Q: I am acquainted with two IO-540 engines with 1,000-plus hours that have bent rods in the #5 cylinder and stuck valves in the #2 cylinder. One aircraft was manufactured in 2007 and the other in 2009. The oil has always been changed at 50 hours using Exxon Elite 20/50. Any particular thoughts?
RICHARD WEICHMAN, via email
A: There are many factors that may lead to bent pushrods as a result of sticking valves, so let’s look at a few of those.
First, I commend you for your oil change frequency. I assume this also includes an oil filter change as well?
It’s important to know that frequent oil and filter changes are one of the best defenses in preventing potential valve sticking, but there are others just as important, if not more so.
When I look at the information you provided, I probably have more questions than answers, but let’s proceed and see what we can come up with as a possible solution to your problem. There is no doubt the engine oils used could possibly be a factor, but my feelings are, if you are using a good aviation grade oil, such as the one you mentioned, I think we can remove that from part of the equation.
My gut feeling is that we have other issues and my first thought is with the age of the aircraft involved. My concern is the condition of the engine baffles and their proper location.
If we fail to maintain the engine baffles, they tend to get weak and do not always remain in the proper location to direct the cooling air in the correct direction for proper cylinder head cooling. If the cooling air is being lost, this may cause the temperature of the valve guides to increase. This condition, in turn, may result in coking of the oil between the exhaust valve stem and the exhaust valve guide.
Typically, if an exhaust valve is going to stick, it will occur at first engine start-up when all engine temperatures are cold. What happens during flight when the engine is operating at normal temperatures is that the oil passing between the exhaust valve stem and the guide is in a liquid form and operation is normal. However, when the engine is shut down, as the oil cools, it hardens between the valve stem and the valve guide and forms the coking material.
The sticking normally occurs when the engine is started up the next time because the clearance between the valve stem and the valve guide was reduced when the oil coked. It actually has a grip on the valve stem, so until we get some heat in the guide to loosen up that coked oil, turning it into a liquid state again, we incur our potential sticking problem.
This sticking valve may result in the engine experiencing an intermittent hesitation or miss or the engine shaking more than usual during a normal start-up. If the shaking is noticed and appears to be unlike a normal start-up, I’d suggest shutting the engine down and taking a close look under the cowl for any bent pushrods.
I’d highly recommend you review a copy of Lycoming Service Instruction 1425A, dated Jan. 19, 1988, which will explain in detail the “Suggested Maintenance Procedures to Reduce the Possibility of Valve Sticking.” Lycoming has also provided other publications covering the subject, and I’d recommend you or your maintenance facility review these and comply with any of them that may be required or recommended for your engines.
The most important publication regarding this subject is Lycoming Service Bulletin 388C dated Nov. 22, 2004, which addresses the “Procedure to Determine Exhaust Valve and Guide Condition.”
However, since you didn’t state whether these engines were factory new, rebuilt, or overhauled engines, I’d suggest you review Lycoming Service Instruction 1485A, dated July 2, 2003, which addresses the valve guide material and a recommended “Time Of Compliance.” If these engines were overhauled in the field, the facility that performed the overhaul should be able to tell you which exhaust valve guide were installed at that time.
One more important thing to mention is the envelope in which the aircraft operates. This may also be a contributing factor to the potential for sticking valves, so this is another thing you may want to review. Avoid rapid descents with reduced power, slow flight which usually means reduced cooling, and take time for a proper cool down following the flight.
Hopefully after reading my thoughts on the subject you’ll be able to resolve your valve sticking situation.
http://generalaviationnews.com/2014/12/28/what-is-causing-bent-rods-and-sticking-valves/?utm_source=The+Pulse+Subscribers&utm_campaign=dca29a79e5-TPoA2014&utm_medium=email&utm_term=0_62525a9780-dca29a79e5-58005
Assuming the writer’s problem is bent valve pushrods and not connecting rods, and cylinder intake and exhaust valves and not some other valve: There is considerable evidence that a lack of dedicated cooling oil to the exposed exhaust valve stems in Lycoming engine rocker boxes leads to valve train distress. Toss in insanely high CHT redlines and the lack of CHT monitoring of all cylinders leads to exhaust valve and occasionally intake valve sticking. Some argue that stuck valves damage cam lobes and followers. On the other hand, poor rocker arm geometry, swapped rocker arms, high CHT limits and the lack of engine monitors lead to rapid valve guide wear and subsequent burned exhaust valves. This localized heating due to poor valve sealing leads to valve head cracking or separation of the valve head from the stem. Exhaust valves run hotter than intake and more prone to distress, but intake valves are not immune.
