Questionthe car works fine, but is wasting plenty of gasoline. A simple trip eats up a quarter tank. There is no smell of gas. Where should I start to look.
Answer First, take all four spark plugs out, one at a time, and check their color. They should be a light brownish-gray. If one of them is very dark or black, then the injector on that cylinder may be malfunctioning by not closing all the way. This can be simple injector failure or a wiring/computer problem. Easiest way to determine which is to switch it with an injector on a cylinder with good color and drive it for a hundred or so miles. If you check the plugs again and the problem has moved to the other cylinder, then the injector is bad. If it stays on the same cylinder, then the problem is either in the harness or the ECM itself. Without going into a lot of technical talk, the computer can fail in a manner that causes the injector to ground out and stay open all of the time.
If all of the plugs are the same color, then the most likely culprit is the coolant temperature sensor. When they fail they cause the computer to think that the car is cold all of the time. So, the computer gives you the over-rich starting mixture all of the time and never goes into closed-loop operation. From what you said about the large amount of fuel being used, this is the most likely scenario, providing that the injectors are all good. So, check the spark plugs as mentioned above first.
The last possibility is that the oxygen sensor is bad. The ECM can not always tell if it is bad, and will compensate by running rich. Almost all failure modes in a computer equipped car, whether the OBDII, like yours, or the earlier types, involve running rich. This is safety measure to prevent the possibility of running lean, which can burn up the valves and even burn holes through the tops of the pistons.
I would be sorely tempted, if the plugs all look the same, to take it to a qualified mechanic with a proper real-time engine analyzer and get him to check all of the sensors and controls to see if he can see what isn't working. It should be a snap for him and could easily save you the cost of a couple of unnecessary parts.
