adverseyaw

IO-360 misfires at idle and low MP

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Hi everyone, I'm trying to track down an occasional stumble I'm seeing at idle and through the bottom half of the power band. The engine, an IO-360-A3B6D in an M20J, runs well at full power.

Symptoms: every few seconds, a cylinder will fail to fire (or fires softly). The engine doesn't afterfire/backfire.

 

In trying to track this down, I had some fuel injectors get clogged due to debris in the fuel bladders. That work is done but the misfire remains.

So far I've:

  • Cleaned or replaced pretty much all of the fuel system (bladders, boost pump, mechanical pump, fuel servo, flow divider, all fuel injectors, all screens). It's clean now, no debris.
  • Pressurized the induction system, no leaks
  • Timed the engine to 20 BTDC
  • Pulled the plugs a few times, they're clear (not sure if my A&P gapped them)
  • Done about 20 runups with pretty much every combination of MP, mixture, and mags. So far, no clear culprit. It misfires at pretty much all mixture and mag settings, and all MP below about 20".

In maybe 10% of the runups I do, it will run like a top (no stumbles!) for a few minutes. So I know it's possible for it to idle cleanly.

This engine has less than 100 hours on it since overhaul so infant mortality of some part or another may be a factor.

Any ideas where to go from here? Thanks much

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Hi everyone, I'm trying to track down an occasional stumble I'm seeing at idle and through the bottom half of the power band. The engine, an IO-360-A3B6D in an M20J, runs well at full power.
Symptoms: every few seconds, a cylinder will fail to fire (or fires softly). The engine doesn't afterfire/backfire.
 
In trying to track this down, I had some fuel injectors get clogged due to debris in the fuel bladders. That work is done but the misfire remains.
So far I've:
  • Cleaned or replaced pretty much all of the fuel system (bladders, boost pump, mechanical pump, fuel servo, flow divider, all fuel injectors, all screens). It's clean now, no debris.
  • Pressurized the induction system, no leaks
  • Timed the engine to 20 BTDC
  • Pulled the plugs a few times, they're clear (not sure if my A&P gapped them)
  • Done about 20 runups with pretty much every combination of MP, mixture, and mags. So far, no clear culprit. It misfires at pretty much all mixture and mag settings, and all MP below about 20".
In maybe 10% of the runups I do, it will run like a top (no stumbles!) for a few minutes. So I know it's possible for it to idle cleanly.
This engine has less than 100 hours on it since overhaul so infant mortality of some part or another may be a factor.
Any ideas where to go from here? Thanks much

In my opinion the injection is primitive and at low flow rate there is little atomization. My after Flight warmed up, does the same. If this engine had a high pressure with electronic injection would run much smoother. I’ve listened to many Mooney’s and seems common.


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The Usual response...  :)

Got any engine monitor details to share?

Best regards,

-a-

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1 minute ago, Mjknick@gmail.com said:


In my opinion the injection is primitive and at low flow rate there is little atomization. My after Flight warmed up, does the same. If this engine had a high pressure with electronic injection would run much smoother. I’ve listened to many Mooney’s and seems common.


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I could be talked into that. That said, I've never experienced it on carbureted aircraft I've flown, nor the M20J I got my complex endorsement in.

Do others encounter this as well?

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Somebody posted a similar question last night... about 24 hours ago.

Best regards,

-a-

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Just now, carusoam said:

The Usual response...  :)

Got any engine monitor details to share?

Best regards,

-a-

Back when I was clogging injectors with debris, I was getting EGTs to peak 100-200 degrees on affected cylinders. I've only run it up twice since then, but no notable EGT changes since then.

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2 minutes ago, carusoam said:

Somebody posted a similar question last night... about 24 hours ago.

Best regards,

-a-

This one, I take it? Am digging through now, thanks much.

 

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The value in sharing the data...

Other people are going to see things not operating the way you may expect.

They read graphs, like reading a book...

Often they can be mechanics, or are so technically ept, they do it for a living....

Some might be engineers, others just know a lot about your engine.

Best regards,

-a-

 

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4 minutes ago, adverseyaw said:

This one, I take it? Am digging through now, thanks much.

 

That looks like 8hrs ago...

Somebody was having a challenge at 2400 rpm with a backfire after a 2hr flight... it was an F or a J ???  Not an Acclaim.

Try the search function...

I’ll try and get back to this, but it’s getting late on the East Coast.

Best regards,

-a-

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Just now, carusoam said:

That looks like 8hrs ago...

Somebody was having a challenge at 2400 rpm with a backfire after a 2hr flight...

Try the search function...

