High CO in Cessna 150M Cabin

Mooney Fan

Mooney Fan

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Mooney Fan
I'm getting high levels of Carbon Monoxide in my cabin. With all the air vents closed it reached ~200ppm on my digital meter. Opening the vents and leaning out the mixture it dropped to about 25ppm.

I returned to the field and need to sort this problem out before flying it again.

SCAT tubing all appears good. Of course, in the summer the the heater has not been used.

Last annual IA said my windshield seal was toast. Essentially, its not there. Could gases be coming in there?

Also, my gear leg fairings that cover the slots in the fuselage where the legs go through have gaps. Looks like some type of clear tape has been used to seal the gaps with some of it missing. Can exhaust gases come into the cabin through those slots?

Cabin door seals are new.

Where is the most common place CO comes into the cabin from?

Thanks for any suggestions.
 
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High CO2? I wouldn’t be worried at all. Now CO on the other hand…

:)
 
Mooney Fan said:
Also, my gear leg fairings that cover the slots in the fuselage where the legs go through have gaps. Looks like some type of clear tape has been used to seal the gaps with some of it missing. Can exhaust gases come into the cabin through those slots?
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It can, but with the 150s it's not a common problem. That leg goes into bushings in bulkheads in the gear box, and there is no real access to the cabin from the outboard end of the box where the first bushing is. That bushing would have to be shot to let CO past it.

More than likely is a cracked muffler. Even with the heat shut off there is still some leakage into the cabin through the heat valve, especially if it's not rigged right.

Canada, being a super-cold country where real pilots go flying even when the temperatures are near freezing:), has an AD that demands inspection of exhaust-type cabin heaters every 12 months/150 hours. I have found cracks. The AD starts with a visual inspection, with pressure and soap-testing of suspect areas, but I went to the soap right off. It revealed leaks that could not be seen. https://wwwapps.tc.gc.ca/Saf-Sec-Sur/2/cawis-swimn/AD_html.aspx?ad=CF-90-03&cn=CF&l=E
 
Why not try a trip around the pattern with cabin heat disconnected?

I’m presuming readings are fine on ground during taxi.

The boots around the steering rods just aft of the Firewall can cause this.

Assure ALL openings in the Firewall are sealed.
 
Man good luck with your investigation.

We had a similar issue in our Cherokee. Ended up with half a new exhaust stack. The slip joints were done as we just needed new pipes. Before the fix we’d get 30-50ppm when nose is up or after landing if winds are right. Now we never hear the monitor goes off.

When you look under the cowling, did you find any white/grey powder near exhaust bits? That was our clue.
 
Magman said:
Why not try a trip around the pattern with cabin heat disconnected?

I’m presuming readings are fine on ground during taxi.

The boots around the steering rods just aft of the Firewall can cause this.

Assure ALL openings in the Firewall are sealed.
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That's a good idea. I'll give it a go with the SCAT off the heater inlet.
 
SebIp said:
Man good luck with your investigation.

We had a similar issue in our Cherokee. Ended up with half a new exhaust stack. The slip joints were done as we just needed new pipes. Before the fix we’d get 30-50ppm when nose is up or after landing if winds are right. Now we never hear the monitor goes off.

When you look under the cowling, did you find any white/grey powder near exhaust bits? That was our clue.
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Not under the cowl. But I do have white near the tips of my exhaust that protrude through the cowl.
 
Glad you found it! CO kills a couple of people a year around here. Not so much airplanes, but generators mostly.
 
Mikey52 said:
A little jb weld and it should last till next inspection.
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When I bought the plane 16 months ago (120/hrs) it had a fresh overhaul and pretty much everything was new including prop, carb to fresh O/H mags etc. But for some reason they slacked on the exhaust.

Going to fix that. She's getting new on both sides. :)
 
Mooney Fan said:
When I bought the plane 16 months ago (120/hrs) it had a fresh overhaul and pretty much everything was new including prop, carb to fresh O/H mags etc. But for some reason they slacked on the exhaust.

