Doug Schumacher

Help with an engine problem

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I'm new to this MooneySpace and this is my first post.

I share a 1981 M20J with 2 other pilots.  Starting in July 2018 we have been trying to find the cause of an intermittent hiccup with the engine.  Some back ground about the plane; about 1500 hrs on engine, 150 hrs on prop, 150 on Mags, and A&P has checked: fuel injector flows,  cleaned plugs, and rebuilt fuel servo.

Problem:  You can fly for a long time (sometimes 2, 5, 10 hours) without any problem, and then all of a sudden you can feel a slight vibration and you can see the RPM fluctuate +/-10 RPM (sometimes a bit more and sometimes less).  This only happens for 1-2 seconds and then it goes away completely, and then it will usually come back again many minutes later.   We have a engine monitor and we were able to videotaped it happening.  Upon close examination you can see cylinder #4 does change temperature several degrees during the hiccup, so it appears cyl-4 is miss-firing for 1-2 seconds.

Any thoughts of suggestions would be great, as we are spending $ without results.    

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Welcome aboard Doug.

You have come to the right place.

Can you start with posting engine monitor graphs?  Which monitor do you have?

Are you familiar with Savvy.com?  Helpful for uploading and sharing monitor data...

Screen shots work well too...

Loading a video around here is often best done using YouTube... then link.

Get started when able...

PP thoughts only, not a mechanic...

Best regards,

-a-

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Follow your plug wires from each magneto to each plug, looking for chafing or worn spots--there may be intermittent grounding of a wire. 

Is it always the same cylinder? Check the plug--lead fouling, high resistance, proper gap.

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1 hour ago, Doug Schumacher said:

I'm new to this MooneySpace and this is my first post.

I share a 1981 M20J with 2 other pilots.  Starting in July 2018 we have been trying to find the cause of an intermittent hiccup with the engine.  Some back ground about the plane; about 1500 hrs on engine, 150 hrs on prop, 150 on Mags, and A&P has checked: fuel injector flows,  cleaned plugs, and rebuilt fuel servo.

Problem:  You can fly for a long time (sometimes 2, 5, 10 hours) without any problem, and then all of a sudden you can feel a slight vibration and you can see the RPM fluctuate +/-10 RPM (sometimes a bit more and sometimes less).  This only happens for 1-2 seconds and then it goes away completely, and then it will usually come back again many minutes later.   We have a engine monitor and we were able to videotaped it happening.  Upon close examination you can see cylinder #4 does change temperature several degrees during the hiccup, so it appears cyl-4 is miss-firing for 1-2 seconds.

Any thoughts of suggestions would be great, as we are spending $ without results.    

What is several degrees? A significant EGT increase would lead me to believe that the cylinder in question is going from dual ignition to single ignition during the “rough phase”.  Hank and Rich have both giving you good advice above. Look forward to learning what you find.

Edited by Shadrach

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Possible grounding of mags due to bad ignition switch

Possible mag failure especially if slick mags

Ignition harness

Bad P leads

clogged injector or bad flow divider  - Trash in fuel system

 

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24 minutes ago, David Lloyd said:

Intermittent stuck valve.

That would tend to occur more in a cold engine, but it still could be of course.

It could also be a weak spark from one of the magnetos, bench testing or in situ testing might be an early thing to test

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Had #4 stick a couple times for several seconds in flight before it stuck one morning at startup.  Found it then.  Had to drive the valve out of the guide with a 7/16" wooden dowel and hammer.  Like knee replacement surgery, best not to watch.

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Can you download from the engine monitor and post data egt, cht, ff, etc when it’s happening?

 

 I had a valve starting to stick last year and noticed it airborne and with the engine monitor before I noticed it on startups.  Maybe that was just me not noticing...  Egt on that cylinder went up about 100 degrees above where it was set in cruise.  When that happens, could also be a mag/spark issue on that cylinder causing single plug to fire as in a mag check.

Edited by Ragsf15e
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Good advice above.  A few other cheap checks: move both plugs to another cylinder; have your mechanic borescope the valves while the plugs are out, one step further would be to do the Lycoming valve wobble test.

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Thank you all for your responses.  The engine monitor does not collect RPM, but we have a digital tachometer.  So I was able to take a video of the engine monitor and the tachometer and then pause the video each second and record the data.  Please see attached graph.  You can see Cyl-4 looks different then 1, 2 and 3 during the vibration and RPM hiccup, and note that the vibration and RPM hiccup only lasts 1-2 seconds.  The hardest part about this problem is that it only happens for 1-2 seconds (never for >3 seconds), and sometimes you have to fly for hours to see it happen.   Also I assume that the tachometer responds a few seconds faster than the EGT temp probes, so the temps should probably be shifted to the right a few seconds. 

