M20J - Exhaust Leak

[WheelPantsOff]

Greeting all,

A month out of annual and I have an exhaust leak.  When using cabin heat, within 5 minutes my mechanical cat's eye goes from tan to black, and the electronic sensor reaches several  hundred ppm and I stop the experiment.

I performed a general visual inspection of the risers and muffler assembly and nothing was out of sorts.  I checked for leaks and I was getting a lot of smoke out of the slip joints and two of the riser to exhaust port mating.  I'm guessing that this is original to the airplane (~43 years).  I understand that in order to take off the muffler assembly the risers have to come off and this is ending up into being a full exhaust repair.  I'd like to replace the riser gaskets and have the muffler replaced.  I can't open it because it has rivets all over the place on the cylindrical cone and I don't want to mess with that.

Given these conditions, who would you recommend it gets sent out to for repair?

Many thanks and have a safe and joyous holiday season!

[Slick Nick]

Don't mess around with carbon monoxide. It could be leaking inside the heat muff even. I'd have it sent out for repair. 

[EricJ]

You're looking for a crack in the inner part of the muffler, which may not be visible without removing the outer heater jacket.   Sometimes you can run a borescope up the tailpipe and see cracks, but the best thing to do is remove the muffler and give it thorough inspection.  

The leaks out the slip-joints and riser joints are a non-issue as they'll close up once the exhaust gets hot.    They're made to be loose when they're cold.

 

[N201MKTurbo]

Twice in 5000 hours of Mooney ownership I had that happen. In both cases it was a split in the muffler inside the heater muff. In one case the split was about 6 inches long and about 1/8 inch wide. It will make you sick as a dog and it will take 3 days to get over it. 

[Shadrach]

 

I would get quotes from both of these companies:

https://planexhaust.com

https://aircraftexhaust.net

[WheelPantsOff]

Thanks, guys.  Time to get it sent out and repaired.  I can't get a good look into the innards with a borescope so I'll just have the A&P remove the risers and hand me the muffler.  On the exterior shroud/jacket, is it normal to have multiple rivets holding the shroud together?  It looks like it may have been repaired before and looks like a 'rough repair.'

Separately, researching shops, I have never seen such an incestuous business segment as aviation exhaust repair stations.  It's like either Hartzell or some other VC-owned player owns all of the big ones.

Thanks again to all!

[PT20J]

7 hours ago, N201MKTurbo said:

Twice in 5000 hours of Mooney ownership I had that happen. In both cases it was a split in the muffler inside the heater muff. In one case the split was about 6 inches long and about 1/8 inch wide. It will make you sick as a dog and it will take 3 days to get over it. 

Do you happen to recall how many hours were on the muffler in each case?

[N201MKTurbo]

The first one had about 1500 hours. It was a ball joint system. I replaced it with a used slip joint system. I don’t know how many hours it had on it all together. It was repaired once and wasn’t jigged up just right when it was welded. It put some stress on the muffler. Back in the day, there were a lot more exhaust repair shops. Now, you have to send them across the country to get them fixed.

The ball joint system would rot out the risers, so you had to get them repaired a lot. The slip joint systems would last a lot longer, but the alignment was more critical.

[LANCECASPER]

13 hours ago, WheelPantsOff said:

Separately, researching shops, I have never seen such an incestuous business segment as aviation exhaust repair stations.  It's like either Hartzell or some other VC-owned player owns all of the big ones.

Some of the guys that used to work at AWI in Minnesota (now owned by Hartzell) started up another aerospace welding company nearby in Rogers, Minnesota.

https://aircraftexhaustsystemsllc.com/contact/

[Shadrach]

11 hours ago, LANCECASPER said:

Some of the guys that used to work at AWI in Minnesota (now owned by Hartzell) started up another aerospace welding company nearby in Rogers, Minnesota.

https://aircraftexhaustsystemsllc.com/contact/

Good to see that there are more and more alternatives to the companies that were part of the Hartzell roll up. 

[takair]

I would consider sending the entire exhaust, including risers. The jigs are all a little different and I have not had the best of luck by piecemeeling the exhaust.  Small misalignments accelerate cracking. At a minimum I would call in advance. Your mechanic can get eyes on things as it comes apart. Hard to say what’s going on in the various parts without pictures. Not uncommon to have repairs on the shroud, but it should still be set up such that inspections can be done under the shroud. 

[WheelPantsOff]

5 hours ago, LANCECASPER said:

Some of the guys that used to work at AWI in Minnesota (now owned by Hartzell) started up another aerospace welding company nearby in Rogers, Minnesota.

https://aircraftexhaustsystemsllc.com/contact/

I spoke with Rich earlier today and am going to send the entire exhaust assembly to him later this month or early Jan to repair.  None of the other Hartzell-owned entities picked up the phones and AES's owner takes the time to spend 30 minutes with me discussing the process and knowledge.  That's time and money well spent for me.

