1993 Buick Century Custom 3.3L Runs Rich, Ruff at idle; 10MPG

This is an engine that is running very rich. Check the throttle body and also the TPS. Check all the spark plugs.

Sounds as if the engine is running rich ( over fueled ). There are many reasons for this issue. I'm suspecting the vehicle is fuel injected? You can check for a leaky fuel pressure regulator on the fuel rail and also a sticky/stuck EGR valve that can cause this problem. These are the quickest and easiest checks to do first. IF all ok, you may require the use of a scan tool to check sensors/codes for proper operation and look at any available data stream that may point you in the right direction

replace the fuel filter then check the air/fuel mix needle valve according to manufacturer specs. Sounds like you have way to rich a fuel mixture.

Depend on the mileage of your car. could be bad O2 sensor ,coolant sensor, too much carbon inside cylinder,Converter catalyst is there for burning gaz that the spark plug have not burn

The regulator is on top of the engine along the gas line. It has a vacuum line on top. Start the truck and pull the vacuum line off the regulator. If fuel comes out of the regulator, it has failed. You can run pretty rich with a busted regulator-especially at idle.

not sure if the 93 has a mass airflow sensor, i know it has a air charge temp sensor that can cause the issue you are describing.

recommend you pull the vacuum hose off the fuel pressure regulator at the end of the fuel rail and check for gas in the line. You can plug the hose with a golf tee and start the car, then see if any gas comes out of the vacuum port of the regulator. This would indicate the regulator diaphram has failed and is causing the engine to run rich. Replace the regulator if it leaks gas out the vacuum port. The regulator looks like the below picture.

fuel lines inside the intake plhenum may be cracked and leaking inside the plhenum. same thing happened to my 1992.sonoma. they are made of a plastic material and tend to crack right in the bend of these lines.

Try a scan, you may have bad o2 sensors which can make it run rich.

Another problem could be a fuel injector that is leaking. You have to remove the plugs one at a time an look at the ignitor see if there is fuel on it to know which injector is leaking.

There can also be problems with idle air controls or even the main computer, but start with the scan. Most Advance and Autozones will do it for free.

Hope this helps.

Need to check for fault codes. Possibly a faulty sensor causing the fuel system to run rich or lean.