Weird Fuel Consumption Problem

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?

How are you measuring the mileage?

You might want to check for a thermostat that’s stuck open. If the thermostat is stuck open it takes forever for the engine to get up to operating temperature thereby the engine uses more fuel.



This is the official government EPA website.

I agree with @Tester . . . check and/or replace that thermostat and make sure all the maintenance items are up to date (filters, plugs, etc.)

I also agree with @JosephEMeehan . . . that website also will tell you the official/correct EPA way to measure fuel economy. Believe it or not, many people do it incorrectly or just guestimate.

"...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."

I can’t figure out what this sentence means. If you are comparing mileage on a long highway trip versus around town short trips where the engine simply doesn’t ever get warmed up, the entire explanation could be purely this difference in usage - for two reasons. When the engine doesn’t reach normal operating temperature, it simply runs less efficiently. Not only that, but stop and go driving means standing still while the engine runs, and then shifting through the lower gears, all of which impacts gas consumption substantially.

As implied by @db4690, you can’t judge your mileage by estimation. Fill your tank, use the car, when your tank gets low, fill the tank exactly the same way: same station, same pump, stop at first click. Do the math to see how many miles you went on that many gallons. For better accuracy, measure this over multiple tanks to reduce the impact of slight variations in where the pump shuts off.

If you have a temperature gauge rather than a light, it should settle somewhere in the middle of the range once the car is warmed up. If it stays on the cold end, that’s a sign of what @Tester suspects, that your thermostat is at fault. That would prevent the engine from reaching normal operating temperature, which will impair fuel mileage. Thermostats do need periodic replacement, they’re cheap, so many people replace them when changing coolant as a preventative. I’d suggest you also replace the radiator cap too, it’s about the simplest repair possible, and I think it could also impact the coolant temperature if it were worn out. But caution: Remove the cap ONLY when the engine is cold!