Hello All - I have a 1959 C172 with a newly overhauled O300 engine.
Been getting bad CO only when using heat. Pulled muffler, pressure tested, no leaks detected. Siliconed all around the heat shroud, replaced the rubber seal in the cabin heat flapper box thing, and made no difference whatsoever. Also replaced all the gaskets that seal the downtubes to the exhaust manifold.

The amount of CO is disturbing. Within 30 seconds of pulling heat, CO spikes to 255 ppm (maxes out the meter). Engine and downtubes have 70 hours on them, muffler appears to be original or close to original.

Has anyone experienced a leaky muffler that passed a leak test? Any other sneaky areas we should be looking into before throwing more $$ at a new muffler assembly?

Side note: I recently had EGTs installed on all six cylinders. I can't imagine a small leak around these probes could contribute the amount of CO we're detecting in the cabin.
I welcome your thoughts and ideas - this has been maddening trying to chase it down.

Thank you!

Parker
1959 C172 N6383E

A leaky muffler can still pass a leak test. Cracks may not show as a leak when the muffler is cold, but open up when it is hot.

Dale Moul wrote:A leaky muffler can still pass a leak test. Cracks may not show as a leak when the muffler is cold, but open up when it is hot.

This.

I had this happen decades ago on a Pacer before electronic detectors were widely available. It caused the chemical detector on the dash to go dark brown within a few short minutes, it caused a landing on a road, I disconnected the SCAT tubing from the cabin, took off for home again, etc etc, and I dealt with a head banger for days. I took the exhaust manifold off and pressure tested it...it held 3 psi for 15 minutes without discernible leakdown. I put it back on, and it turned another detector dark brown by the time I was done running up at the runway end.

A dye check on the inside of the flanges revealed several tiny cracks. It was welded, and then tack welded to a steel plate and stress relieved so it would be coplanar, pressure tested again, then yellow tagged. No more issues. The misalignment that had formed surely had allowed those cracks to open up. I have no idea what caused the misalignment, but this did happen after a trip through some moderate rain.

My right side muffler was found cracked yesterday underneath the muffler around the outlet.

It has 811 tach hours since this muffler was installed new in 2005. Have you taken your muffler shroud completely off the muffler and done a visual inspection for cracks?

Thanks for the info, all.
We had cleaned and reinstalled the muffler, then flew for about 5 hours with no cabin heat and no CO issues. We are hoping the cleaned muffler will show signs of soot or white trace from where the exhaust must be leaking. We will be pulling it off next week for a more complete inspection. I’m already expecting to need a new muffler and a few shroud parts. I’ll report back with findings.

Thanks
Parker

Parker wrote:Thanks for the info, all.
We had cleaned and reinstalled the muffler, then flew for about 5 hours with no cabin heat and no CO issues. We are hoping the cleaned muffler will show signs of soot or white trace from where the exhaust must be leaking. We will be pulling it off next week for a more complete inspection. I’m already expecting to need a new muffler and a few shroud parts. I’ll report back with findings.

Thanks
Parker

Please stop flying the airplane immediately. There is only one place to have that much CO coming in when you put the heat on and you really answered your own question saying the muffler looks 60 years old. Siliconing the heat shroud on the muffler makes no sense whatsoever. replacing the seal on the flapper make no sense either for a CO issue. I hope your A/P didn't suggest that - if so you might need a new A/P as well as a new muffler. Was there actual carbon tracking from the down tubes at the heads? I really wouldn't waste another hr of labor time chasing this, like how do you clean a muffler?? Order a muffler and stop throwing money away and possibly your life. There is a great youtube video on a Mooney pilots experience with CO in the cockpit - please watch

Old mufflers are like old cylinders, better decommissioned than repairing. Opinion.

8GCBC wrote:Old mufflers are like old cylinders, better decommissioned than repairing. Opinion.

That is good advice. My cracked muffler story.... mine from an O-300 in my old 170 didn't look quite as bad as Aryana's but had a welded repair. One day I pulled the cabin heat knob and my lips were instantly numb. It happened so fast that it was shocking. I might not be the sharpest tool in the shed but I knew enough to turn the cabin heat off and open some fresh air vents. I flew the rest of the way cold. I'm lucky that the exhaust leak was so massive that it was not an insidious subtle incapacitation.

The old weld had cracked. I would never repair an old muffler after that.

Thankfully I never use cabin heat. These had 811 hours since installed new in 2005. I'll replace them as a pair again.

Check out this quote to replace everything (risers, muffler, shroud, clamps, pipe) on the right side.

This hobby is really great if you hate money.

I should have clarified, we have narrowed the issue down to be a cracked muffler, even though it passed a pressure check. We cleaned the muffler and ran the plane (with no heat, there are no CO issues. Meter reads zero ppm). We did this so we can hopefully see white traces from an exhaust leak so we can see where it is leaking from. It has been cold flying, but fine with me if it means we can find the leak.

New muffler will be on order this week, along with a few shroud parts.

Parker

You must have called Acorn just minutes before I did last week. They said I was the third call for the same parts - and I have the exact quote in my inbox right now. Ouch. Good luck!

Alright folks, update:
We installed a brand new muffler, taped all seams on risers/clamps, and redid a lot of the sheet metal on the shroud.
Started plane. CO spiked to 255 ppm. ZERO difference from before.

After some more structured troubleshooting, cursing, and seriously considering setting it ablaze and walking away, we decided to remove the tailpipe and try. I'll be damned, CO dropped, still high, but made an improvement.
Our theory is that the backpressure in the tail pipe was forcing a little exhaust out past the joint, up into the shroud. Turns out, the tailpipe was inserted far into the muffler and a sloppy joint didn't make a good, solid seal from tailpipe to muffler. Exhaust was escaping UP, past the joint, into the shroud, and sucked right into the cabin.

We took the tailpipe off, cut it so it didn't protrude up into the heat shroud, and installed a new clamp so it sealed firm.
CO maxed out a 8 ppm.

So, make sure to check your tailpipe connections to the muffler. If they are sloppy and protrude up into the shroud, good chance you're blasting exhaust gas into the shroud, right into the cabin.

After weeks of troubleshooting and head scratching, we have it figured out. Needless to say, we CRANKED the heat until we were sweating, just cus we could . Hope this helps someone in the future.

PD

Did you buy a muffler from Acorn?

Glad you got it figured out.

I ended up with a new muffler from Nicrocraft and some other parts from Acorn. Both companies were incredibly helpful and had some great tips.

PD

Parker, thank you for the follow up, I’m sure it will help someone out in the future. Glad you figured it out, too bad it took so long to solve the puzzle.

Kurt