Exhaust Smell and CO in Cabin

You may want to drop the muffler off the engine. Then plug up the 4 ports and then get some soapy water and then you can use a shop vac in reversed air mode and blow into the exhaust pipe and while doing that apply your soapy water to look for any leaks. I almost had a guy die here at my airfield  two months ago that flew in for an annual. He passed out on the taxi way and veered onto the grassy area. I had to open the door and kill the engine. Then we picked him up out of the cabin because he was unconscious. Another 5 minutes in the air and he probably would have passed out and crashed. Check the muffler for leaks. If you have any questions give me a call. It's very easy to do. In his Flight Design we found an 1.5" "U" shaped crack on the muffler under his cabin heat shroud. He was very lucky.

I second all of Rogers advise and would go a little further.

I had a cracked muffler/silencer. The crack was so faint, only the light from a strong beam, held at a slight angle to the surface, allowed it to be seen.

Take no chances, Rogers suggestion of slight positive air pressure & soap is a good one.

Remember; The smell of exhaust will almost certainly indicate high CO levels BUT high CO levels are not always accompanied by an exhaust smell.

CO gas is a colourless, odorless, slightly denser than air. In the absence of good ventilation, will first collect in foot wells and other low points - that's why CO indicators should be placed low in the cabin (not, as is common, high on the instrument panel). 

This person with CO poisoning got lucky he had his mechanic and paramedic waiting for him. LOL

This pilot was completely unconscious and red faced. It took him 5 days to fully shake off the CO bonding to his red blood cells. CO has an affinity 200 times greater then oxygen to red blood cells that transport oxygen and it takes days to get rid of or a hyperberic chamber is sometimes used. The responding paramedics and the hospital he was taken to both missed a CO diagnosis. The beat red face was what tipped me off to possible CO poisoning. That's why I pulled the muffler and tested it. In many places testing for CO muffler leaks on certified aircraft is very normal at annuals. 

I also agree with Sean that CO monitors should be placed low and not up by your head. Early warning is key. It's better to have one of the digital monitors than just a plastic card type and like he said CO is not always noticeable by smell.

In fact carbon monoxide is slightly less dense than air, it's carbon dioxide that's denser.

However CO is so close in density to air that it's generally assumed to disperse in the enclosed air mass rather than rise. So it's arguably best to have the detector in your head area. Cockpit draft conditions might lead to a local concentration somewhere and if that happens to be near your bodily air inlet you want to know!

e.g. https://www.covesmart.com/resources/diy-home-security/is-co-heavier-than-air/ just picking the first reference that came up.

Stories like Roger's are a good reminder of just how easy it is to get CO in the cabin.  In the light sport world, cracks and other failures in the exhaust system are all too common.  A good CO detector not only tells you if you have CO present, but also how urgent the situation is so that you can make an informed decision. In my opinion it should be standard equipment.  My first CO detector was battery operated and worked great, but often I would forget to shut it off and it would be dead when I returned for the next flight. I later upgraded to a fully installed unit that reports via my EFIS and I would never go back.