231 Carbon Monoxide Problem

[tomatl]

Hi all, perhaps someone has an idea on how to solve my carbon monoxide problem. My CO monitor shows up to 50 ppm in the cockpit, particularly during approach. (At cruise it‘s down to 15ppm). It seems to come from exterior exhaust gases, not through the firewall. The air from the cabin heat is not contaminated and the exhaust system has been checked for leaks. I resealed cabin and baggage doors, changed the „rat boots“ in the gear cavities, and reapplied the tapes under the front and rear seats in the cabin. All without success, levels essentially remained unchanged. Grateful for any ideas, I am at a loss here.

 

 

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[carusoam]

Congrats on your first post, Tom.

Lets start with what device you are using for a CO monitor?

This will give us an idea of how comparable your numbers are with other M20 owners...

Are your elevated numbers a new change from where they have been for a while?

Did the high numbers begin after any maintenance being done?

 

TC’d motors have a higher rate of exhaust system wear than other planes... look for leaking seals, they tend to leave gray deposits...

Any idea on how many hours are on the exhaust system..?

Really old TC exhausts will start to show thinning of the tubes between the exhaust valve and the TC.

PP thoughts only, not a mechanic...

Best regards,

-a-

 

[whiskytango]

You may want to use an aspirating sample pump type carbon monoxide sensor, such as a Bacharach Monoxer, while the plane is in various flight modes.  This instrument provides real-time CO readings on the meter for the location of the sensor end of the sample probe.  This would allow you (or better yet, a passenger seated next to you) to take localized air samples throughout the plane in flight to identify the exact location of CO entry.  There are companies that do short-term rentals of these meters.  Make sure that it is an aspirating sample pump meter.  I discovered a path though a gap in the door seal that was allowing significant CO infiltration while in climb.

[LANCECASPER]

I would check the exhaust system again more carefully with soap/water spray and pressurize it. 15 ppm at cruise is way too high but might be acceptable at slow speed taxi. After reading @DanM20C 's story I started paying a lot more attention to this. I show 0 ppm in cruise and 2 or 3 on the ground.

 

[tomatl]

Thanks for the response, this forum is great! I have a Draeger Pac CO monitor. I have the problem since I bought the airplane last year. The exhaust is brand new, I just had it replaced but it did not make any difference. Using my monitor, it seems that the levels increase towards the back of the cabin. So I thought that CO enters under the back seats. Taping and sealing the floor there last week, however, made the problem worse. Maybe the gas vented out of these openings ...


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[carusoam]

Mooney cabins are vented from the baggage compartment areas...

Short bodies, air goes out a floor vent, visible from the outside... makes a nice icebox drain...

Longer bodied Mooneys have some vents on the side? (I believe) visible slats...

Air intending to enter the cabin comes from eyeball vents up front, or ceiling vents above.

On newer Mooneys the air enters the cabin through a NACA vent in the tail area, and is ducted forward to the ceiling vents...

Rat sox (or fabric) are not the least bit CO resistant... if CO is coming through here, it is a sign of CO possibly leaking up front...

Some people have reported success in using their portable CO monitor to find the source of CO...  similar to what WT/Ken has described above...

CO sensors usually have a lifetime limit to them measured in years...  if you have had yours this long, it might need calibration...  

Remember, the whole cabin is running under vacuum slightly, air can be drawn in from anywhere while moving at speed... this is evident while opening that pilot’s window...

See if you can borrow another CO meter while your are re-checking everything?

PP thoughts only, not a mechanic...

Best regards,

-a-

[tomatl]

Thanks A.! I will do that. I spent already tons of money with reputable shops, they still have no idea of how to resolve my problem. My monitor is quite new, well within the lifetime of the sensor. Today, I had 30+ in cruise and 50+ on approach. I will get a precision sensor and try to isolate the entry point. What would be the best course of action, if CO came in through the wheel wells?


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[carusoam]

there is some ability to update the tape that was originally used to seal seems in the airplane...

modern aluminum tape from the HVAC industry is lightyears better than the 60’s version of the duct tape that was used...

Somebody posted pics around here regarding re-sealing some locations... expect that to be at the wing root... but could be in several places...

We can invite @M20Doc to see if he has additional insight on sealing out CO from a plane that has been chasing the issue for a while.

Best regards,

-a-

[Guest]

One area to check is in the tailcone.  Remove the battery access door on the left side of the plane and look for sealing of the wire pass through from the tailcone to the baggage compartment.

Clarence

[takair]

You mentioned it is worse on approach, is that gear up or down?  Does flap position make a difference? Do you have a picture of your exhaust orientation?  

[cliffy]

m29doc hit it on the head. The wire run hole just fwd of the battery on the left side is a prime place for air to come forward from the tailcone. Take a look for leaks there.  

