has a new o2 sensor and coolant temp sensor. car runs fine but its fouling out plugs and thining the oil to the point where if I put 40 weight oil in it 3 days later its so thin that the lifters are ticking. And needless to say it gets shitty gas mileage.

Where should I start my troubleshooting? Im not sure where to look for the problem. Oh and the check engine light is not on.