My car started running rough a few weeks ago. It starts fine but once it warms up it stumbles,almost stalls but catches itself before it does.You can feel it tug a little when driving. Last night the check engine light finally came on. I did the Home diagnostic and the system is flashing a Code 45 - oxygen sensor rich. Is this a new problem or could this be causing the stumbling(or did the stumbling cause this code?). Does the O2 sensor need to be replaced, or do I have more serious problems? I had the disrtibutor out a few weeks back to check on a leak, maybe its a tooth out, timming has not been set. Are these factors?

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1990 Iroc convertible,305 tpi,Black with black leather interior,lowered,Taylor 8mm wires,K&N's

 

I have the Haynes Repair Manual. It says code 45 is a rich exhaust. Possible causes are:
1. Restricted air filter.
2. Fuel in charcoal canister or crankcase.
3. Malfunctioning carb or injector.
4. High fuel pressure.
5. Defective TPS.
6. Defective MAP.
7. Defective Ox sensor.
8. Defective ECM.

Hope this helps. It's a lotta stuff, but I guess it a vague code. They usually list stuff in ease of repair.

 

First thing you need to do is to set the timing to factory spec. Also recheck plug wire routing on the cap to the plugs.

Pull a few plugs and see if it's really running rich, if so, then look into why.

Beside what's listed above, injector(s), fuel pressure regulator, CTS could also contribute. weak spark, etc...lotsa of stuff to look for, if in fact it is running rich.