1987 Mercury Grand Marquis mileage: 132,000. I cannot pass emissions testing. I first brought my car in and failed on HC and CO. About a week later I tried again burning premium fuel and after adding some emission system cleaner. I was still high on CO and HC. So then, I changed the air filter spark plugs and ignition wires. I then went back and tested again. I added some rubbing alcohol to my fuel tank to try to get it to burn a bit cleaner. My CO and HC jumped about 50% each. I do not want to sink too much money into this car. What should I do? Oxygen Sensor? Get the computer checked?