Theres a major issue with this method.
First things first, don't expect doing this with a diesel engine, so we are talking only about gasoline engines or its variants (ethanol, E85, ...)
People don't understand that CO is one of the major gases that come out of the exhaust and the concentration depends highly on how the engine is running (cold, hot, idle, full load and partial load).
Ok now comes the thing to consider: your body will react very quickly for unburned hydrocarbons HC and NOx.
These 2 gases are EXTREMELY difficult to overcome, meaning your throat, nose and the eyes will be irritated. I experienced that sometime ago (no for suicidal purposes) and I had to catch fresh air, because of how much my eyes were irritated.
So what I'm saying, is not that this method doesnt work, far from it btw, CO is extremely harmful and has irreversible side effects, it is toxic for 1/1000 part per volume. But before you'll get what you want there would be other things to overcome before.