Mechanics

GAS MILEAGE DROP GS300

2000 Lexus GS 300 • 6 cylinder 2WD Automatic • 180,000 miles

Gas mileage suddenly drop in 2000 Lexus current miles on car 180000. Lost 60 miles on a full tank of gas. This was a sudden drop. Replaced oxygen sensors as recommemded by the dealer but still no improvement in mileage. Car always serviced by dealer since purchased.
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Patel.nic
August 1, 2011.




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.

Caradiodoc
Aug 1, 2011.
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

thanks
aatef
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Tiny
Aatefalikhan
Sep 4, 2013.
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

Tiny
Aatefalikhan
Sep 4, 2013.
Your mechanic will do many of the things I listed, typically starting with checking if there are any diagnostic fault codes. Next he will likely look at the short and long-term fuel trim numbers. If they are high positive, the computer is adding fuel beyond what was programmed in at the factory in response to something. He has to figure out what that something is. If the numbers are high negative, the computer sees there's too much fuel going into the engine and is trying to reduce that amount without success. The question then is why did the computer lose control of fuel metering or how is that extra fuel getting into the engine?

If the fuel trim numbers are close to zero, the computer is happy with the way the engine is running and there is some other cause for the poor fuel mileage. Most commonly that is due to a dragging brake or an automatic transmission's torque converter that isn't locking up at highway speeds. A misadjusted brake light switch can cause the torque converter to unlock repeatedly. You can watch for that if your car has a tachometer. With the engine warmed up, in third or fourth gear, and above 45 - 50 mph, the torque converter should be locked up. Maintain the car's speed and hold the accelerator pedal steady, then tap the brake pedal with your other foot. You should see engine speed increase about 200 rpm for a couple of seconds, then go back down when the torque converter relocks. If the engine speed doesn't change when you tap the brake pedal, the torque converter is not locking up and fuel mileage will drop. Try holding the brake pedal up with your toes. If that makes engine speed drop, suspect the brake light switch is out of adjustment.

Caradiodoc
Sep 4, 2013.

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