[cbarry]

Does it seem to make a difference depending on cowl flap settings?  If the reading is higher when the cowl flaps are closed, you still could have an exhaust leak (bad gasket, misfitted tailpipe joint, hairline crack...) that is registering more and that is seeping through into the cockpit simply due to the build up in pressure under the cowling and the  “leaked exhaust gases” not exiting like they would with the cowl flaps fully open.  Just a thought.

 

[tomatl]

Thanks Doc! I'll check the wire pass through and will get back with some pictures. I also ordered the Bacharach Monoxer (thanks for the suggestions whiskeytango!) to see if I can pinpoint the entry point and check levels for various flight modes. All I know so far is that levels are higher in landing configuration and also much higher when I hold the sensor back over the rear seat. I'll keep you posted.

[tomatl]

Here is a picture of the inside of the tail cone with the pass through for all the wires. Is that supposed to be taped up?


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[takair]

1 hour ago, tomatl said:

Here is a picture of the inside of the tail cone with the pass through for all the wires. Is that supposed to be taped up?


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Yes, supposed to be sealed.  

[tomatl]

What is the proper way to seal this safely (given that objects could fall into the flight control components in the tail cone)?


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[Guest]

4 hours ago, tomatl said:

Here is a picture of the inside of the tail cone with the pass through for all the wires. Is that supposed to be taped up?


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I don’t know what Mooney used when these were born, but I have used a product called “Panduit Duct Seal”. You’ll have to remove the baggage compartment side panel to work the sealant from both sides.

Clarence

[takair]

Going way back, the wire bundle was wrapped in leather and the leather was potted with some type of pre-RTV.  The concept still would work.  Wrap in leather or some other conduit and then perhaps use what Clarence suggested....maybe with some RtV to finish off.

[Jim F]

I was at the hanger today and here is the pic. I have a 81’ 231. It looks like modeling clay and is still playable.  I don’t see any harm in just filling the gap in with clay. 

[carusoam]

Expect that it is a special modeling clay that doesn’t...

• freeze @20k’ in winter over N. Dakota and fall out...
• melt when it gets TX hot and sag out...
• stick irreversibly to all those individual wires...

We’ll find out how serious this challenge really is when Tom the OP comes back with the hole plugged and a bunch of zeros on his CO meter.

Then we could ask Siri what wire duct clay looks like...

Might be better to ask dan at Lasar or a MM or parts manual...

Siri found this... so we know this stuff exists for some environments.  Our environment is a bit special and comes with added costs if it fails... also make sure it is not corrosive with aluminum...

https://www.homedepot.com/p/Gardner-Bender-1-lb-Plug-Duct-Seal-Compound-DS-110/100212441

PP thoughts only, not a CO expert.

Best regards,

-a-

 

[cliffy]

Actually a plug from plain ol' upholstery foam woks pretty good also. There's no pressure there to speak of. Its just a draft.  

[tomatl]

Hi all, my mechanic filled the gap with some type of putty (only from the tail cone side) after I wrapped the cables with a piece of leather (that I cut out from the underside of our couch cushion when my wife was not looking). Unfortunately, the CO levels did not change. I am at a loss here. When I flew the plane back home from the shop I got 0 on takeoff and climb out, 12-20 on cruise in the front part of the cabin (more in the back).

Perhaps the door seal is still leaking?


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[Andy95W]

On 6/2/2019 at 9:38 AM, cbarry said:

Does it seem to make a difference depending on cowl flap settings?  If the reading is higher when the cowl flaps are closed, you still could have an exhaust leak (bad gasket, misfitted tailpipe joint, hairline crack...) that is registering more and that is seeping through into the cockpit simply due to the build up in pressure under the cowling and the  “leaked exhaust gases” not exiting like they would with the cowl flaps fully open.  Just a thought.

 

Tom- you never answered the cowl flap question.  There's a fair amount of pressure in the cowling at cruise with the cowl flaps closed.  A very small exhaust leak could be pushed through a very small breach in the firewall.

[tomatl]

Made some progress in solving the CO issue. I taped the edge of the wing roots with duct tape. Also, I found a belly panel screw missing under the plane right downstream of the exhaust, which I also taped (see pictures below). With this, CO dropped to 0 ppm in cruise, with both cowl flaps in or out. I still have a little before landing with gear/flaps down. Next time I fly, I‘ll remove the tape on the wing roots and see if I still get zero. I this case, the fix would be to just put a bigger screw into the worn out thread. If it‘s the roots, they would need to be removed and re-taped below. That would be quite costly. I hope, it‘s just the screw.






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[carusoam]

Glad to see you are making headway with the CO challenge...

Check What those screws are supposed to screw into... a larger screw probably isn’t the right thing to do... swapping out the worn hardware on the other side would be better...

It is probably some form of tinnerman (speed nut...).

PP thoughts only, not a mechanic...

Best regards,

-a-