There are dozen of things that reduce Toyota Corolla's gas mileage. Unfortunately, though, most are very difficult to diagnose. Nevertheless, let's see what we can determine.
Possible Causes of High Fuel Consumption in Toyota Corolla
(1) Underinflated tires.
(2) Clogged air filter.
(3) Clogged PCV System.
(4) Faulty oxygen sensor.
Check the tire pressure with a gauge or simply inspect the tires. If they are sort of bulging out around the buttons, they are low. Now if the tires on this car are low on air then underinflated tires causing poor gas mileage. The lower the tire pressure, the more friction or drag between the tires and the road. This excessive friction/drag causes the engine to work harder. Hence, gas mileage suffers.
What to do?
Add some air to the tires. Make sure you don't exceed the pressure level as specified in this car owner's manual.
However, if the tires on this car not underinflated, then the tires aren't the problem. Incidentally, if the tires were low, they'd create excessive drag and thus cause poor gas mileage but since the tire is not low next to check is the filter.
Now, Locate the air filter. Refer to a manual that covers your car if you need help finding it. Remove the filter and closely examine it.
If there a lot of dirt in the filter then clogged air filter is causing poor gas mileage. A clogged air filter essentially starves an engine of air. This means the air/fuel mixture entering the cylinders will be too rich (i.e. too much gas in proportion to air). As a result, the engine burns more gas than necessary poor gas mileage.
What to do?
Replace the air filter. Any auto parts store will carry a wide selection of filters.
However, if the filter is clean, next to check is the PCV valve. Locate the PCV valve. Refer to a repair manual for this car if you need help finding it. Then, start the engine and let it run for several minutes until it reaches operating temperature. With the engine idling, pinch the hose leading to the PCV valve so as to prevent air from flowing through it.
If the PCV system is working, there should be a noticeable drop in the speed of the engine as you pinch the hose. This is normal. However, if the PCV system is clogged, there won't be a drop in engine speed as you pinch the hose. In other words, the engine speed will remain constant.
Now, if the engine speed remains constant a clogged PCV system is causing poor gas mileage.
As indicated above, the engine speed remained constant as the hose was being pinched. Unfortunately, this isn't normal. You see, the engine speed should have gone down as the hose was being pinched. However, since it didn't, the PCV system must be clogged somewhere, probably in the hose or the PCV valve. A clogged PCV system causes the air/fuel mixture to be too rich (too much gas, not enough air). As a result, the engine burns more gas than necessary (which of course translates into poor gas mileage).
What to do?
You'll need to have a mechanic replace either the PCV valve or its connecting hose, whichever he determines is clogged. Have this problem fixed the soonest. If you ignore it, you will waste money because of poor gas mileage and worse, the overly rich air/fuel mixture will damage the catalytic converter.
In a situation like this, some mechanics may offer to replace several other items related to the above. For now, just stick with the replacement of the PCV valve or its hose. This should correct the problem. You can always return at another time for the other items the mechanic suggested.
If the engine speed remains constant then the oxygen sensor could be faulty. An oxygen sensor is a gadget which constantly measures the oxygen level in the exhaust gases as a way to indirectly determine how rich or lean the engine's air/fuel mixture is. The oxygen sensor's data is sent to the vehicle's on-board computer which then makes a calculation and adjusts the air/fuel mixture if necessary.
Clearly then, if the oxygen sensor is faulty, it will send bad data (or none at all) to the on-board computer. This will adversely affect the resulting air/fuel mixture. For example, the air/fuel mixture could end up being too rich (too much gas, not enough air), therefore causing poor gas mileage.
What to do?
First off, we must determine if the oxygen sensor is actually faulty. So, take this vehicle to a garage for a scope-based diagnostic checkup. The mechanic will hook your engine up to a scope analyzer from which he should be able to tell if the oxygen sensor is the culprit. If so, have it replaced.
Try to find a garage that is approved by the AAA. Such garages tend to be reputable and, best of all, if you have a dispute after any work is done, the AAA has the policy to investigate the situation and resolve it.
How to Replace Toyota Oxygen Sensor
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