Strange Fuel Consumption Problem

[mrkjdotson]

I've recently purchased my 96 corolla from a private owner. The mileage on the vehicle is still relatively low for the age of the vehicle and the car runs pretty well. Doesn't sputter, doesn't hesitate, no smoke from the tail pipe. Starts every time.

Soon after purchasing the car I realized that it goes through fuel very quickly. I've had a diagnostic check done at my local shop and they could find no code indicating the cause of the fuel consumption and they had no ideas.

To date I've had all engine leaks repaired and had the spark plugs changed. That appeared to improve the problem somewhat. Strangely enough when the vehicle stays in motion and never sits idle, the car gets roughly 21 mpg. When the car sits idle for 1 - 3 hours fuel consumption plummets to 10-14 mpg.

From this point, my mechanic has suggested changing the map sensor and both of the O2 sensors. Can anyone think of anything else that might help?

 

[Scout706]

Do you have a locking gas cap? Where is it sitting idle? If there is no puddle of gas, or gas odor, then the fuel might be going back up the filler spout. Into someone else's car.

 

[Scott O'Kashan]

Don't waste your money changing anything.

Diagnose.

If any of the sensors were causing problems, they would cause a check engine light to come on. Since that isn't the case, the sensors aren't causing the problem and would be a total waste of money to replace.

How many miles are on the car?

Has anyone checked for fuel leaks?

How is the fuel economy being calculated?

 

[mrkjdotson]

Thanks for the input

In response to scout706:
I don't have a locking gas cap. It only sits idle when I'm in the car taking a nap before I go into the gym or go into work. No puddles or odors ever.

To Scott O:
Currently the car has 168,749 miles on it. No one has checked for any fuel leaks. I have been calculating the fuel economy by filling the tank and setting the trip meter to 0. Once the gas gauge gets down to half I calculate the mileage at that point. Most internet searches that I've done have said that the car has a 13.5 gallon tank.

 

[Scott O'Kashan]

That's not an accurate way to calculate fuel economy because gas gauges can be rather vague and also most gas tanks have a reserve above their listed capacity, which will totally skew your calculations. In other words a 13.5 gallon tank could actually be 14 gallons.

Fill the gas tank all the way to the brim so you can see the gas at the top of the fuel filler neck. Doing this will eliminate the variable of the gas pump having different pressure cut off settings and not refill the tank to the same level it was before. Record the odometer reading. Drive the car until you are at 1/4 tank or less, then refill the gas tank all the way to the top of the filler neck again. Record the odometer reading and the quantity of gas needed to refill the tank.

Now divide the miles traveled on that tank of fuel by the gallons of fuel it took to refill the gas tank. This is the only way to accurately calculate fuel economy. I have a strong suspicion you will discover your vehicles fuel economy is normal.

 

[mrkjdotson]

Thanks

Thanks. I will do that.