Going to fix that. She's getting new on both sides. :)
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Check the risers between the mufflers and the exhaust ports on the heads. Those crack next to the welds, too.
 
Mooney Fan said:
When I bought the plane 16 months ago (120/hrs) it had a fresh overhaul and pretty much everything was new including prop, carb to fresh O/H mags etc. But for some reason they slacked on the exhaust.

Going to fix that. She's getting new on both sides. :)
Click to expand...
And this wasn’t caught on prebuy?

I fully understand from experience, having done my part of elbow grease on the tail fixing up, how many items come up after you buy. The exhaust cracking is like an obvious and must check item I think on a purchase though. What else might have been missed?
 
Just what was in the PB Agreement?

Unfortunately; these things seem to work backwards.

“ How much for a Pre Buy Survey?” is how they begin.

The price then will generally dictate the Depth of the task.

Often folks only want a Comp Check and pull the Screen followed by

Airframe Visual with no panels pulled.

Often the Buyer says they will check ADs and other papers.

There are a LOT of ways it can be done but it is up to the

Buyer to dictate the scope before agreeing on a price.

If the Buyer does not understand this they should not own any aircraft.
 
SebIp said:
And this wasn’t caught on prebuy?

I fully understand from experience, having done my part of elbow grease on the tail fixing up, how many items come up after you buy. The exhaust cracking is like an obvious and must check item I think on a purchase though. What else might have been missed?
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Nope, or in annual back in Feb. I'll go out on a limb and say its happened sometime between Feb and now because the shop has a good reputation around here.
 
Mooney Fan said:
I'll go out on a limb and say its happened sometime between Feb and now because the shop has a good reputation around here.
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You'd have to ask them if they had that shroud off the muffler at annual. They likely didn't. I have found the screws rusted into the tinnerman nuts on those things even though we have the AD that forces the inspection. And that rust doesn't get that bad in one year. Nor did that crack form just since the last annual; they take time to get as long as that.

A Cherokee owner I spoke to took his airplane to another shop for the annual. He'd only owned the airplane for six or eight months. The mechanic took the muffler shroud off to do the inspection, and found a hole in the muffler he could stick his fist into. "No wonder I was getting headaches," said the owner. The logs showed that the inspection had been done at the last annual. Sure it was. Like pencil-whipped inspection. A fist-sized hole in a few months and hours? Nope.

These things can be a pain to do, so they sometimes get cheated.
 
Dan Thomas said:
You'd have to ask them if they had that shroud off the muffler at annual. They likely didn't. I have found the screws rusted into the tinnerman nuts on those things even though we have the AD that forces the inspection. And that rust doesn't get that bad in one year. Nor did that crack form just since the last annual; they take time to get as long as that.

A Cherokee owner I spoke to took his airplane to another shop for the annual. He'd only owned the airplane for six or eight months. The mechanic took the muffler shroud off to do the inspection, and found a hole in the muffler he could stick his fist into. "No wonder I was getting headaches," said the owner. The logs showed that the inspection had been done at the last annual. Sure it was. Like pencil-whipped inspection. A fist-sized hole in a few months and hours? Nope.

These things can be a pain to do, so they sometimes get cheated.
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And why inspections cost so much done right. You’re mechanic should work on your plane more than once a year.

In 2018 when I got my plane the rumor was it was 3000 for a 172 type plane with owner assist at my airport for an annual inspection.At the time I thought that was high, not anymore even without inflation.
 
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A Pressure Test in place with a shop vac of leaf blower would have

disclosed the crack even w/o removal.

Quick and easy to check the whole system.

My suggest is to do in September or October before heating season.

The muffler should have the baffles checked sometime but that’s

more involved.

Mufflers go bad really quick with the mag switch OFF then back ON.

My collection includes one from a 150 with the end blown off!
 
I keep seeing the title when I scroll thru the new postings and interpret to mean

trying to fly high altitude thru the mountain passes in Colorado in a 152.....
 
