods of high CHT’s and peak EGT’s, with no adverse signs of wear in the valve assembly.

    Chris D’Acosta
    CEO – Swift Fuels, LLC

    • But if the chemistry of SWIFT fuel has elements that burn more slowly, then the exhaust valves bear the brunt of that. It’s sorta like running too rich in a hotrod and the rich exhausts fumes are still combusting as it leaves the engine and the valves/headers start glowing red hot.

    • You don’t know anything about engine or exhaust gas temperatures unless you have an engine monitor with individual sensors to each cylinder and exhaust port. Your statement makes sense if the aircraft has the single engine temperature probe that was installed on older aircraft. If your theory is correct, running CHTs in lower 300s and EGTs closer to 1300-1350 should not cause this recession to occur I would suggest.

  15. Chris: First let me say that I can’t wait to use unleaded fuel. And the take advantage of the concomitant use of synthetic oil.
    However, I wonder about Lycoming’s statement and your response:
    “Lycoming has recently suggested that operating an engine at lean conditions for extended periods could starve the engine of fuel needed for cooling, thus contributing hotter temperatures in the cylinder, resulting in more rapid wear of the exhaust valve guide and valve assemblies. On this basis, Swift Fuels would advise pilots to limit extended flight operations at peak EGT or lean of peak”
    If I operate/cruise my Lycoming O320 D2J at just lean of peak ~65% power (7.5-8g/hr) with CHTs around 330 degrees and EGTs around 1490- 1520, I don’t see that “lean of peak” condition creating high cylinder temps. Now the question is: would UL94 cause my CHTs to increase to close to 400 running at 65%? or what would be a reasonable increase temp?
    2nd question: Doesn’t lean of peak reduce CHT from Peak power regardless of fuel chemistry?
    My Cont IO520 in my C210 cruises (always less than 70%) at a higher CHT – around 375, EGTs around 1520. Similar concerns?

  16. John — Point taken. Operating an aircraft on unleaded avgas at peak EGT’s is certainly hotter on the cylinders and therefore more prone to create challenges when running on a extended use basis – if operating without an anti-VSR additive. Yes, I would agree that properly running lean of peak should be less problematic for both CHT’s and EGT’s.

    • Chris, thank you for your input on this rather confusing issue. I should have read your comments before I inserted my previous response on the use of benzene in motor fuels. Anyway, I am curious about one issue regarding the “particulates” mentioned in the Lycoming response. If toluene and xylene exhibit slower burn rates that result in creation of soot, it seems to me that running rich of peak would actually exacerbate the problem and running LOP would reduce the volume of such particulates (i.e. more complete combustion). Also, running at peak EGT puts the maximum temperature stress on exhaust components. Leaning the engine beyond peak actually causes the EGTs and CHTs to go down. I know that Lycoming has consistently opposed pilots from running their engines LOP, but GAMI and others have maintained for years that it does not harm an engine – Lycoming or Continental. Seems to me that there is more going on here than meets the eye.