Carbon monoxide

deyoung

deyoung

Line Up and Wait
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Nov 13, 2014
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Tucson, AZ
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Chris
What's the most common reason for CO in the cabin? I added a CO detector and recently I've noticed it gradually creeping up; it's one of the yellw/green/blue button ones, and it takes an hour or so to go from yellow to green-toward-blue, but seems to do it pretty consistently.

Heater and defroster are off. It's possible I haven't noticed it before becaus being winter I'm now usually leaving the cabin vents closed; during the summer they are usually pretty wide open.

This is on a PA-28-235.

Thanks!
 
Cracked muffler is the most common cause. It lets fumes into the muffler shroud and then into cabin heat box. Exhaust fumes drafting into the cabin from the tail pipe is an unlikely but possible source. The first thing I'd do would be to get a new CO detector. Validate that you have a problem. Borrow a digital CO instrument if you can find one.
 
One source are the firewall seals for control cables and fuel lines but it's almost certainly from your muffler. The heat control is nothing more than a flat aluminum plate at the firewall. Ram air pressurizes the heat exchanger shroud at all times during flight and the firewall shutoff is not an airtight seal. You can test this theory by opening the heat and observing if your levels rise more rapidly.
 
There are inexpensive digital CO detectors for the home which may be used in the plane to get a digital reading. If you have propane in your home you should have a CO detector anyway.

Google limits but the limits are for a certain ppm concentration for a certain duration. Be aware CO is colorless and odorless. Be aware your pulse oximeter will read incorrectly high if you have been inhaling CO.
 
Running rich of peak sends the engine out CO through the roof. From there, leaks in your exhaust system or even exhaust from the tailpipe leaking into the cabin will give the inevitable result.
 
Capt. Geoffrey Thorpe said:
Running rich of peak sends the engine out CO through the roof. From there, leaks in your exhaust system or even exhaust from the tailpipe leaking into the cabin will give the inevitable result.
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Or the landing gear. Talked to a 150 owner at one point who found he was getting CO in where the main gear entered the fuselage.

Ron Wanttaja
 
Also check the boots on the control rods from the rudder pedals out to the front steering assembly.
 
I use a digital meter, since my muffler shroud is subject to cracking. I would test. The cabin to be sure you are getting CO in the cabin. If you are have an experienced A+P check for the source.
 
Silvaire said:
One source are the firewall seals for control cables and fuel lines but it's almost certainly from your muffler. The heat control is nothing more than a flat aluminum plate at the firewall. Ram air pressurizes the heat exchanger shroud at all times during flight and the firewall shutoff is not an airtight seal. You can test this theory by opening the heat and observing if your levels rise more rapidly.
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Yes -- but if you think you may have a problem and want to do further testing before grounding the plane, you really should have a digital CO monitor. I was getting suspicious as I was smelling exhaust whenever I cracked open the heat, but the giveaway was seeing the reading rise consistently from 5 ppm or so up to 15-20 in the first few minutes after opening the heat control. I continued to fly with the heat off since I knew what the levels were like, until I had a chance to get the plane serviced.

The diagnosis was indeed a cracked muffler. It is a good thing it was caught too, since the muffler was banging/scraping on one of the side engine mounts, and starting to cause chafing. Now we're just waiting on a part for an unrelated problem discovered incidentally, but the CO problem, and the engine mount impingement, should be a thing of the past.

Don't continue to fly a plane with a suspected cracked muffler unless you have a reliable way to monitor CO levels. As Silvaire says, the firewall is not airtight and even with the heat shut off may still be letting dangerous levels of CO into the cabin.
 
Not specifically saying it's the case in a 28-235, but once, on a Comanche, exhaust was getting into the tail cone and entering the cabin. A good seal at the aft baggage bulkhead cured that. Took us a while to discover where it was coming from.
 
Clark1961 said:
what prompted you to type that?
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Didn't mean to go off topic, just suggesting that there are inexpensive digital CO detectors available for the home which could be used temporarily in the plane to get an indication of the CO level in the plane. I own apartment buildings and as such have purchased many CO detectors. Fuels commonly in a home such as propane and others can lead to elevated CO levels in the home; if these fuels are present it is prudent to buy one of these inexpensive CO detectors. Search Amazon for digital carbon monoxide detector; there are plenty available.
 
OP, if that detector is changing colors, you have a serious problem on your hands. Unless you are open cockpit, I'd have the plane at MX pronto.

With CO, it basically prevent oxygen molecules from attaching itselft to blood cells. Like having a drink of liquor, once the CO environment is removed, you still have CO in your blood and it takes a while to dissipate.

I would strongly caution you about flying that plane any distance trips until you solve the issues.
 
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