Throttle to idle, small pops on final

[PierceM20F]

Hi everyone! I’ve got a 1967 M20F / Lycoming IO360-A1A with about 300 SMOH. Noticed on my past few flights that going to idle either on the downwind leg or final produces some small, consistently spaced out “pops”. I wouldn’t call them backfires as they are not that strong at all. Will usually only last a couple seconds or so. I also noticed some light oil streaks coming from the #2 cylinder part of the cowl. So I investigated. I found what looks to be some oil leaking somewhere near the intake area OR could be the valve cover gasket. The gaskets are cork from around 2014 (previous owner barely flew the plane). 
 

Thoughts on changing out the gaskets to silicone ones in order to solve the problem? Engine does not run rough, and passes idle check during a ground run up. I also did not see any glaring cracks in the intake manifold. I will note, the #2 cylinder is always my “hot” cylinder on climb out but all cylinder temps even out once in cruise. I also put a quart of oil in about every 10 hours so I know it’s not a huge leak. Added pic of new silicone gasket to show what I’d purchase. 
 

Thanks all! Happy flying!
 

 

[Andy95W]

1.) I’d be a lot more concerned by what appears to be an exhaust leak blowing on your #2 bottom spark plug, and the poor installation of the EGT and CHT wiring harness.  The exhaust leak would also explain your high CHT on #2 that you mentioned.

2.) Yes, the silicone gaskets will probably fix that oil leak.  However, the picture of the one you posted is for a parallel valve Lycoming, your M20F has angle head valves.  The picture below is the one you want, Aircraft Spruce is a much better vendor for most airplane owners:

https://www.aircraftspruce.com/catalog/eppages/valvegasket05-11181-6.php

3.) That popping at low power settings isn’t uncommon at idle power.  It is more fuel related than oil or exhaust.

Edited December 5, 2021 by Andy95W

[PierceM20F]

33 minutes ago, Andy95W said:

1.) I’d be a lot more concerned by what appears to be an exhaust leak blowing on your #2 bottom spark plug, and the poor installation of the EGT and CHT wiring harness.  The exhaust leak would also explain your high CHT on #2 that you mentioned.

2.) Yes, the silicone gaskets will probably fix that oil leak.  However, the picture of the one you posted is for a parallel valve Lycoming, your M20F has angle head valves.  The picture below is the one you want, Aircraft Spruce is a much better vendor for most airplane owners:

https://www.aircraftspruce.com/catalog/eppages/valvegasket05-11181-6.php

3.) That popping at low power settings isn’t uncommon at idle power.  It is more fuel related than oil or exhaust.

Thanks Andy! I did notice that there is a nut missing from one of the studs for the exhaust manifold. I’ll see if I can get a nut and tighten that back down. I’m thinking that’s why there is an exhaust leak in that spot. 
 

As far as the popping, that makes sense. I know when on the ground for a while on a hot day, the engine will “hiccup” slightly as in a drop in RPM for a half second or so due to fuel vaporization. Goes away when increasing MP. 
 

Looks like some new seals and a nut are in order!

[Guest]

I’d replace bot the intake flange gasket AEL71973 and the exhaust flange gasket 77611, new nuts STD1410 new lock washers MS35333-41.  When finished use a shop vacuum discharge hose to pressurize the exhaust system from the tailpipe to check for additional leaks.

Clarence

[PierceM20F]

27 minutes ago, M20Doc said:

I’d replace bot the intake flange gasket AEL71973 and the exhaust flange gasket 77611, new nuts STD1410 new lock washers MS35333-41.  When finished use a shop vacuum discharge hose to pressurize the exhaust system from the tailpipe to check for additional leaks.

Clarence

Hi Clarence. Thank you for the help! Is there an illustrated parts catalog in general for the engine? Both you and Andy got P/N information from somewhere! A Mooney IPC would also be handy I’m sure. 

[Ragsf15e]

5 minutes ago, PierceM20F said:

Hi Clarence. Thank you for the help! Is there an illustrated parts catalog in general for the engine? Both you and Andy got P/N information from somewhere! A Mooney IPC would also be handy I’m sure. 

There might be one in our downloads section, but you should be able to find a maintenance manual and parts catalog electronic copies online if not in the downloads section.

You definitely want to get a nut on that exhaust because you’re pouring hot exhaust out onto that plug.  Also, you’re pouring out carbon monoxide.  How sealed is your firewall?  Do you have a good detector? I lost an exhaust stud nut once and my detector was reading much higher than normal.

Finally, clarence called out your intake gasket as well because of all that blue stain on the intake riser.  It’s not uncommon there but it’s fuel residue.  A new gasket there will keep the intake sealed better.

 

[PierceM20F]

1 hour ago, Ragsf15e said:

There might be one in our downloads section, but you should be able to find a maintenance manual and parts catalog electronic copies online if not in the downloads section.

