New issue with new Lycoming engine--IO-360. . .

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Robster

Robster

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May 12, 2004
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Cumberland County, Pennsylvania
Last Friday while doing a run-up on the new engine prior to take-off, I advanced throttle from 1500 to 2100 RPMs for the mag check. When it hit 2000 RPMs, the RPMs did not increase with a corresponding forward movement of the throttle. . .in fact, the engine began to stumble and lost 100-200 RPMs and then jumped to 2300 RPMs all at once. It did this twice and then ran fine.

Since then my mechanics have been able to consistently duplicate the behavior on a cold engine, but as soon as it warms up it runs normally. The mechanics have checked the fuel filters (two), removed and cleaned the injectors, checked the airbox, prop governor, mags, alternate air control, spark plugs. . .and called Lycoming, who indicated it may be a stuck valve and recommended an oil additive to lube the upper cylinders (similar to Marvel Mystery Oil).

Any A&Ps out there got any ideas? We'll be test-running it again tomorrow morning to see if the additive did anything. . .the engine has over 100 hrs. on it since installation and has run fine up to this point.
 
could be sticky valves. Cold they stick, hot they dont.
They wont slide easily, at idle they dont have to move so fast.
I would think that once they are sliding the friction to valve stem and guide would heat them up and would stop sticking quickly if they are moving. That would happen in less than 4 minutes.
So does it do that stumble till what temp for the engine? How long to warm up before it is fine?
 
I'm certain you tried it individually on the Left and Right mags. Certainly, one mag firing way out of time has been responsible for all sorts of weirdness.

But, that almost sounds like a lean stumble. My IO-360 will do something similar if I taxi with the mixture leaned, then try to quickly accelerate, to cross a runway, for example.

I recall your engines were recently overhauled. Did you also overhaul the fuel servo (the throttle body) and the spider?
 
A stuck valve often results in very rough running, regardless of throttle position. Your engine seems a bit young for "morning sickness". But, you never know...

What oil are you using? I'm using Aeroshell 15W-50 with good results. I'm also using Camguard. While I'm not a "true believer" with regard to Camguard, I do borescope my engines and there is no question that corrosion is significantly reduced now. Of course, I've also changed my operating habits. If I'm not flying, I don't run up the engines anymore. I simply let them sit.
 
Originally Posted By: Cujet
I'm certain you tried it individually on the Left and Right mags. Certainly, one mag firing way out of time has been responsible for all sorts of weirdness.

But, that almost sounds like a lean stumble. My IO-360 will do something similar if I taxi with the mixture leaned, then try to quickly accelerate, to cross a runway, for example.

I recall your engines were recently overhauled. Did you also overhaul the fuel servo (the throttle body) and the spider?


Yeah. . .mags were checked. . very minimal drops and smooth running on each alone. The engine was a factory reman. . .came with new fuel distribution. Thanks for the input though!
 
Originally Posted By: Cujet
A stuck valve often results in very rough running, regardless of throttle position. Your engine seems a bit young for "morning sickness". But, you never know...

What oil are you using? I'm using Aeroshell 15W-50 with good results. I'm also using Camguard. While I'm not a "true believer" with regard to Camguard, I do borescope my engines and there is no question that corrosion is significantly reduced now. Of course, I've also changed my operating habits. If I'm not flying, I don't run up the engines anymore. I simply let them sit.


We use AeroShell 15w-50. . have used that oil for decades without issues. Prior to now we've never used any oil additives. This plane gets used a LOT, so doesn't sit around much. For what it's worth, the new engine is the newer roller tappet design. . my first experience with that. . .not sure if it's relevant or not. We'll continue the troubleshooting today and will let you know what comes of it. . .
 
Originally Posted By: Cujet
Does the electric fuel pump make any difference?


Nope. . .ruled that out early on. . . called the Lycoming tech rep who advised the mechanic to test the injector spray patterns, which were normal. Lycoming also seems to think it could be prop governor-related, but I can't quite wrap my head around that scenario. . .I think that would be a long shot.
 
UPDATE: We're still troubleshooting this issue. Lycoming authorized our mechanics to inspect the exhaust valves / guides / lifters. We found some fouling in one of the guides and two of the lifters had some slight scuff marks on them. We replaced the lifters, honed the guides and put 'er back together. . . .and continued to have the same issue. Tomorrow the prop-governor and prop will be removed and inspected at the prop shop. . .their findings should be interesting. If they don't find anything, I'm outta ideas. . .In 30+ years of flying, I've never encountered this particular issue. . .
 
What's tripping me up in your description is that you said the engine "stumbles". It's natural to assume a misfire is responsible for RPM loss.

However, if I set the prop control at 2300 RPM on my IO-360A1B6 powered Cessna, then advance the throttle to full, I momentarily get more than 2300RPM, then the prop gov drags it down to the setting I've chosen.

Last week, I did a prop balance at 2300RPM and this was my experience every time I advanced the throttle.

Just guessing here: Is the engine actually misfiring? or are you simply asking a very cold engine to produce power and hearing characteristic "lean" operation unstable combustion due to the cold cylinders?

Remember, the mag check is done at a particular, and modest, RPM for a reason. It involves fairly high cylinder pressures. So, it's a good RPM to check for a weak spark, as the higher the cylinder pressure, the more difficult it is to fire a plug. Plus, the mag is not at full RPM, so it's generated voltage is lower, possibly exposing weakness. Plus, it helps get your engine up to operating temperature without too much stress on a cold engine.

I sure hope it's just your prop gov.

