Oxygen Sensor related to reduced fuel economy?

[KingstonU]

My 2004 Kia Rio with 100,000 km ( 62,000 miles ) has lost some fuel economy over the last year, going from an average of 4.7 l/100k ( 50 MPG ) to now being closer to 6.5 l/100km ( 36 MPG ).

What could be causing this? Oxygen sensor going bad? I should be able to replace this myself right?

Could anything else be causing it?

Thanks,

 

[Vette73]

Do you have a K&N filter (or some other high flow filter)? If so could be dirty mass airflow sensor.

Have you replaced (if it has one) Fuel filter, air filter, PCV valve, and thermostat and radiator cap yet?
If not do all of those. Good 60k items that can affect performance/fuel economy. Also spray out the throttle body with carb cleaner.
Check tire pressure and also did you replace your tire right before this drop?

Also no engine lights on right? and did this drop in MPG just happen or slowly come on?

 

[Throckmorton]

Could be. That's pretty common... or maybe it's just something cheap like a clogged air filter. Try swapping in a new filter (high quality brand name paper filter, not K&N).

 

[KingstonU]

It's been happening slowly over the last year (putting 25,000km on it in the process). No engine light.

When it first started decreasing I though it was due to it being winter weather + winter tires, but then summer came + summer tires and the fuel economy didn't improve. Tire pressure is good.

I do oil change every ~6,000 km, air filter change every 12,000 km (though I think I need to replace it soon, and standard air filter, not K&N).

The fuel filter seems cannot be replaced on my car.

I have not changed the PCV valve, and thermostat and radiator cap. How much would all these cost and are they difficult to do?

Could doing all these restore fuel economy back or close to 50 MPG again?

Thanks

 

[Vette73]

^

Maybe. Start with the easy and basic stuff and worry about the costly and hard stuff later.

They are easy and cheap to do yourself and could help. Also spray out the throttle body to clean it up. And spray a little into the hose for the PCV valve.


EDIT:

Just went to rock auto and the Rio in 04 has a fuel filter listed. Only cost $15ish

 

[JeffreyLebowski]

^

Maybe. Start with the easy and basic stuff and worry about the costly and hard stuff later.

They are easy and cheap to do yourself and could help. Also spray out the throttle body to clean it up. And spray a little into the hose for the PCV valve.


EDIT:

Just went to rock auto and the Rio in 04 has a fuel filter listed. Only cost $15ish

But where is it? Is it in tank or inline? If it's intank, you're best off dealing with the crap gas milage than spending the money to have someone drop the tank, pull the fuel pump and change the filter.

 

[Vette73]

But where is it? Is it in tank or inline? If it's intank, you're best off dealing with the crap gas milage than spending the money to have someone drop the tank, pull the fuel pump and change the filter.

No its an external one. Looks like a regular external one. You can even buy it with the bracket attached to make it easier to install.

 

[KingstonU]

No its an external one. Looks like a regular external one. You can even buy it with the bracket attached to make it easier to install.

Hmmm, I could have sworn that last time I looked into this it was not viable, I think do to it being inside the gas tank, and other people online with this model came to the same conclusion, but I will look into it again.

I'll replace that air filter and then try and do the other stuff this spring, thanks for the advice!

 

[wirednuts]

50mpg? are you sure? 36mpg sounds more like what its supposed to get.

 

[Silex]

A broken O2 sensor in the exhaust manifold will lead to lowered fuel economy. The one (or more) in the cat just compares the AFR between the sensors and throws a CEL if it's beyond a certain threshold.

 

[SuperSix]

Fuel filter is possibly located on firewall. I don't know of any mass production cars that don't have fuel filters, or that are located in the gas tank.

There's a strainer/sock in the gas tank, but not the actual fuel filter.

 

[Viperoni]

$20 says its not the fuel filter on a car with only 60k miles.

I would start with replacing the PCV valve, cleaning the TB, replacing the spark plugs, cleaning the MAF sensor (if there is one) and then the o2.

 

[KingstonU]

I should clarify how I measure my fuel economy. I have a ScanGaugeII hooked up to my OBD-II port, it reports back average fuel economy in liters per 100 kilometers (L/100 km). A year ago it would always report between 4.5 and 4.9 L/100 km avg (60,000 km on the car then). That number has increased over the last year and now it reports between 6.1 and 6.9 L/100 km (100,000 km on the car now).

This is on the same route and I can also confirm just by how much distance I cover before my tank is empty.

 

[5150MyU]

Lazy O2 sensor-but alot of other things go(dumb) neutral also.-won't kick code.
Fuel filter is on drivers side front of tank.

 

[T2urtle]

I should clarify how I measure my fuel economy. I have a ScanGaugeII hooked up to my OBD-II port, it reports back average fuel economy in liters per 100 kilometers (L/100 km). A year ago it would always report between 4.5 and 4.9 L/100 km avg (60,000 km on the car then). That number has increased over the last year and now it reports between 6.1 and 6.9 L/100 km (100,000 km on the car now).

This is on the same route and I can also confirm just by how much distance I cover before my tank is empty.

not saying the SG2 isnt a decently accurate thing to read out MPG, its better then most but doing it by hand is always more accurate. miles driven divided by gallons filled.

Yes your o2 sensor could be the trigger, a CEL happens when the ECU knows that the o2 sensor is reading COMPLETELY wrong. By completely wrong i mean way way too high/low then normal operating numbers. If your ECU is a bit slow or reading just a small bit higher/lower then normal then it might not set a fault in the ECU. But your MPG could be effected.

You can replace the o2 sensors yourself. You need a special socket, or a modified one, go under and find the sensor, unclip and unbolt off. Rust might be a big thing holding you back or strength to get it off. Also you have 2 sensors, Up/front/pre-cat sensor and down/rear/post-cat sensor. Most cases i would just replace the front. Since the rear just compared numbers with the front.