You definitely want to get a nut on that exhaust because you’re pouring hot exhaust out onto that plug.  Also, you’re pouring out carbon monoxide.  How sealed is your firewall?  Do you have a good detector? I lost an exhaust stud nut once and my detector was reading much higher than normal.

Finally, clarence called out your intake gasket as well because of all that blue stain on the intake riser.  It’s not uncommon there but it’s fuel residue.  A new gasket there will keep the intake sealed better.

 

I’ve got a basic detector on the panel. The firewall is sealed fairly well from what I can tell. Haven’t had any symptoms but then again I’ve been on relatively short flights recently. I’ll be ordering these parts ASAP! New owner so continuing to learn all there is about ownership and maintenance. 

[Ragsf15e]

17 minutes ago, PierceM20F said:

I’ve got a basic detector on the panel. The firewall is sealed fairly well from what I can tell. Haven’t had any symptoms but then again I’ve been on relatively short flights recently. I’ll be ordering these parts ASAP! New owner so continuing to learn all there is about ownership and maintenance. 

Yeah, there’s lots to sort out on “new” ones.  I also have an F, ‘68 model.  Still sorting stuff out 7 years in.  Up to you, but lots of us use Sensorcon CO detectors.  You can search MS for our discount code.  Very sensitive detector. Good piece of mind as CO is a silent killer.  You’ll find lots of good help on here.  Heck, Clarence who replied to you above owns a well respected Mooney Service Center and is extremely knowledgeable.  Just watch out, because he has a strange taste for much bigger engines than you see on the typical Mooney…

[Guest]

1 hour ago, PierceM20F said:

Hi Clarence. Thank you for the help! Is there an illustrated parts catalog in general for the engine? Both you and Andy got P/N information from somewhere! A Mooney IPC would also be handy I’m sure. 

As mentioned, a quick internet search should help you locate a manual.  After a few decades doing this, a few numbers just stick in my mind, however I can’t find my phone!

Clarence

[MikeOH]

Make sure you torque that exhaust nut properly.  I had the very same (#2 inside exhaust) come loose (I caught it before it fell off or caused the thermal damage/stains you have).  When I went to torque down the nut, I couldn't achieve the proper torque.  Turns out the threads in the cylinder head were stripped! Which is likely why the nut came loose in the first place.  I had to have a heli-coil installed, new gasket, stud, and nut and then got proper torque.  No issues since (a couple of years and hours ago).

[Prior owner]

you can also take the exhaust from the vacuum cleaner and use it to detect leaks in the intake… open the ram air and insert the vacuum hose, being sure to open the throttle all the way. Stuff a rag around the hose and spray soapy water mix over your intake flanges at the top, and lower intake tube seals. Yes, it’s pressurizing rather than sucking, but if it’s a bad Intake leak, it’ll usually show up bubbling. Just don’t use too much pressure- intake boots are sometimes worn out and very expensive to replace!

A common problem with aging Lycoming engines that didn’t get overhauled at the lycoming factory is that the face of the intake tube is no longer sitting above the flange, as it should, in order to compress the gasket around it- the shoulder of the tube gets worn, as well as the flange, and now they are equal in height, or worse- the tube is sitting below the flange! Getting it to seal properly will be difficult after that happens. 

[Guest]

34 minutes ago, PilotCoyote said:

you can also take the exhaust from the vacuum cleaner and use it to detect leaks in the intake… open the ram air and insert the vacuum hose, being sure to open the throttle all the way. Stuff a rag around the hose and spray soapy water mix over your intake flanges at the top, and lower intake tube seals. Yes, it’s pressurizing rather than sucking, but if it’s a bad Intake leak, it’ll usually show up bubbling. Just don’t use too much pressure- intake boots are sometimes worn out and very expensive to replace!

A common problem with aging Lycoming engines that didn’t get overhauled at the lycoming factory is that the face of the intake tube is no longer sitting above the flange, as it should, in order to compress the gasket around it- the shoulder of the tube gets worn, as well as the flange, and now they are equal in height, or worse- the tube is sitting below the flange! Getting it to seal properly will be difficult after that happens. 

That’s in some of the old TCM engine training manuals as well.

[PierceM20F]

I was able to replace the intake gasket relatively easily and torque it. However, the remaining exhaust manifold nut was stuck to the stud and the stud started to unscrew. So, I decided to just tighten the nut down instead of changing the gasket. I then added the missing nut and tightened the whole manifold back down. A problem I ran in to was the weld on the exhaust manifold. Someone must have welded a crack because it is a THICK weld. To the point where I could not tighten either nuts down any further because the socket wouldn't go over the nut due to the essentially zero clearance between the nut and the weld. The manifold is tight like the others so I'm expecting the issue to be resolved. I'll report back once I take it for a flight, most likely tomorrow evening. 

