Carbon monoxide poisoning is one. Unfortunately it's cumilative and you can slowly get it over the span of several days (even weeks). It will cause you to feel tired and if it's severe enough, it can make you fall asleep (if you're driving at night this is a very bad thing). That's why some people will run a car in an enclosed area to kill themselves, they fall asleep and carbon monoxide takes the place of oxygen in your blood stream and causes the brain to eventually shut down from suffocation.
You may not have a problem, but it can also kill you.
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Mustang Parts Specialties ripped me off, it could happen to you as well.
'84 Mustang 5.0 LX
my LX
'89 Mustang GT
my GT