IO-360-L2A - 2350 Max RPM in the Air with Vibration

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Thandie's picture
IO-360-L2A - 2350 Max RPM in the Air with Vibration

I've been troubleshooting this engine for months now, and I'm out of ideas.  Each time this aircraft gets written up I've done the basics, compressions, plugs, etc.
First time I found No. 4 exhaust valve sticking, zero compression.  Replaced cylinder. 
Second time, found the mags were at 1100+ hours with no 500 hr inspection.  Put two different fully inspected mags on.
Third time found No. 2 exhaust valve sticking, zero compression.  Replaced cylinder.
During this time, I had to keep leaning the servo back in order to get the appropriate idle/mixture settings.  It kept creeping up.  Finally I started noticing blue stain on the ground under the airbox drain line.  I determined the servo to be going bad and not shutting fuel fully off, replaced servo.  I also ran through a thorough fuel system check per the Precision Airmotive troubleshooting guide.  No defects noted.
Finally, everything is checking fine, compressions are holding in the high 70s, idle mixture is stable, mag drops are fine after a clearing run, ground run is 2350 static rpm, but in the air pilots can't get more that 2350 RPM and are getting a vibrating tach, a noticeable vibration in the engine through the 1800 - 2100 rpm range you can feel and hear, and the fuel flow needle is bouncing. 
So this week, grasping at straws, I check my valve clearances after shut down.  I get anywhere between 0.010" - 0.050" and most are a little spongy.  I replaced all 8 tappet plungers, checked dry plunger clearance, 3 were in the high 0.070" range and 3 in the 0.080" range, 2 around 0.40".  Replaced the 6 above 0.070".  I'm a little annoyed feeling that I've gotten a little gipped by my overhaul guy, but happy that I've probably figured it out.
Test flight revealed no change in symptoms.
Beyond pulling the engine, or putting new mags on, I'm out of ideas.  I do have a optical tach I'm going to use to verify RPM, but the vibration makes me believe that it's probably not going to reveal anything.
Any thoughts?

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Thandie's picture

Just did a run with the optical tach. 2340 - 2400, with 2350 indicated on the aircraft tach.

One symptom persists on the ground and that is the plugs foul fairly quickly. Meaning if I let it sit full rich for a while on the ground, especially at lower rpm settings, the drops are out of limits. A clearing run solves this no problem. I have been attributing this to oil fouling due to the two new cylinders which are still on the break in period.

A theory I have is that with the continual leaning of the servo to obtain proper idle mixture settings caused the engine to run excessively lean at higher RPMs. One of the flights prior to changing the servo the pilot reported back firing in addition to the vibration and high oil temp. This seems consistent, but he didn't report an over temp, even when asked. If it was running lean it only had about 10-15 hours (best judgement) in that state. Could that have caused damage to the engine that is causing this symptom? Perhaps warped an internal component or galled a journal? How can I tell without a complete tear down? Would it be worth pulling the prop and doing a dial indicator on the flange? I feel like I'm missing something obvious and over thinking this issue.

I've considered new mags, but I can't see that a mag would be causing this. Even if the internal timing was off on one, that would just make a weak spark. Shouldn't the engine perform on one mag?

bob.pasch's picture

Thandie, please identify the model of aircraft and the original discrepancy you're troubleshooting.

1-One thing I would recommend is NOT run full rich until T/O.

2-Have you checked the prop/blade angles?

3-Have you had the prop balanced?

4-You might try a hotter set of plugs [if authorized].

Thandie's picture


Thanks for the response.

The aircraft is a 2004 172S Nav II. The engine has about 1350 SMOH.

1. I have spoken to the operator (Chief Pilot of a flight school and his instructors) about keeping it leaned until take off, and also if they get bad drops to do a clearing run. We've been having hot weather which has been causing the fleet as a whole to deal with rich mixtures. We've adjusted the fleet for the seasonal temperature changes.

2 and 3. We have a known good prop coming from a sister location on Monday and we are going to put it on to test the prop theory. It's a McCauley fixed pitch and there is no history or indication of damage. The only thing we did recently to that area is put a new ring gear on the starter ring gear support assembly. The thought crossed my mind that maybe some how we threw it out of balance, but I can't think how. To cover our bases though we will test a different prop.

4. I like this idea, and hadn't thought about that at all. I will research this and see what's approved for this AC.

One thing I did fail to mention, because I didn't think much of it at the time, but a couple flight after we had changed the mags out, when I changed the servo, I was going through the paces, including checking mag to engine timing and noted the LH mag was 4-5 degrees off. I am sure the mags had been installed and timed correctly, and the torque seal hadn't broken. I adjusted it again at the time, but it still didn't change anything with the engine issue. If the LH mag slipped internally it could explain the higher than normal bad mag drops on the left. Meaning when both mags are dropping out of limits, the RH drops about 200 and the LH drops about 300. The issue I have with that is that a clearing run gets both mags to drop between 90 and 100, additionally (correct me if I'm wrong) an internal slip will only effect the strength of the spark, not the timing of it relative to the engine, the points set that. So worst case with a slightly internally mistimed mag, could that really cause a 200-300 RPM decrease in max RPM and a vibration when both mags are firing? I know AC are sensitive machines, but it's hard for me to see that. Before the prop gets in, I am going to swap it out, just to say I did.

