89 oldsmobile custom cruiser, 307 engine, 225000 with carb. Its not a fuel injection engine hi, recently I've made a modification on the exhaust. I have cut the Y pipe and put a double exhaust, double mufflers. I know you can't do that on a newly car. Mine don't have a sensor on the cat. When I start the engine, a little light of check engine appear and the car run fine but its extremely hard on gas.30$ is nothing! Before it was economical like a V6! Someone told me it was the 02 sensor in the manifold. I've check and I got one. I don't want to replace the Y pipe and everything. Do you have solutions? Thanks alot!
If you can pull a code, that would be helpful. Also, there could be a 1 wire or 2 wire O2 sensor.
If the O2 sensor is bad, it could effect the dwell and the Mixture Control Solenoid. The MC solenoid controls carburator feedback system to maintain Air/Fuel Mixture. This solenoid on-time (dwell) is controlled by the PCM based on inputs from O2 sensor, Vacuum switches, ECT, IAT, and vehicle speed. If you have a replacement O2 sensor and no way to test the existing one, Make sure the harness plug supplies 450mV reference signal from the PCM to the O2 sensor. If it does, swap them and see if it helps. That would be a quick test. Otherwise, you should get the code(s) that tripped the MIL light on and go from there. Dueling the exhaust, (Although may be a federal no-no) shouldn't effect the fuel economy negatively. With the miles on the vehicle, it could be a multitude of things including EGR, the carburator itself, internal engine component wear, etc. You need