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Problems That Can Be Causing a Fuel Economy Drop

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Sometimes, the symptom is something less substantial like a loss of fuel economy. Symptoms like this can take a while to notice and then baffle the DIY mechanic who's wondering why it is that measured MPG is suddenly lower than it has been in the past.

Assuming obvious things like low tire inflation, bad fuel, and a defective MPG readout (if using your built-in trip computer) have been ruled out, there are other things to check.

Sensors to Test

As oxygen sensors age, they can begin to become slow in their returns to the engine computer, which will likely be interpreted as a lean mixture. The computer compensates by adding more fuel to the mix. This can go on for some time and as the fuel being added is relatively small, the engine can burn it off without anything else going awry. So the only indication that this is happening is a loss of fuel economy. Pulling the first and (if a V-style engine) second O2 sensor(s) for a quick test can solve this issue or rule it out as a problem.

A coolant sensor going bad can also cause fuel economy loss. If the sensor reads low, keeping the computer thinking the engine is "cold" longer than it should, then the higher RPM used to bring the engine up to temperature when started cold will last longer. This, in turn, means more fuel is used. A cold engine requires higher amounts of fuel in order to keep from stalling.

Valves to Check

Valves involved in the intake of air, distribution of coolant, or change in air:fuel mixture are also suspect. Check the thermostat, which could be faulty and opening early. It could also be stuck open.

Check the EGR Valve for leaks. This could be causing less than expected amounts of exhaust gas to return to the air:fuel mix, which in turn causes the sensors to read "lean" and the computer to add more fuel. If the change is small enough, no codes will trip in the computer.

Check the intake manifold for vacuum leaks. As with the EGR valve, any disturbance in the air:fuel mixture can cause it to become lean, or read lean, resulting in more fuel being used. The smallest of pinhole leaks in the intake or EGR can throw things off.


A misfire is often unnoticed by the driver, especially if it's relatively consistent and only happens at higher speeds when the engine is running at higher RPM. Misfires during idle are usually easy to feel, but at higher speeds, not so much.

Misfiring is suspect if the fuel economy of the car has suddenly dropped, especially highway economy. Most misfires will trigger a code and engine light, but you can't always count on that. Scan tools that are sophisticated enough to read the engine as it runs, even at speed, can find a misfire where the engine's computer may not be.

Additionally, beware the "random misfire code" (P0300), as this is more likely a vacuum leak causing an occasional misfire from a lean mix.

Other Causes

Worn and fouled spark plugs, dirty injectors, oil taht is too thick for the engine, a dirty air filter, clogged or restricted exhaust, or low engine compression are also causes of low MPG returns.

Other things to check are dragging brakes, a clutch slip, or transmission hesitation. A faulty torque converter can be a common problem in older vehicles that suddenly begin returning bad fuel economy. Other symptoms will usually include a hesitation or reluctance to speed up on the highway or a "slipping" of gears when slowing from speed.