The most common reason is a dirty air filter in cars made before 1990. Running rich is caused by too little oxygen and too much fuel in the combustion chamber, resulting in incomplete combustion, carbon black soot, carbon-monoxide and increased fuel consumption, reduced power. The most common part that can cause a car to run rich or have a rough idle in any make or model is the O2 sensors. There are many other causes, a leak in the intake, leak in exhaust before the O2 sensors, cracked or disconnected vacuum hoses, fuel regulator, fuel pump, clogged fuel lines etc. Any sensor can throw the computer off. These common problems are very difficult to diagnose without a good scanner that will allow you to get feedback (in real time) while the car is running. Get yourself a Haynes Repair Manual and compare the suspected sensor input to regular specs. You could find a good scanner on-line probably cheaper than in Harbor Freight. If not, take the car to a local shop see if they have a good method of scanning for codes and reading the sensors in real time. I don't recommend Autozone they are only good if you have the check engine light on. For that just get a cheap code reader and you can check for yourself any time. This is what I recommend to any one, DO NOT DO ANY GUESSING for any reason its too expensive. You'll start replacing parts and you'll end up with a completely rebuild engine that still having a problem. Get a good scanner not a code reader, Haynes Manual and don't replace anything without diagnosing it, its a waste of time and money.