Sudden drop in gas mileage on a 2004 Chevy Cavalier

I suddenly experienced about an 8 - 10 mpg drop in gas mileage. The car seems to run fine. Any ideas what is thew cause?? Thank you!

Do you do a lot of short trip driving?
How long have you owned this car?
Have you checked your tire pressure?
Is the Check Engine Light illuminated?
What is the maintenance history of this car?
What is the viscosity of the oil in your crankcase?
How long do you warm up the engine before driving it?
Do you use drive-up windows at banks, fast food joints, etc?

It is all highway driving.
Owned the car for 2 years.
Tire pressure is checked weekly.
No engine light.
Car maintained well.
5W-30 oil is used.
Engine warmup is 1 minute or less.
No major idling.
This problem has just recently reared its ugly head.

Do you. like me, live in a place that recently experienced a too-long cold snap? Somewhere on the east coast perhaps…all the way down to Miami?

Does the car produce the same amount of heat that it always has? Has its temperature gauge been behaving normally?

You might want to elaborate a little on “maintained well.” Some people sort of think that regular oil changes at the local Jiffy Lube counts as good maintenance. Spark plugs? Wires? Air filter? Fuel filter? Alignment?

You should expect some mpg drop in the winter regardless, though 8-10 is probably too much. How do you figure your mileage?

Two things should be checked when there’s a sudden drop in fuel mileage. These are the thermostat and the coolant temperature sensor for the computer.

If the thermostat is stuck partially open, the engine may not get up to full operating temperature to go into the closed loop mode. So the engine runs rich using more fuel.

If the the coolant temperature sensor for the computer has failed where it’s telling the computer the coolant temperature never gets to full operating temperature when it actually does, this will cause the engine to run rich using more fuel.


I live in Cleveland, Ohio in the “snowbelt”

This is actually my son’s car. We bought it off a guy whose dad owns a Chevy dealer and all maintenance up to 80,000 was done at the dealer. Then we took over and it has 93,000 on it now. I will have to get back to you on this after I ask him some of these questions. Thanks for your input.

If coolant temp sensor fails, will it trigger the check engine light???

No. A faulty coolant temp sensor won’t necessarily cause the check engine light to come on. A failed one will.

The coolant temp sensor is one of the primary inputs to the computer. If the computer see’s a signal from the coolant temp sensor it thinks it’s valid. So the computer adjusts the fuel mixture based on the invalid signal from the faulty coolant temp sensor.


A faulty oxygen sensor severely dropped the mileage on my minivan about 8000 miles before it triggered the check engine light.

How long have you had this car? If you got it last summer, it may just be the winter/summer difference and totally normal.

Over how long a period have you checked this mileage? If it was just one tankful, the pump on the previous tank may have tripped off too soon and then the next time you filled the tank, more gasoline went into the tank. Therefore, it seemed as though you went a shorter distance on more gasoline.

I assume you are computing mileage by dividing the distance between fill-ups by the amount of gasoline to fill the tank. If your car has an miles per gallon readout, somethimes these things don’t give an accurate reading.

Finally, has anyone else been driving the car? Wnen my son was a teen-ager, we had a Ford Aerostar. He could milk more miles from a gallon of gasoline then anyone else I’ve ever seen. The mileage was always better on the Aerostar after he had been using it than what I would get with the same vehicle under the same conditions. You may have the reverse situation where someone doesn’t drive conservatively that uses the car.

Well for 2 months this winter it was getting about 30 mpg and just this past week is when the mpg dropped significantly.

I know where you are coming from with the “teenage” angle, but he doesn’t have a lot of money and drives carefully to “save” money. We will keep monitoring it. We will run thru this tank of gas and fill up where we normally do and see what happens mpg-wise. Thanks

NEW INFO…We have determined that gas mileage drops dramatically when the switch is in the “windshield defrost” position. I believe in this position the compressor runs all the time. Is this crazy or what??

Yes, in the defrost position, the compressor runs to help dehumidify the air to clear the windshield more quickly. Unless there is something wrong with the compressor causing a severe drag on the engine, there isn’t much you can do. I would guess you are driving around town in cold weather. This will cause the mileage to really drop.

Open the hood. With the engine running, put the heat and a/c controls to OFF. Look at the a/c compressor. Is it running? Put the controls on “windshield defrost” position. Does the a/c compressor engage, and the engine rpm remains the same (or, maybe rpm increases)?
An a/c compressor doesn’t run for free, and the engine computer boosts fuel injection when the a/c pressure switch shows the a/c is operating. These actions require additional fuel.

I’m the son of cardud.

My weekly drive looks like this:
Mon: 80 miles (4 city, rest highway)
Tue-Thur: 66 miles (2 city, rest highway)
Sat: 24 miles (all, light city)

As you can see, the majority of the time, I am driving on the highway.

As my father has stated, I believe I narrowed this problem down to the defroster.

Yesterday’s Trip:

Drove ~34 miles with the heater on (front and bottom vents) - Gas Gauge was decreasing normally - Tank was slightly below the Full mark

–2 Hour Break–
Drove ~7 miles heater on - no problems
–3 Hour Break–

Drove less than 20 miles with the defroster on - Gas Gauge was decreasing rapidly. - Tank was slightly above the 3/4 mark.
Drove ~20 miles with heater on - Gas Gauge decreased at a normal rate - Tank was at the 3/4 mark.

As you can see, with the defroster a lot of gas was being used rapidly. My tanks holds about 14.1 gallons. A 1/4 of tank would be 3.5 gallons. I would estimate that I probably used about 1.5-2.0 gallons for the 20 miles that I drove.

I realize that the gas gauge isn’t always completely accurate, but it was appearant that the gas gauge was decreasing much faster than normal with the defroster on.

I used use my defroster all the time during the winter months and particularly the last two months without any noticeable loss in gas mileage.

Thank you to everyone who has give advice so far!

The car is producing heat like normal. The temperature gauge is in the middle at 195.