Another thought: During a recent flight of an experimental (with a new Lyc IO-540 parallel valve) the engine would misfire on downwind every time. Otherwise it ran fine. That misfire was not just on one cylinder, and it seemed random. Ground mag checks showed nothing. In air mag checks were inclusive, but did show a little something, with more even EGT's when switching.

On my suggestion, we installed 12 Champion fine wire plugs. Problem solved, 100%. The plugs were probably internally cracked from mis-handling, somewhere along the line. The engine was not shipped with the plugs. The owner installed them, and the owner won't admit that he dropped them. We threw out 12 plugs with less than 10 hours on them. You never know...
 
Years back I had an occasional rough engine. In this case it was a bad plug wire. The shop was able to find it. I did have a cylinder fail years ago too, it had to be replaced.

If the fuel supply is fine I'd be looking over the ignition system, mags, plug wires & plugs.

I do let the engine warm up for a bit before any mag checks. Just the normal taxi to the run-up area should do it though.
 
Originally Posted By: Cujet
What's tripping me up in your description is that you said the engine "stumbles". It's natural to assume a misfire is responsible for RPM loss.

However, if I set the prop control at 2300 RPM on my IO-360A1B6 powered Cessna, then advance the throttle to full, I momentarily get more than 2300RPM, then the prop gov drags it down to the setting I've chosen.

Last week, I did a prop balance at 2300RPM and this was my experience every time I advanced the throttle.

Just guessing here: Is the engine actually misfiring? or are you simply asking a very cold engine to produce power and hearing characteristic "lean" operation unstable combustion due to the cold cylinders?

Remember, the mag check is done at a particular, and modest, RPM for a reason. It involves fairly high cylinder pressures. So, it's a good RPM to check for a weak spark, as the higher the cylinder pressure, the more difficult it is to fire a plug. Plus, the mag is not at full RPM, so it's generated voltage is lower, possibly exposing weakness. Plus, it helps get your engine up to operating temperature without too much stress on a cold engine.

I sure hope it's just your prop gov.

Another thought: During a recent flight of an experimental (with a new Lyc IO-540 parallel valve) the engine would misfire on downwind every time. Otherwise it ran fine. That misfire was not just on one cylinder, and it seemed random. Ground mag checks showed nothing. In air mag checks were inclusive, but did show a little something, with more even EGT's when switching.

On my suggestion, we installed 12 Champion fine wire plugs. Problem solved, 100%. The plugs were probably internally cracked from mis-handling, somewhere along the line. The engine was not shipped with the plugs. The owner installed them, and the owner won't admit that he dropped them. We threw out 12 plugs with less than 10 hours on them. You never know...



Thanks for the info. . .the stumble that I've observed I believe is due to the addition of throttle w/out a corresponding increase in RPM (overly rich for the RPM setting), although I'm just guessing. When the RPMs hang at the 2000 mark, I can increase throttle and the MP increases as you'd expect. . . .although I was skeptical at first, I'm thinking this could be a prop / governor issue. . .I've just never had one "hang" like that at a particular RPM. Should know today. . will update later if I have any answers.
 
For many years now, there have been certain prop gov's that have a "hang up" with the "new improved" parts. Maybe you have one of those?
 
UPDATE: The prop and governor checked out within spec. Some small parts in the hub were replaced because they showed some wear, but the prop mechanic (correctly) noted that they wouldn't have impacted performance. We put the prop and governor back on and the problem persists.

Lycoming suggested that we next look at the fuel servo, as I guess they had a batch of bad ones during our engine production date. Mechanic is swapping left to right as I type this to see if the problem follows the servo to the other engine. Stay tuned. . .
 
Update

It turns out that the fuel servo was bad. We should have a new unit on it today. Apparently there's a SB out on some Precision fuel servos (I can get that info if anyone's interested) that may include the unit on our aircraft. We've been troubleshooting this issue for weeks, but I'm glad we stayed on it. . . a bad fuel servo can ruin your day. . . .

Lycoming is covering the troubleshooting labor and most of the parts under warranty, thankfully! Oddly enough, Lycoming won't / can't sell a fuel servo off the shelf directly without testing it on the EXACT model engine that it's being used on, which pretty much precludes getting a new one from them in a timely manner. They authorized us to get a rebuilt unit from a distributor though. Found that interesting. . .must be a liability thing?
 
Well, I sure am glad you found the problem. It would be nice to know the exact internal problem.

My servo (RSA5) has about 350 hours on it. I swear it's operation is "funky". I can watch the output fuel pressure slowly rise and fall about 0.5 psi in cruise. Ugh, gremlins.
 
Originally Posted By: Cujet
Well, I sure am glad you found the problem. It would be nice to know the exact internal problem.

My servo (RSA5) has about 350 hours on it. I swear it's operation is "funky". I can watch the output fuel pressure slowly rise and fall about 0.5 psi in cruise. Ugh, gremlins.


CuJet. . .check yours ASAP. . .it's a diaphragm in the servo that is made by AvStar. . .yours could be affected. It seems like they're not too sure of the units affected because the lists seem to change. I will tell you that Lycoming made it CLEAR that they wanted our unit back for examination and we weren't specifically on that list of affected servos.

Here's a link to SB 596: http://www.lycoming.com/support/publications/service-bulletins/pdfs/SB596.pdf

Here's a link to the ensuing AD note: http://www.airweb.faa.gov/Regulatory_and...&Count=100#

Let us know if your ship is affected. . .just curious.

--Rob
 
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