For such a sudden change in fuel mileage, either the engine has a running problem or it is working harder to overcome something. That something could be a dragging brake. A sticking parking brake cable or sticking caliper piston could be the culprit. Stop on a slight incline, shift to neutral, release the brake pedal, then see if the car creeps downhill on its own. If it does not, look for a brake that feels hot after driving on the highway.
The oxygen sensors are rarely the cause of low fuel mileage. When they fail to respond properly, a diagnostic fault code will be set in the Engine Computer and that will turn on the Check Engine light. Most of the time it just reports what it sees, so replacing them isn't going to change what's in the exhaust.
Anything that causes extra unburned oxygen in the exhaust will lead to higher fuel consumption. A vacuum leak will introduce more air into the engine that the computer doesn't know about. The unburned oxygen will show up in the exhaust and be reported as a lean condition. The computer will respond by requesting more fuel. A misfire can have the same result. Unburned fuel and oxygen show up in the exhaust, but O2 sensors don't detect fuel, just oxygen. They will report that unburned oxygen from the misfiring cylinder as a lean condition, then the computer will request more fuel to all the cylinders on that side of the engine. No matter how much extra fuel is requested, there will still be that unburned oxygen being detected. You might even smell the unburned fuel at the tail pipe.
A leak in the exhaust system ahead of the first oxygen sensor can also cause high fuel consumption. In between the pulses of exhaust flow, the momentum creates pulses of vacuum that can draw in outside air. The oxygen in that air will be detected, again, as a lean condition, and the computer will request more fuel. No matter how much extra fuel it adds, there will still be that unburned oxygen sneaking in so it will continue to report a lean condition even though there's way too much extra fuel going into the engine.
Answered by caradiodoc (expert)
Hi caradiodic !
Got the same problem with my GS 300 (2000) since 6 months back. Before that it was running fine with a 375+ miles on a full tank. Now it have dropped between 285 and 320 miles per full tank. With all the causes u recommended can u specifically point which things need to be checked with the mechanic
Wednesday, September 4th, 2013 AT 1:56 AM