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Gas Mileage Tanked

Hi all. I own a 04’ Jeep Wrangler and the gas mileage has taken a sudden drop. It was running around 17mpg but over a couple of months dropped to now about 9mpg. The check engine light only came on intermittently and when I took it to autozone they said the TPS (throttle positioning sensor) needed to be replaced and should fix my gas mileage problem. But since being replaced it has not made a difference.

The car seems to be running fine except once in a blue moon it will die on me without warning. Besides that it shows no other signs that it is damaged. Any suggestions on what the problem is or how to fix it?

Check for a dragging brake (stuck caliper or e-brake) Have a modern, up to date shop do a live scan of the vehicles fuel system controls, including the oxygen sensor(s) to be sure the engine is operating properly in “closed loop” mode…

Is the engine temperature behaving normally? Getting up to normal temps? Have good interior heat?

Are we to presume that the engine light is now off? Anytime you get those codes read, write down and keep the exact code - format “P1234.” If a new TPS did fix it then you probably got a little lucky. Don’t let auto parts stores people tell you what a code means. No code can tell you that you need to replace a part.

Temperatures are all reading fine. And engine light has stayed off. But its still been acting up.

In the future I’ll remember to write down the code.

Have you checked your tire pressure lately? Use the owner’s manual or the placard on your door post. Low tire pressure can kill fuel economy.

The easiest way to check for a dragging brake is to feel the wheels after a drive. A dragging brake will heat the wheel up.

Caddyman’s suggestion was a good one. When your engine is cold, the ECU ignores the upstream oxygen sensor and runs the engine rich. That’s called “open loop mode”. When the engine warms up, the ECU factors in the osygen sensor reading. That’s called “closed loop mode”. If your temp sensor is bad, the ECU may not know the engine is warmed up and will continue to operate the engine rich. Your symptoms fit this possibility perfectly.

To illustrate how much of a difference it can make, I’ve attached a technical document for my own car. As you can see, when the engine is really cold the ECU even operates the injectors twice as much as normal. That’ll kill mileage and cause operating problems if the engine is at operating temperature.

A bad upstream 02 sensor will cause a significant drop in fuel efficiency. I’ve seen them cause a drop of 25-35% in gas mileage on my cars. The only thing is if the 02 sensor was bad it should be throwing a code related to the 02 sensor instead of for the TP sensor. Did they erase the code when they checked it? If not the code should still be stored. If you don’t have a code reader Auto Zone will check codes free. You might also want to look around under the hood to see if you can find a vacuum leak.

A dirty MAF sensor will also kill the fuel mileage. A $5 can of MAF sensor cleaner can solve that problem. Don’t use throttle body cleaner or carb cleaner. And clean it with the engine cold. Let it dry completely before starting it up.