According to John Schwaner at the Sacramento Sky ranch, an intake or exhaust valve stuck shut can bend valve pushrods. As the afflicted pushrod bends it can break pushrod shrouds so oil goes overboard. Stuck valves can also result in valve rocker arm and rocker mount boss damage. But more insidiously, a ‘stuck’ valve can crack the thin but very hard nitride coating on cam lobes and followers.
If the valve train manages to pry a sticky valve open and it stays stuck open, the valve train unloads. Then the valve train hammers cam lobes and followers while pushrods fall out of their sockets and and valve rotators fall off the valve. http://www.avweb.com/news/maint/182894-1.html?redirected=1 John’s hypothesis is that the cracks in the cam lobe and follower surfaces leads to spalling, which exposes the soft steel beneath. Then rapid wear begins, and the cam lobe rounds off while producing lots of metal.
Performing SB388C ‘valve wobble check’ every 400 hrs on both intake and exhaust valves might also prolong cam and follower service. http://www.lycoming.com/Portals/0/techpublications/servicebulletins/SB%20388C%20%2811-22-2004%29/Procedure%20to%20Determine%20Exhaust%20Value%20and%20Guide%20Condition.pdf
Stuck valves that damage the nitride layer on cam lobes and followers might help explain the rash of rounded cam lobes GA is experiencing. Engine shops report getting engines with low hours – some under 500 – for prop-strike teardown and find the cam lobes and followers already damaged, cause unknown. Aviation Consumer addressed the possible causes, but not the stuck valve theory.
The cam lobes in Lycoming engines with ‘mushroom head’ cam followers cannot be inspected without splitting the crankcase. Some Lycs and all Continentals have barrel followers that can be removed, allowing cam and follower inspection.
Unless someone inspects the oil filters and suction screens for magnetic particles nobody will know the cam lobe is rounding, and there is no requirement to dial cam lift, but doing it along with the valve wobble check is easy. Otherwise you won’t know the engine has lost power.
Lycs share intake cam lobes between opposing intake valves so one would think that rounding the shared lobe a serious problem and easily noticed. It isn’t, at least not with constant speed props compensating for the reduced power. Will the metal particles damage the engine? Not if it has a full flow filter or pressure screen; suction screen and prop governor screen. Here’s a generic Lyc oil flow diagram: http://wiki.velocityoba.com/index.php?title=Nose_Oil_Cooling
Lycoming exhaust valves rely on cooling thru the stem as well as valve head-to-seat to cool the valve, so the stem and guide are a critical cooling path. Most Lyc valve stems are hollow, filled with elemental sodium to help with heat transfer to the valve guide. Hence, Lyc exhaust valve guides deal with more heat than Continental valve guides, and the hollow valve stem is a point of failure.
I might point out that as a general rule, Lyc cylinder heads are designed to ‘heat soak’ vs Continentals, which are designed to heat shed. Lycs have few or no fins around the exhaust and intake valve area whereas Continentals have many very fine fins that draw off heat quickly and might be the reason for the shock cooling theories.
The valve wobble test SB 388c is a Goldilocks test to make sure there is enough valve stem to guide clearance to prevent sticking, but not enough to affect valve head-to-seat concentricity. Excess wobble leads to poor stem to guide cooling. It also leads to poor valve seating and hot spots. Hot spots can lead to burned valves and possibly a broken valve. https://www.youtube.com/watch?v=x6OyfoV1Z2I
SB388c addresses only the exhaust valve, but losing an exhaust valve only shuts down one cylinder. Stick open or lose an intake valve and it shuts down the whole engine, so my thoughts are to verify the intake valve wobble is suitable as well. SB388C is optional for part 91 operators, and there are ways to verify ‘some, but not too much’ wobble without dis-assembling the valve train and springs.
Oil choices: IIRC Exon Elite is a semi-synthetic. Synthetics have great wear properties but zero lead solvency: Mineral oils have good lead solvency, hence synthetic oils are blended with mineral oil. This lead deposit problem is the best reason I can think of to run mogas when able. That said, great gobs of lead/sludge goo can clog oil galleries in the crankshaft but I think unburned hydrocarbons and high temps are the cause of stuck valves, not lead deposits. LOP operations and maintaining CHT below 400F minimizes it.