[RobertGary1]

10 minutes ago, PierceM20F said:

I was able to replace the intake gasket relatively easily and torque it. However, the remaining exhaust manifold nut was stuck to the stud and the stud started to unscrew. So, I decided to just tighten the nut down instead of changing the gasket. I then added the missing nut and tightened the whole manifold back down. A problem I ran in to was the weld on the exhaust manifold. Someone must have welded a crack because it is a THICK weld. To the point where I could not tighten either nuts down any further because the socket wouldn't go over the nut due to the essentially zero clearance between the nut and the weld. The manifold is tight like the others so I'm expecting the issue to be resolved. I'll report back once I take it for a flight, most likely tomorrow evening. 

Can you get a crow's foot on it?

[PierceM20F]

11 minutes ago, RobertGary1 said:

Can you get a crow's foot on it?

That's what I used to get it to where it is now as far as tightness goes on one side. The other side can only be tightened by socket and it's tight as well but I would guess slightly less tight than the other side of the manifold.

[Guest]

2 hours ago, PierceM20F said:

I was able to replace the intake gasket relatively easily and torque it. However, the remaining exhaust manifold nut was stuck to the stud and the stud started to unscrew. So, I decided to just tighten the nut down instead of changing the gasket. I then added the missing nut and tightened the whole manifold back down. A problem I ran in to was the weld on the exhaust manifold. Someone must have welded a crack because it is a THICK weld. To the point where I could not tighten either nuts down any further because the socket wouldn't go over the nut due to the essentially zero clearance between the nut and the weld. The manifold is tight like the others so I'm expecting the issue to be resolved. I'll report back once I take it for a flight, most likely tomorrow evening. 

Application of a hot blue wrench helps loosen stuck exhaust nuts.

Clarence

[67 m20F chump]

I have a special socket for that stud.  I took a 12 point socket and put it on the grinder.  It’s now a thin wall socket and just fits.  
 

Edited December 7, 2021 by 67 m20F chump

[Culver LFA]

17 hours ago, PierceM20F said:

I was able to replace the intake gasket relatively easily and torque it. However, the remaining exhaust manifold nut was stuck to the stud and the stud started to unscrew. So, I decided to just tighten the nut down instead of changing the gasket. I then added the missing nut and tightened the whole manifold back down. A problem I ran in to was the weld on the exhaust manifold. Someone must have welded a crack because it is a THICK weld. To the point where I could not tighten either nuts down any further because the socket wouldn't go over the nut due to the essentially zero clearance between the nut and the weld. The manifold is tight like the others so I'm expecting the issue to be resolved. I'll report back once I take it for a flight, most likely tomorrow evening. 

This solves all exhaust install clearance problems, for experimental airplanes only of course.

 

[SSimpson77]

Those “pops” you hear are usually a sign of intake leak. Sometimes also referred to as “lean pop”. But sounds like you got that taken care of. 
Making specialty tools is part of the fun of working in a Mooney if you ask me. 

[BRBENNETT]

in my engine (same as yours 1967 M20 F, IO360-A!A), I had the same issue with oil streaking out of the flange of intake tube.  it turned out to be a bad valve guide.  my engine only had about 300 hrs on it at the time.  it was a factory defect part. 

[PierceM20F]

2 hours ago, BRBENNETT said:

in my engine (same as yours 1967 M20 F, IO360-A!A), I had the same issue with oil streaking out of the flange of intake tube.  it turned out to be a bad valve guide.  my engine only had about 300 hrs on it at the time.  it was a factory defect part. 

I can confirm that changing the intake seal, and adding nut/retightening the exhaust manifold has solved the popping issue! Was a very quick and inexpensive fix. 

[Yetti]

yep ground down socket.    Sometimes smooth throttle movements solve things.  I live on downwind to an airport.   Chopping the throttle is more common than you think.

[RobertGary1]

10 hours ago, Yetti said:

yep ground down socket.    Sometimes smooth throttle movements solve things.  I live on downwind to an airport.   Chopping the throttle is more common than you think.

And lean.  I have the quadrant so I lean for landing. This quadrant makes it easy to push all 3 forward for a go around 

[WAFI]

22 hours ago, RobertGary1 said:

And lean.  I have the quadrant so I lean for landing. This quadrant makes it easy to push all 3 forward for a go around 

Have you had lean pops? I’m having all my intake gasket change tomorrow. I seem to have a rough idle and random lean pop after a flight and landing during the heat of the day.

[MikeOH]

2 hours ago, WAFI said:

Have you had lean pops? I’m having all my intake gasket change tomorrow. I seem to have a rough idle and random lean pop after a flight and landing during the heat of the day.

I commonly have lean pops while taxiing, especially in hot weather, but I aggressively lean on the ground.  However, idle is always smooth.