One or two items that may be of value. The problem seems to get more pronounced as the engine heats up. The oil temperature never red lines, but it does rise fairly quickly and stays in the highest section of the green arc. Oil analysis trends have given no indication of an increase in any metal. Every oil filter has been cut open and no metal or excessive coking noted. In fact the oil filter seldom even has any flakes of coke in it. Mag drops after taxiing back from a flight are the worst and do include engine stumble. This as I've mentioned I believe is because of the two new cylinders allowing oil to fowl the plugs.

The more I explain the more I like that hotter plug idea. I'll report back as I work this. Any other thoughts are appreciated!

One thing I would like to ask the community as a whole, has any other Lycoming maintainers noticed a decrease in quality? It's not just this engine that has been giving us problems. We've had a rash of sticking exhaust valves starting about 12-16 months ago. I have another engine with 250 hours SMOH with a valve that starting to stick, can't get above 60 on compression. Lycoming doesn't warranty valves sticking, but my overhaul guy said he'd fix it for the cost of parts. We're pulling it at the next 100 hour and sending it out, but that's ridiculous!

Same thing with the fleet of Seminoles, the engines and the props are junk. The low pitch stop keeps coming loose, the last one destroyed the prop cylinder and the spinner, a $3000 or so repair all told. I could go on. It just feels like quality has tanked over the past 1-2 years.

bob.pasch's picture

Three things:

1-You kinda slipped over the prop balance. You said you have "a known good prop". That may be however, prop balancing is a misnomer. You're actually balancing the entire system! So when you change ANY component [ring gear, prop, spinner, bulkhead etc.] you MUST rebalance "the prop" system. It's not unusual for a ring gear to throw it out of balance especially if the ring gear is used as the balancing station. Also, if the prop is a 2006 era prop, it's possible to have been dressed one too many times! I'd send it to a prop shop for a serviceability check.

2-You're a straight man for my Green Arc story. Any time an owner or instructor squawks oil [or any] temp on the high side of the green range, I tell them that manufacturers spend millions of dollars on GREEN paint for a reason! If it's LOW green, if it's HIGH green or any where in between - IT'S GOOD! GREEN = GOOD, don't interpret that as anything else unless there's a corresponding change elsewhere that MAY be related.

3-When was the last SB388 [whatever edition?] performed? 388 is a must as far as I'm concerned especially on trainers.

Lemme know...

Thandie's picture


Thanks again for the response.

I about the entire system needing a dynamic balance, but now that I think about it it makes sense. I've been maintaining a fleet of 172S for 8 years now and never once had an issue with the prop. We comply with SB 240E every 1000 hours and I believe the props are balanced at that time. I will look into a dynamic balance of the prop on the AC on Monday. I believe our sister location has equipment for that.

I agree with the green = good and generally I tell the operator the same thing, but at the same time if engine parameters change from normal operation I like to take it into account when troubleshooting. In this situation I don't put too much into the oil temp, it's green and stable, good.

SB388 I will look into. We do not comply with it. Thank you for turning me on to it!

Any thoughts on the low pitch stop issue on the Seminoles?

bob.pasch's picture

Thandie please tell me you're NOT signing off SB 240E every 1000 hours. First of all it's up to Revision "W" not E. Second in order to fully comply with this SB, you need to split the case. I doubt that you've been splitting the case half way to TBO. That could cause you big headaches if the Feds read that!

What's your issue with the Seminole props?

Thandie's picture


I apologize for not being more clear. We are not doing Lycoming SB 240E, but Mccauley SB 240, the dye pen inspection. I am not sure but I believe the prop is balanced by the facility we use at that time.

The Seminole props in our fleet keep loosening the low pitch stop retaining nuts. The last one here we didn't catch until a pilot found the nut on the ground under the AC. It had been operating this was for a couple hours and wore out the prop cylinder and spinner, costing around $3000 in repairs. It's a two piece spinner and Hartzell does not have a one piece options for it. We've had a rep come out and look at our installs and he wasn't able to determine a cause for us. We are pushing for a single piece prop, but that's going to be slow in coming. What I can't understand is why this is now happening. We never had issues with this before. They are torqued and safetied, and still for some reason work loose with the safety still attached. I thought maybe they were negative because its a LH rotating prop and should be reverse threaded, but they aren't and were safetied correctly given the threading. I am recommending to the operator we do more frequent inspections until we figure out why they are working loose. I'm curious if it's just us, or if other people are experiencing the same.

Dave Zavoina (not verified)
Anonymous's picture

Have you ever checked the valve guide wear, Lyc SB 388?

bob.pasch's picture

Roger on McCauley SB. So you're actually complying with AD 2010-04-05, correct?

You're still not too clear on the balancing either, my friend. Are there balance weights on the spinner [or elsewhere]? Is the a decal indicating balancing? Is there a LOG BOOK entry in one or the other log books? If not, you need to eliminate one source of trouble before you move on to the sublime.