[WheelPantsOff]

Had the exhaust assembly removed today.  Two cracks in the muffler:  one you could slide a quarter through and another smaller one.  The shroud appears to have been 'modified' by the prior owner and cut on one end.  The adventure never ends!

[N201MKTurbo]

45 minutes ago, WheelPantsOff said:

Had the exhaust assembly removed today.  Two cracks in the muffler:  one you could slide a quarter through and another smaller one.  The shroud appears to have been 'modified' by the prior owner and cut on one end.  The adventure never ends!

Glad you are Ok. The CO sickness is like a very bad hangover that doesn’t go away for three days. And that is the good outcome.

[MikeOH]

18 hours ago, N201MKTurbo said:

Glad you are Ok. The CO sickness is like a very bad hangover that doesn’t go away for three days. And that is the good outcome.

And, here is a little "PSA" information to further convince those with no monitor or just the 'dot':

Carbon monoxide (CO) symptoms often persist even after carboxyhemoglobin (COHb) levels fall to normal because CO poisoning is not just an oxygen-delivery problem. It causes direct cellular, neurologic, and inflammatory injurythat takes time to resolve—and sometimes evolves after exposure ends.

Below are the main mechanisms, from fastest-reversing to longest-lasting.

 

1. Cellular Hypoxia Doesn’t End When COHb Normalizes

Even after CO unbinds from hemoglobin:

• Oxygen release from hemoglobin remains impaired
  CO shifts the oxyhemoglobin dissociation curve left, so tissues stay relatively hypoxic.
• Mitochondria were already deprived of oxygen → ATP depletion

⏱ Recovery: hours

 

2. Direct Mitochondrial Poisoning

CO binds cytochrome c oxidase (Complex IV) in mitochondria:

• Blocks oxidative phosphorylation
• Forces cells into anaerobic metabolism
• Causes lactic acidosis

This persists independently of COHb level.

⏱ Recovery: hours to days

 

3. Nitric Oxide & Oxidative Stress Injury

CO exposure triggers:

• Excess nitric oxide (NO) production
• Formation of peroxynitrite (a powerful oxidant)
• Lipid peroxidation and protein damage

This creates ongoing oxidative injury even after oxygenation improves.

⏱ Recovery: days

 

4. Neuroinflammation & Delayed Brain Injury

CO initiates an immune-mediated inflammatory response in the brain:

• Microglial activation
• Delayed demyelination (especially white matter)
• Apoptosis (programmed neuronal death)

This explains Delayed Neurologic Syndrome (DNS).

⏱ Onset: 2–40 days
⏱ Duration: weeks to months

 

5. Cardiac & Skeletal Muscle Injury

The heart is highly oxygen-dependent:

• CO causes myocardial stunning
• Increases risk of arrhythmias and MI
• Muscle hypoxia → prolonged fatigue

Importantly, cardiac injury predicts long-term mortality even when neurologic recovery seems complete.

⏱ Recovery: days to weeks

 

6. CO Binds Other Proteins Besides Hemoglobin

CO binds to:

• Myoglobin → impairs muscle oxygen storage
• Neuroglobin → disrupts neuronal oxygen handling
• Cytochromes → prolonged metabolic dysfunction

These bindings release more slowly than COHb.

⏱ Recovery: days

[N201MKTurbo]

2 hours ago, MikeOH said:

And, here is a little "PSA" information to further convince those with no monitor or just the 'dot':

Carbon monoxide (CO) symptoms often persist even after carboxyhemoglobin (COHb) levels fall to normal because CO poisoning is not just an oxygen-delivery problem. It causes direct cellular, neurologic, and inflammatory injurythat takes time to resolve—and sometimes evolves after exposure ends.

Below are the main mechanisms, from fastest-reversing to longest-lasting.

 

1. Cellular Hypoxia Doesn’t End When COHb Normalizes

Even after CO unbinds from hemoglobin:

• Oxygen release from hemoglobin remains impaired
  CO shifts the oxyhemoglobin dissociation curve left, so tissues stay relatively hypoxic.
• Mitochondria were already deprived of oxygen → ATP depletion

⏱ Recovery: hours

 

2. Direct Mitochondrial Poisoning

CO binds cytochrome c oxidase (Complex IV) in mitochondria:

• Blocks oxidative phosphorylation
• Forces cells into anaerobic metabolism
• Causes lactic acidosis

This persists independently of COHb level.