GA doesn’t usually wear out the ‘bottom end’ of aero engines, they corrode and suffer damage from deposits. We damage the top end with high CHTs. Flowing lots of oil to the rocker boxes cools the valve stems, but most Lycomings are very bad at it. More on that later.
To minimize lead deposits, the current wisdom is to us Philips XC 20W50 – a very good mineral oil with excellent lead solvency – and add Camguard for corrosion protection. Camguard is FAA approved with a growing body of evidence to show that it meets expectations for corrosion protection and reduced deposits. http://www.aviationconsumer.com/newspics/pdfs/35-5-Cam.pdf
In Feb,May and July 2005 issues of Aviationconsumer.com you’ll find extensive testing of various oils and additives for corrosion and wear protection: Well worth a read. http://www.aviationconsumer.com/issues/35_5/maintenancematters/5446-1.html
Bill Marvel has a very good discussion on rocker box oiling and why it’s important to prevent valve failures. Sadly, for many Lycs, rocker box oiling is as much by accident as by design. The quick fix is to set the oil pressure to run just below redline at cruise RPM and oil temp: Doing so should maximum oil to the rocker boxes for valve stem cooling. He also recommends installing hydraulic lifter units that provide better top-end oiling. http://occonline.occ.cccd.edu/online/rfoster/The%20Rest%20of%20the%20Story-%20Valve%20Lifters%20and%20Tappets.PDF
Engine monitors are by far the best investment an owner will ever make in a plane. Airplanes with only one CHT or – like early Cardinals – no CHT- are flying blind. Data logging allows playback on a PC for analysis. The on-line engine management course at http://www.advancedpilot.com/ teaches how to run aero engines for maximum longevity. I recommend it.
Advanced Pilot is a subsidiary of http://www.gami.com/ The Gami site is a wealth of knowledge based on millions of airline hours operating with complex piston engines. Gami adds to that with data from flat aero engines run in their test cell.
The guys at GAMI/Advanced pilot seminars and Mike Busch at http://www.savvyaviator.com/ agree that high CHT is a major factor in exhaust valve failures. Factory CHT redlines in the 475F range are an invitation for disaster, and OEM advice to run 50F ROP puts the engine smack-dab at it’s highest CHT. GAMI’s test stand shows that no conforming aero engine will detonate or go into pre-ignition if the CHTs are kept below 400F, so that has become their recommended climb CHT limit, with 380F for cruise, running lean of peak in cruise to minimize valve stem deposits by cooling with air instead of fuel.
To conclude, an engine monitor with CHT readouts for each cylinder; engine management (and maintenance) that keeps CHTs below 400F as an absolute limit and: Adjusting oil pressure to the legal maximum for your installation; and run straight mineral oil for maximum lead solvency. These steps appear to reduce valve train distress. Finally, at overhaul insist on valve hydraulic units that maximize oil flow to the rocker boxes.
I have a 1978 201J model that I just had to have 2 cylinders overhauled on and noticed this: on the #3 cylinder the previous owner had stated that they were having an issue with a sticking valve several times on that specific cyl. He stated that as soon as he started using Marvel Mystery Oil this problem ceased to happen again. (During winter oil changes I always used this also to help with easier starts and prevent sticking valves on my C model- although not officially approved for use, I never had a sticking valve either). Anyway, back to my J model. When the cylinders came off I noticed that I couldn’t manually move the rocker arm up and down. The cause was that during assembly at overhaul(or sometime shortly after) the bushing that the rocker arm rotates up and down in worked itself partially out or was never totally pressed in and seated (bottomed out). This caused significant binding in the valve train, hence valve sticking open. In your case, maybe closed causing a bent push rod. Just a thought because of my experience, I hope this helps.
Richard asked about ‘bent rods’ in the #5 cyl and stuck valves on the #2 cylinder.
What kind of ‘rod’ is the submitter talking about?
Which valves?
Sometimes, nomenclature matters.
I have read that operating too rich can also cause sticking valves. Like during taxi with the mixture always full rich. The excess unburned lead can result in carbon build up on the valve stems and elsewhere.