Motor car exhaust may have contained up to 25% carbon monoxide. However, catalytic converters found on all modern automobiles eliminate over 99% of carbon monoxide produced.
en.wikipedia.org/wiki/Suicide_methods
I have never heard of suicide via CO2. I do not think it is possible.
There is one site on a hemoglobin chain that can absorb an O2 molecule in the lungs and release the O2 in the body where the O2 concentration is low. Hemoglobin has a much greater affinity for CO than for O2 at the site; that is, a CO molecule can strongly bind to that site and prevent the site to pick up O2. In other words, one CO molecule means a loss of one hemoglobin molecule.
CO2, on the other hand, is basically inert and does not react with a hemoglobin molecule. If the CO2 completely replaces O2, then of course, the body will succumb due to the lack of O2. But CO poisoning is much more effective than CO2/O2 replacement.
There are better methods - consult your PCP.