⏱ Recovery: hours to days

 

3. Nitric Oxide & Oxidative Stress Injury

CO exposure triggers:

• Excess nitric oxide (NO) production
• Formation of peroxynitrite (a powerful oxidant)
• Lipid peroxidation and protein damage

This creates ongoing oxidative injury even after oxygenation improves.

⏱ Recovery: days

 

4. Neuroinflammation & Delayed Brain Injury

CO initiates an immune-mediated inflammatory response in the brain:

• Microglial activation
• Delayed demyelination (especially white matter)
• Apoptosis (programmed neuronal death)

This explains Delayed Neurologic Syndrome (DNS).

⏱ Onset: 2–40 days
⏱ Duration: weeks to months

 

5. Cardiac & Skeletal Muscle Injury

The heart is highly oxygen-dependent:

• CO causes myocardial stunning
• Increases risk of arrhythmias and MI
• Muscle hypoxia → prolonged fatigue

Importantly, cardiac injury predicts long-term mortality even when neurologic recovery seems complete.

⏱ Recovery: days to weeks

 

6. CO Binds Other Proteins Besides Hemoglobin

CO binds to:

• Myoglobin → impairs muscle oxygen storage
• Neuroglobin → disrupts neuronal oxygen handling
• Cytochromes → prolonged metabolic dysfunction

These bindings release more slowly than COHb.

⏱ Recovery: days

#4 explains a lot.....

[N201MKTurbo]

My 3 day sickness came from flying a 2 hour cross country (IFR if I recall) with the heater on. I felt really crappy when I got there. I thought it might be CO, so I bought a spot detector. It was 1984, there was nothing better. I stuck it by the heater outlet and fired up the engine. It turned black in about 5 seconds. I flew home with the heater off and all the vents open. It was -5F outside. The shivering kept me alert. 

BTW, with the heater off the spot stayed tan.

[skykrawler]

Unless there was some event, like a backfire, it's hard to understand how cracks like that could form a month out of annual.  Unless of course you are operating many hours per month.

Owners need to look the maintainer in the eye ask them if they checked the muffler for cracks during the annual.  It takes time to remove and reinstall the shroud, especially on some of the C and E models.   It make one wonder what other important inspections are being skipped.

There are so many detectors out there now that it inexpensive - there's no longer an excuse.

[N201MKTurbo]

I believe that muffler cracks can happen very quickly. The muffler can have considerable stress on it. Especially if it was assembled imperfectly. It isn’t unusual for one or two risers to need a bit of push and shove to get them on their studs and when the nuts are tightened it will increase the stress on the exhaust system. With the engine vibrating constantly, the tiniest little flaw can cause a split to quickly form. The split will relieve the built up stress.

Luckily, the exhaust systems are very tough and reliable and these things rarely happen.

 

[EricJ]

Cracks can propagate quickly since the heat cycles on a muffler are pretty extreme.   It doesn't take much to start a crack on an old muffler, and they live in pretty harsh circumstances.    This is why it's a good idea to check them during annual inspection, and have a CO monitor the rest of the time.

 

[LANCECASPER]

It doesn't take too many backfires either to cause damage to an older muffler than looked fine recently.

[Fly Boomer]

22 hours ago, MikeOH said:

And, here is a little "PSA" information to further convince those with no monitor or just the 'dot':

That's some good stuff.  Where did that come from?

[MikeOH]

6 minutes ago, Fly Boomer said:

That's some good stuff.  Where did that come from?

Surprisingly, ChatGPT pulled that up with a simple inquiry, "what is the half life of carbon monoxide in the bloodstream?"

[skykrawler]

22 hours ago, EricJ said:

This is why it's a good idea to check them during annual inspection, and have a CO monitor the rest of the time.

 

Good idea?   It's mandatory isn't it?

[WheelPantsOff]

On 12/31/2025 at 7:21 AM, skykrawler said:

Unless there was some event, like a backfire, it's hard to understand how cracks like that could form a month out of annual.  Unless of course you are operating many hours per month.

Owners need to look the maintainer in the eye ask them if they checked the muffler for cracks during the annual.  It takes time to remove and reinstall the shroud, especially on some of the C and E models.   It make one wonder what other important inspections are being skipped.

There are so many detectors out there now that it inexpensive - there's no longer an excuse.

Sorry if this posts twice.  I don't believe the first response didn't get posted.  I'm highly confident that the crack didn't suddenly develop a month and less than 10 hours after the annual.  Either way, I can't prove it one way or the other.  Based upon the removal, I would also say that this was the first time in years that the exhaust/muffler was removed.  In 30 years and overseeing hundreds of inspections, and the last few of these at MSC's, I don't think the muffler/exhaust was ever removed and the inspection goes something like, 'the muffler's there, no squawks, inspection complete.'  I do see in the checklist that it's a required item.