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Drastic drop in mileage

This Echo gets mileage in the 30s or even over 40 with lots of highway driving (I know, I keep a notebook!). Suddenly, with no change in driving habits, it dropped to 24! Also, after the low gas light was on for a very short time, it took over 10 galls to fill it up (was almost bone dry). Normally I can drive around for days with that light on.

Could this have been an error at the pump? Or is it more likely to be something under the hood?


There are probably 1000 possible reasons for a drop in gas mileage, includng but not limited to overdue maintenance, low tire pressure, a failing sensor (MAF,MAP, and O2 are the most common suspects), low fuel pressure (from a failing pump perhaps), change in weather, and even a dragging brake. There’s insufficient information here to speculate.

One recommendation I can make with the information provided is to stop driving around for days with the “need fuel” warning light lit. You’re pushing your luck here. The pump uses the gas to keep itself cool, and you may just fry the pump.

In addition to mountainbike’s sound advice, here is another possibility to consider:
Your recent “fill-up” may not have really filled the tank, and as a result you may just have been running around with a lower volume of gas than you thought.

Calculate your mileage over at least 3 full tanks of gas in order to come up with an accurate mpg figure. Just be sure not to overfill the tank. When the pump clicks off for the first time, don’t force any additional gas into your tank.

“The pump uses the gas to keep itself cool, and you may just fry the pump.”

I keep reading this and I just can’t get my head around it. If you can’t ever run out of gas without making it impossible to start the car, how is this not a major design flaw?

It doesn’t “fry” the pump immediately, but repeatedly running the tank dry can cause premature failure of the fuel pump. This is over a period of years, not minutes.

Thanks everyone for these ideas. Tires may be low-ish. The check engine light, a sensitive creature, has not come on, so I can rule out sensors, right?
I was not aware that apart from being dumb for the obvious reasons, driving around with the low fuel light could hurt the fuel pump. Heard and understood.
This car has 137K and I bought it with 101K. I have not changed the fuel pump. Does anyone know the life span of those things?
And, VDCdriver, I calculate every time using the trip odometer, and my notebook has a running list. Are you saying to run the trip odometer for 3 full cycles and just keep track of the gallons over those three?
I did notice that my last calculation seemed high, so maybe that was it. I am anxiously awaiting the next one.
Hey, while we are here, this is a commute car, and my commute is about 2.3 miles round trip, 4 days a week. Needless to say, we go a whole year before we drive 5000 miles. I have always gone with Tom and Ray’s advice that 5000 is a better number for oil change than 3000, but if that means almost a year, then do I need to revert to the ‘suggested’ 3 months? What about 6 months instead?

I would change the oil at least every 6 mos. But if your low yearly average means a lot of short trips then you should do it more frequently. 5,000 miles in short bursts is worse for a car than 20,000 long trip miles. This won’t have much impact on your gas mileage though - a little, maybe.

In addition to the things mentioned already, a healthy and well-functioning cooling system is one of the most important things in fuel economy. Having that evaluated - especially thermostat evaluation and the coolant temperature sensor - is a good idea.

If your previous fuel mileage was unusually high you have your answer. For whatever reason the previous fill up didn’t fill the tank.

Cigroller mentioned a sensor that if bad won’t throw a code…and will caue a drop in mileage…the temp sensor! Nice catch Cig. Other good comments too…I often don’t think of the cooling system with these type of posts.

The fuel pump should last the life of the car. Since you don’t mention ever actually running out of gas, I have to assume that even though the light was on you’ve had enough to keep it cool. Most of the heat actually gets dissipated from the gas poassing inside the pump, so if you haven’t run out the pump should be healthy.

Every 6 months is also good advice.