Data from July-2018 video.pdf

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Due to the relatively consistent failure period, it “Feels” electronic to me which I think would most likely be in the mag.  The issue could be plug, plug wires or fuel related in that order but I feel that Fuel is s longshot, again due to the consistent failure period.  I like the suggestion of rotating (and of course inspecting) the plugs to see if the EGT issue follows the plugs.  If the dynamic cylinder stays the same, I would IRAN the mags and double check the wires for that cylinder.

I think a sticking valve or a plugged injector in a 4 cyl would yield stronger symptoms.

 

 

 

Edited by 81X

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Start with the cheapest and most common, spark plugs. I have seen numerous posts here and beechtalk where an owner spent lots of money chasing similar gremlins only to have it be the spark plugs. If you don’t already have a set I would suggest tempest fine wire plugs. If that doesn’t do it mags and the spark plug wire harness are next.

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24 minutes ago, larryb said:

Start with the cheapest and most common, spark plugs. I have seen numerous posts here and beechtalk where an owner spent lots of money chasing similar gremlins only to have it be the spark plugs. If you don’t already have a set I would suggest tempest fine wire plugs. If that doesn’t do it mags and the spark plug wire harness are next.

Plug harness is much le$$ expen$ive than magneto IRAN, install and timing . . . And he can do it himself with A&P supervision.

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Well.... that’s a first!  Hand manipulating data and graphing using excel... :)

We are looking at 40 seconds of raw EGT data, in cruise...

Things that are of interest, usually...

  • air
  • gas
  • spark
  • Ignition timing
  • Other quirky mechanical things

 

Just looking at the rpm data...  

  • most analog instruments would be unable to show a 10rpm swing...
  • most digital instruments would measure a 1 rpm swing and then round it to the nearest 10...
  • something caused the prop rpm to drift slowly down, an over correction occurs, then the prop rpm begins drifting slowly down again... it isn’t a one time blip... it seems it may be going on longer than the 40 second time frame.

Looking at EGT...

  • raw EGT data is so dependent on probe and location, not much can be discerned from the raw data...
  • Peak data is usually what gets used to tell a lot about what is happening with fuel distribution in the engine.
  • having one EGT being higher than the others doesn’t say enough.
  • Lean to peak, share some data, FF is greatly helpful for each peak....

Can you share the actual video?

Do you hear the prop changing rpm while this happens? Or at any time during the rpm drift....during flight?

It would be a whole lot better to post engine monitor graphs and state a few things you know about the monitor itself...

An actual video shared with the sound of what is happening would be good too...

A longer collection of data showing the rpm drift could be telling something as well...

How long has it been since the prop governor has seen any work?

Basically, you have posted some data that indicates something is not working as expected... but not enough data to indicate what is not right.

What people do in this case is collect engine monitor data from start-up to shut down, including a well orchestrated run-up...

If you can post that kind of detail... somebody can tell you what spark plug isn’t working out of eight. What mag isn’t working out of two.

If a valve is sticking, a JPI EGT graph shows an indicative pattern that is pretty recognizable...

Without the data, it is more like pin the tail on the donkey...

What monitor do you have?

What is the rate of data collection set point?

Do you want to post the data?

Do you need help with that?

PP thoughts only, trying to be of assistance, not a mechanic...

Best regards,

-a-

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Those are pretty small changes in EGT.  I am not a mechanic, and I have much less experience with these matters than others, but I'll relate a similar situation we had with our engine.

When we first got the plane and I started cruising LOP, I would notice that fairly frequently the engine would feel like it skipped a beat.  As the flight progressed it became less and less common.  As time passed and more of my partners also starting running LOP, the problem disappeared.

I attribute it to carbon build up, most likely on the plugs, which were massive Champion when we got it.  Since then we have also switch to Tempest fine wire and almost never have the plane skip a beat.  If it does, it is usually after a flight by one of my partners who is not as religious about LOP as I am.

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Thanks for the additional comments.  Prop and Mags were rebuilt at last annual (about 100 hours ago).  I've posted the video to youtube and the link is below.  The video is under-whelming to watch, but when you are in the plane in cruise and the problem happens you know immediately something "wrong" happened.  You can feel a slight vibration as if the engine missed a beat.  But it goes back to perfect in 2 sec or less.

It took me a long time to notice that the RPM was changing, as all I knew for sure was that it felt like the engine pulsated.  Normally the RPM holds rock solid, maybe +/- 1 rpm.  So in this video before and after the hiccup the RPM is still not as solid as usual.  In other words I think this video caught a big hiccup but there were still smaller ones (or settling down) before and after the big one.  Before I graphed the data I could not really see any difference between the cylinder exhaust temps.  If the hiccup lasted longer than 2 seconds (like 10 sec) I think the temperatures would vary alot more.