I’ll try and get back to this, but it’s getting late on the East Coast.

Best regards,

-a-

Yeah, I did a bunch of searching before I posted. Found some threads but nothing that stuck. Most of the ideas were things I've tried.

EDM data attached. My EGT4 connection is toast, so unfortunately that data is not useful. Blue line is fuel flow and green line is RPM.

image.thumb.png.c7b31979cbc9f84513d6fbca1db4a913.png

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Your engine should be timed to 25 BTDC. What kind of plugs are you using? If they are Champion massives, I'd strongly suggest switching to Tempest...preferably fine wire, but if budget is a concern then their massives will be an improvement.

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3 minutes ago, KSMooniac said:

Your engine should be timed to 25 BTDC. What kind of plugs are you using? If they are Champion massives, I'd strongly suggest switching to Tempest...preferably fine wire, but if budget is a concern then their massives will be an improvement.

Sent from my LG-US996 using Tapatalk
 

Went to 20 BTDC per Lycoming SB for reducing CHTs.

Those are good suggestions. I actually don't know what plugs are on the plane. I'll check soon. Thanks!

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You'll really want 25 degrees! @jetdriven has a thread somewhere comparing the performance differences between the two.

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A usual flight contains some interesting things...

Start-up, run-up, climb, Cruise....

If a plug isn’t working, it shows immediately in the data...

 

So many things are ocuring in this brief piece of data, power, FF and related rpm, when did the oddity occur?  

Is it in this collection of data? Can you point out the time when it occurs?

There is a service called Savvy that can be helpful.  Check out their site...

Best regards,

-a-

 

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3 minutes ago, KSMooniac said:

You'll really want 25 degrees! @jetdriven has a thread somewhere comparing the performance differences between the two.

Sent from my LG-US996 using Tapatalk
 

I'm running 10:1 compression pistons and do a lot of work to keep the CHTs in check. Unless timing 20 BTDC will adversely affect idle, it's probably a good discussion for another day :-)

(Unfortunately the pistons aren't a variable I can change, and I haven't heard of more issues at idle, so for now I'm exploring what I can do with the other facilities on the engine.)

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That would be an important piece of info when discussing your challenge...  :)

There is a thread by somebody trying to learn more about the 10:1 piston STC, if you want to lend a hand there...

Best regards,

-a-

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Just now, carusoam said:

So many things are ocuring in this brief piece of data, power, FF and related rpm, when did the oddity occur?  

Is it in this collection of data? Can you point out the time when it occurs?

There is a service called Savvy that can be helpful.  Check out their site...

Best regards,

-a-

 

The problem occurred within a few seconds of engine start, and continued through the entire data log. The log is about 30 minutes long and the sputter probably happened a few hundred times, most noticeable at idle and at RPMs below 1700 or so.

This is off of the Savvy website so could ask them. Getting details on the plugs like you suggested is probably a good first step though.

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Looking at the rpm line...

1,000 to 1700 a few times... what is that? Or what is causing that?

If the engine is warm and running steady at idle for a minute....the ignition skips?

Got any data during that time?

if something skips, the EGT usually catches it.

It will be subtle, and can be masked by other things like moving the throttle around.

It may take some additional effort to catch a good data set that doesn’t Accidently mask what you are looking for.

Best regards,

-a-

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I can't tell from my phone, but if your data recording interval is set for 6 seconds it might not catch your troubles...that is worth checking and correcting.

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This is the midnight thread from last night.... be sure to start at the top, it looks like it may start where I left off reading...

 

-a-

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Since this is a relatively new engine, check everything is tight: fuel injectors, intake tubes, ignition wires...

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No signs of misfire in the data, but the 6 sec default sampling rate is too slow to give much chance of seeing any. The sampling rate needs to be updated to its fastest rate of 1-2 sec to provide diagnostic value. Ground runs aren't very informative but we should be able to get some better data by isolating the mags for a good 30 sec each and see if that shows any misfire. But nothing in this data points at ignition yet; nor is it conclusive to eliminate it.

Its kinda hard to overlook EGT4 is cold. We know the cyl isn't really dead because of CHT4 but we have nothing to detect a mixture or ignition issue without EGT.

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9 hours ago, adverseyaw said:

Went to 20 BTDC per Lycoming SB for reducing CHTs.

Those are good suggestions. I actually don't know what plugs are on the plane. I'll check soon. Thanks!

If your engine is in fact the A3B6D, then it’s not approved for 20 degrees of timing.  This is widely-known information. 

 

820766A1-4C8F-4318-8E1B-823E32F61948.jpeg

Edited by jetdriven

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