Kia Rio Poor Mileage



Hi Folks. My 2003 Kia Rio, 31K miles, was getting good mileage (35-40 mpg) until a few months ago, when the mileage and performance steadily dropped, in a few months, to about the 22 mpg range, where it is today. There has been no change to my driving habits. The mileage is similar with or without a/c running. The acceleration is boggy in the midrange–it gets going ok, then the acceleration gets mushy until I stomp on the gas pedal, when it increases again.

I have had the car inspected, have used several bottles of fuel injector cleaner, and last November, replaced the plugs. Yesterday, I checked those plugs, and they are covered with black soot. I replaced them again, and the mileage has improved (I went on a 450-mi car trip yesterday), going up to about 35 mpg with the a/c on.

The smartest car people I know say there has to be something wrong with the fuel mixture, and they think there is most likely a problem with 1. oxygen sensor; or 2. catalytic converter. (one person said a far-fetched possibility is 3. timing belt). The check engine light is off.

My questions: A. Would the check engine light come on if the O2 sensor had failed? B. What about if the timing were bad–would that necessarily turn on the check engine light (oh for the days of timing lights)? C. How should I proceed in getting this car repaired? Everybody, even mechanics, seem confused and they don’t think that 22 mpg is enough reason to do anything. Somebody has suggested a fuel-system cleaning, but I don’t think that’s going to help, considering what I saw on the 9-month old plugs yesterday.

Thank you for all your help.


The black soot on the spark plugs does indicate an overly rich fuel mixture. Have you changed the air filter? If not, this might be a place to start.


Something is making the computer adjust the fuel mixture extra rich, which is why the plugs are black.

Is the engine reaching correct operating temperature? A bad thermostat would keep the engine running too cool, which might cause the computer to keep the mixture rich, as it does at cold start.

Another possibility is a faulty engine temp sensor sending a bad signal to the computer.


[b]There are a couple of things to check when the fuel mileage drops.

The thermostat. If the thermostat is stuck partially open, the engine doesn’t come up to full operating temperature. This causes the engine management system to stay in the open loop mode which uses more fuel.

A defective coolant temp sensor. If the coolant temp sensor for the computer has failed to where it’s telling the computer that the coolant temperature never gets above -10 degrees, the engine management system will stay in the open loop mode.



All routine maintenance has been done–new platinum plugs, wires, air filter, oil changes, premium gas, blah blah blah. I took the car to an independent mechanic today, and he thought that the problem was likely due to a thermo problem, a bad coil, or a problem with the air-fuel mix. And he said I probably won’t know what it is unless I take the car to the dealer for diagnostics. So that’s what I’ll do.