Let's say for argument's sake that there's no external indication of having it balanced [which is possible it didn't need weight], why don't you ask them if they have balancing equipment?

As for your Seminoles, are you seeing any other discrepancies like busted baffles, oil cooler brackets, spinners/bulkheads, alternator brackets, or premature component failures like vacuum pump or mags?

You're probably doing 25hr oil changes [or at least additives] so inspect them when you do. But if it were me, I'd hand the instructors a screw driver and have them pop the spinner cap every flight until you find something definitive...

PT6Guy (not verified)
Anonymous's picture

On the older 172's (and pretty much every other aircraft out there), if the prop was installed in the wrong position, you may get a vibration due to the reaction between the prop and the crank as it spun. I would double check that since the ring gear (and I assume the prop) were off.

Scott (not verified)
Anonymous's picture

Check these bulletins on Lycomings website > Support > Technical Publications:
Lycoming SI 1497A Engine Procedures for Flight Training Operations.

Lycoming SL L264A Engine Start Problems Due to Drift in Magneto-to-Engine Timing.

Any servo that has been contaminated by excess fuel may cause calibration issues and lead to rough running operation. This is more evident with training aircraft because they spend more time idling on the ground but any servo that is getting contaminated may show these symptoms. Excess fuel at idle may coat the induction pipes and on shutdown will drain out the lowest part of the induction system which on updraft induction systems will be the carburetor or fuel servo. This fuel/dye/oil contamination may get into the orifice between the air diaphragm sections of the servo and cause a rough running engine. In that case the servo has to come apart and be cleaned and calibrated due to the disassembly.

Changing the servo out may work for a while, but if the root cause of the issue isn't addressed then the replacement unit will probably get contaminated and cause the same symptoms.

Slick mags had a timing drift issue that seems to have been resolved. The timing would usually drift in the advance direction. The Champion SL requires that any change in mag timing angle be recorded in the engine logbook. If the timing drift is more than 4 degrees in one change then the mags need to be removed, investigated and corrected. If the timing change is found to be 5 degrees since original install (example: a 2 degree advance at one inspection and a 3 degree advance at a following inspection) then the mags need to be removed, investigated and corrected.

In my experience the mags will run fine at idle and full RPM but the efficiency of the mags comes into question. I've had many customers complain of hard starting issues and when the mags have been sent to me I've found the timing to be almost at magnetic neutral instead of the regular timing position of the mags. Changing the points to the latest design and resetting the internal timing of the mags fixes the magneto issues.

Hope this helps out anyone with these issues!
Scott Norsworthy
Pro Aero Aviation

bob.pasch's picture

Excellent idea!

Narendra (not verified)
Anonymous's picture

Have this problem too, found that induction door trend to open ( weak spring ) and the heat air from muffler enter the induction system

Generic Reader (not verified)
Anonymous's picture

I am NOT a certificated A&P, just a quiet gasoline engine engineer with 30+ years of experience in mall air craft engines. I Do Not Know! my very best guess it that this is a balance (prop or other parts?) issue, but without benching the entire engine and prop, or sending the works to a higher level facility, you'll probably never know. I hope that ALL events are logged in the MX log book, by every pilot and instructor who flies the bird. If I was a student - or instructor at your client's facility, I would NOT accept that airplane for any purpose. Something is wrong and even after multiple 'fixes,' the problem apparently remains.
I'd re-inspect the new parts, yet again. If you are not able to do a complete MOH, send the entire engine - with prop still mounted, to a known good rebuild shop. (That will be expensive, but still far less than a NEW engine.) Sorry, but as is, I would not fly, ride in or instruct in that airplane. I wish I could be of more help, but sight unseen, I refuse to guess beyond what a re-inspection may show. Measure and test everything, again and good luck!
When you are finished, PLEASE tell us what - if anything - was finally found. I still think a prop mounting error at some point, but the symptoms are far from perfect. -C.

Scott Norsworthy (not verified)
Anonymous's picture

Was the original complaint after any major work to the engine? Another thing that came to mind was the crankshaft prop bushing locations. If the prop is mounted incorrectly on the crankshaft due to improper bushing installation then you could have issues there too.
Have a look at Lycoming's S.I. 1098J Propeller Flange Bushing Location for your engine model to inspect the bushing installation when you have the prop off.

jelecroy's picture

If I was getting report of excessive vibration on an engine right after changing the ring gear, the first thing I'd check (because it's easy) is prop tracking. If the prop went on the flange out of plane that will definitely cause vibration and performance loss. Next check the propeller clocking on the crank. I think the IO-360 has a preferred clocking for a 2-blade prop. Unsteady RPM at full throttle points to a problem with either spark or fuel distribution. If the mag timing has changed as much as five degrees I'd do an inspection of the mags, looking for a bad bearing on the mag that allows points timing to vary. I'd also give a hard look at the mag drive gears in the accessory case, in case one of the gears have spalled teeth that allow timing to hunt around.

Jerry LeCroy
based KMDQ

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