Dan Thomas said:
Check the risers between the mufflers and the exhaust ports on the heads. Those crack next to the welds, too.
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Thanks Dan. I'm behind the curve in understanding the airflow from the exhaust to the heater. When the air reaches the housing on the firewall, and the heater is off, where does the heated air get vented out of the housing? I noticed in heater off condition the scoop door is closed but there is a small 3/8" gap. I can reach the lever and pull it and it closes to the stop.
 
Mooney Fan said:
Thanks Dan. I'm behind the curve in understanding the airflow from the exhaust to the heater. When the air reaches the housing on the firewall, and the heater is off, where does the heated air get vented out of the housing? I noticed in heater off condition the scoop door is closed but there is a small 3/8" gap. I can reach the lever and pull it and it closes to the stop.
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The valve should block the cabin opening when closed, and block the outflow when open. Your valve should have that outflow provision. If it doesn't the new muffler won't last long. It needs constant airflow around it to cool it.

Older 150s used a valve like this:

1725313798269.png
1725313828469.png

The flange was screwed to the firewall, and the hose from the muffler shroud went to the nipple. When the valve was opened (heat on) the firewall openings opened up and the rectangular dump port in the side closed.

Later on, we see this:

1725313986453.png

The lever works the flap inside the valve. When the heat is off, a port in the firewall is closed by the flap, and the hot air comes out of the rectangular dump hole. Pull the heat knob, and the flap comes off that firewall heater port and closes the dump hole.

There are some other variations on the same theme. They all have a provision to dump the hot air from the shroud into the engine compartment. But it sounds like you might be looking at this:

1725314205736.png

It's a fresh air valve, located on the inside of the cowling. Someone might have made the mistake of substituting something like that for a busted heater valve. It has no dump feature and that muffler won't appreciate it.

What serial number is your airplane? Got a picture of the heater valve?
 
Most 150’s have 2 Intake Ports on the forward baffle.

The one with the scoop is for Cabin Heat and the other for Carb Heat.

The Carb Heat is “ sheltered” from impact ice and debris by the

lower cowling.


A few 150’s and Conti 172’s are equipped with the “ Winter Package”

that provides heat from BOTH mufflers. This is accomplished by moving

heated air THROUGH the Carb Air Box to provide twice the amount of heat.

IIRC on these certain 172’s there are 2 “flapper doors” and the 150 has

a cap over the “ Heat Dump” outlet. It’s important to recognize these

components are not interchangeable with Standard Configuration parts.
 
Dan Thomas said:
The valve should block the cabin opening when closed, and block the outflow when open. Your valve should have that outflow provision. If it doesn't the new muffler won't last long. It needs constant airflow around it to cool it.

Older 150s used a valve like this:

View attachment 133066
View attachment 133067

The flange was screwed to the firewall, and the hose from the muffler shroud went to the nipple. When the valve was opened (heat on) the firewall openings opened up and the rectangular dump port in the side closed.

Later on, we see this:

View attachment 133068

The lever works the flap inside the valve. When the heat is off, a port in the firewall is closed by the flap, and the hot air comes out of the rectangular dump hole. Pull the heat knob, and the flap comes off that firewall heater port and closes the dump hole.

There are some other variations on the same theme. They all have a provision to dump the hot air from the shroud into the engine compartment. But it sounds like you might be looking at this:

View attachment 133069

It's a fresh air valve, located on the inside of the cowling. Someone might have made the mistake of substituting something like that for a busted heater valve. It has no dump feature and that muffler won't appreciate it.

What serial number is your airplane? Got a picture of the heater valve?
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Thanks. I have the second set-up srewed to the firewall. Makes sense now after your explanation. I plan to lube up the hinge/lever etc when I go back out to the plane.
 
Mooney Fan said:
Thanks Dan. I'm behind the curve in understanding the airflow from the exhaust to the heater. When the air reaches the housing on the firewall, and the heater is off, where does the heated air get vented out of the housing? I noticed in heater off condition the scoop door is closed but there is a small 3/8" gap. I can reach the lever and pull it and it closes to the stop.
Click to expand...
Adjust the cable to take up the last 3/8"
 
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