The first time I noticed it I was flying from Laconia NH to York PA and everything was fine for a hour or so and all of a sudden there was a slight vibration for 1-2 sec.  I could not see anything out of place and it did not happen again for about another hour.   Of coarse I thought I was imagining things.  I then had my phone ready to record if it happened again, and this time it happened a few times with several minutes between each occurrence.  I decided to stay overnight in PA and have an A&P have a look.  We spent $1,200 with the A&P at York.  Bore scope, compressions, replaced intake tube o-ring gaskets, and adjusted idle mixture (he may have checked a few other items).  The entire flight home and the next several flights the problem did not show up.  But it happened again in September.   We checked plugs and fuel injectors and test flew, and it happened a few hours later.   We sent out the fuel servo for rebuild and reinstalled a few weeks ago.  We had a few successful flights, but then it happened to me again in 12/8/18.

https://youtu.be/XkIehU2X-7U

 

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We do have the fine spark plug wires.  We have the ability to download the data from the engine monitor but we did not think it would be helpful where the problem only happens a most a few times for 1-2 sec over a 3 hour flight, and we do not know where in the data the problem happened as the RPM is not one of the data streams.

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Doug,

Did you know that you have one of the most modern engine monitors available?

It can collect tons of data.  And display it all graphically!

Using the graphic nature of data, Engineers and mechanics, lawyers and doctors, around here can review hours of data in about a minute and a half.  Anything that causes enough vibration that you stand ready with a video camera has got to leave a signature, somewhere...

Have you been to the Savvy site yet? 

Have you set the data collection rate to 2 seconds or less?

Any chance of getting the data out of this monitor? It may require a PC, a cable, and a download device... all available from JPI if you need to...

 

 

People here can see things that you might not be able to.

If you could see it, you would be posting the data with a note saying... hey guys, look what I noticed in my flight’s JPI data, check this out... :)

What did you learn with the $1200 expense?

 

 

When problem solving engine problems... a few things are very important...

  • MP
  • RPM
  • FF
  • EGTs
  • CHTs

The MP is incredibly stable while it is in the video (briefly)... the rpm looks like it could have a loose sensor... some digital rpm instruments put an averaging mechanism in their program.  This tach doesn’t.  You got some interesting data detail when you graphed it, that I can’t see watching the numbers flip by...in real time.

Putting all this data together you will be able to separate out the real issue from a bad sensor or crummy program issue...

Can you give better detail on the few seconds of vibration you were sensing?

You might want to start with the normal psychology of the engine before focusing on the abnormal psych.

Run through what is working first. Then find the data where the engine does something odd.

if the engine misses producing power, the prop is going to make adjustments, trying to keep the rpm stable.

The mechanical governor will be challenged to keep up... with an engine misfire... But a JPI will probably see it...

I think you are focusing on the prop rpm, but the prop might just be following a bigger problem upstream.

If the prop is doing something odd, while MP and FF are stable... this is the sign of a governor mis behaving... it has a pair of fly weights that run the govenor’s oil valve up and down... if that valve is sticking because of wear it can cause rpm issues... check your logs to see when the gov was touched last...

How does your GAMI spread look? (The next question that usually gets asked)

My experience of having a strong vibration... It was loud, we new something was wrong, the vibration was tremendous, the exhaust valve was crashing into the piston on departure... it was a land now kind of moment.  As seconds elapsed, our land straight ahead plan was extended to land on the next available runway... I had no engine monitor to help determine what was happening....

PP thoughts only.  One of these days we are going to discuss vibration analysis.... an interesting machine Maintenance methodology... I’m not a mechanic...

Best regards,

-a-

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Carusoam, thanks for all the feed back.  This not a strong vibration.  I will see about downloading the data, we did it once but not sure we still have this July flight still on the device.  I will look into the Savvy site.  There is no doubt in our minds that the engine hiccup/vibration/misfire is trying to be compensated by the prop governor and that is why we see the rpm vary.

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On 12/10/2018 at 1:36 PM, David Lloyd said:

Intermittent stuck valve.

 

On 12/10/2018 at 2:00 PM, jaylw314 said:

That would tend to occur more in a cold engine, but it still could be of course.

It could also be a weak spark from one of the magnetos, bench testing or in situ testing might be an early thing to test

In this case... Bore-scope with the plugs removed. I have this:

https://www.aircraftspruce.com/catalog/topages/vividia12-04891.php?gclid=Cj0KCQiA3b3gBRDAARIsAL6D-N-WIsb8knsxBpa9qqCItmylgp1CZz2LTXMtMxfiZiOCODA-QVS7ergaAtYsEALw_wcB 

I take pictures of my valves at annual or any time the opportunity presents itself. The endoscope sees a ton of action among my A&P friends, so at least they are getting my monies worth...

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