1998 Corolla Fuel Mileage Issue - Toyota Corolla, Chevy Prizm (1998-2008)

Hey everyone.

I looked through the last few pages of posts and did not see this topic so I thought I would ask as a new thread.

I drive a 98 Corolla CE 5 speed. When I first bought it 2 years ago I was averaging right around 38 mpg. Lately I have noticed a steady drop in fuel efficiency, and as of my last fill up was down to 31 mpg. As you can see that's like a 20% drop. It has started to get cold here in Wisconsin, but the mileage decrease started while it was still relatively warm. I also noticed the last couple of days that when I start the car after it's been sitting (overnight or all day at work) it does not want to run on it's own. What I mean is I have to have my foot on the accelerator and keep it pressed a little to keep the car going. My other vehicles (Venture (spit) and Sienna) seem to adjust themselves and the engine revs stay up on their own.

What has been done to the car lately in an attempt to get things back on track: I put in a K&N air filter. I have changed the plugs (asked about it here). I am running Mobil 1 5w30 synthetic.

Other than the mileage and start issues, the car seems to be running fine. Good power, idles good when warm, no CEL, etc...

My question is, what could be causing my low mileage and cold start issue, and is it possible the same thing is causing both?

Thanks for any help and/or advice you might have.

I know that some areas start rolling out the "winterized" gasoline well before it starts to get too cold - many areas start in September, others might start shortly after summer - usually running this gas until spring, before they switch back to "summer" gasoline. In the past, running this oxygenated fuel can easily knock off 20-25% fuel economy, due to the lower power potential and particular blending.

Also, depending on your mileage - possible that the upstream O2 sensor is starting to die on you. If the mileage starts to take a nosedive after a couple of weeks, this is usually a sign of a bad O2 sensor. Some cases the change is very gradual - so called "lazy" O2 sensors - many cases, they can be close to a failure point, but not completely fail. They generally will not trigger a CEL either.

Since you mentioned the car is stalling after a cold soak and you need to keep feeding it gas to keep it running, could be a vacuum leak somewhere. From either a worn, loose vacuum hose or thermal expansion/contraction events.

Thanks Fish.

I will look for a vacuum leak.

If I chose to replace the upstream O2 sensor, any recommendations on brand?

Thanks.

If you plan on replacing it - highly recommend sticking with an OEM style (already pre-wired with the connector) Denso sensor. Costs a little more than the other aftermarket - Bosch and others, but no one has complained that the Denso one ran "differently" then the one they had. If you shop around online, might be able to score one that is within $10-$20 of a cheaper aftermarket sensor.

Thanks Fish.

I finally got some warm weather and was able to work on the car.

It had an after market cruise control system in it that I never used, so I pulled it out.

The system did tie into the vacuum system of the car so that has been removed and the hose sealed. I'm still on the first tank of gas since pulling it out so I don't know how it has affected the mileage. It's still cold in the morning (below freezing) and the car does seem to start better. Time will tell on that issue.

I had been looking at O2 sensors and was already leaning toward the Denso, thanks for the confirmation.