2000 Corolla mpg going down

[Nil]

Hi,

This is my first car (bought with 127K miles on it, now has 128300). I have owned it for about 3 months now (since Jun 2009) and initially it gave a mpg of around 27-28. However, only very recently (past 2 weeks) the mpg has gone down to about 22-23. I don't drive much on the freeway, mostly inside the city in SLC. Since this is a CE model, it does not have a tachometer, so I can't tell the RPM, but I have not changed my driving habits at all.
Also I recently did my oil change, the tires are at 32 psi and replaced the air filter.
What could be wrong and where do I start looking? I heard that the o2 sensor maybe a potential culprit, but how can I isolate the problem.

Thanks a lot for your help.

[Nil]

Also the exhaust is coated with heavy black soot.

[fishexpo101]

There are many possible culprits for lower fuel economy.

Only way to tell for sure if the O2 sensor has gone or not - is to backprobe the sensor. Of course, you can just replace the sensor - the car is of the right age and mileage to suspect a failing O2 sensor. It is highly recommended to replace with an OEM Denso O2 sensor - there have been mixed results with aftermarket and universal sensors - cost difference is minimal if you shop online.

Tire air pressure can be plumped up to 35PSI - seems to be a good compromise of comfort and fuel economy for most people. Spark plugs have been replaced recently? Comprehensive tune up might help - cleaning the throttle body, clean/replace PCV valve, clean MAF sensor, etc.

Have you noted any oil consumption? How often do you change the oil and what viscosity do you use? OEM recommends 5w-30 motor oil - some brands run "thicker" than others - can make a slight difference in fuel economy.

May just need to run the engine out. Lots of slow speed, city driving can cause the plugs to foul up easily. May need to run the car on the freeway every once and while to blow the carbon deposits out of the engine. A bottle of good fuel injector clean can help, especially in areas where gasoline quality varies.

Soot inside tail pipe is pretty normal - when you notice smoke pouring out of the exhaust, then you should worry.

[Nil]

Thanks fishexpo,
so I revved my engine and a lot of white smoke came out of the exhaust - now I should really worry. What can this be due to ?

[Nil]

So my understanding is this is being caused by the coolant getting into the engine and getting evaporated.

If that is the case then some gasket is messed up .. can you tell me how much I am looking at for repairs ?

[The ///Man]

race cars can require a little extra maintenance over your ordinary car.

[fishexpo101]

Originally Posted by Nil

So my understanding is this is being caused by the coolant getting into the engine and getting evaporated.

If that is the case then some gasket is messed up .. can you tell me how much I am looking at for repairs ?

White smoke from a cold start could be from condensation, which is normal. But if you get this after the engine is fully warmed up and it doesn't have the usually gasoline odor - could be either a coolant leak into the engine or heavy oil consumption. You notice that you have to constantly top off the oil or if you notice that coolant is disappearing - that will tell you what might be happening.

If the leak is significant - your motor oil should start taking on the consistency of mayonnaise and or have a lighter color than usual.

Really need to do chemical testing to find out for sure. There are several used oil analysis places that can tell you what type of contamination you have in the motor oil. I personally use Blackstone Labs - good price and quick turnaround.

Headgasket blowing out is not common on the 1ZZ-FE engine, but it can happen. Cost wise, depends on where you live at - but could be anywhere from a few hundred to several hundreds of dollars to fix it. Find out if it is a head gasket leak before you start worrying about how much it will cost.

[Nil]

I took my car to a mechanic who said that my spark plugs & upstream 02 sensor was messed up. He stressed that the oil and the coolant condition does not indicate a coolant leak into the engine. The mechanic guy also said that if it were a blown head gasket then my engine would significantly overheat - which it does not. However he said that there might be some leak in the valve seals and hence some oil maybe leaking into the combustion chamber causing the bluish white smoke on high acceleration. I infact smelled the white smoke that came out when I revved the engine and it smelt like burnt oil, so I am assuming that it is not a coolant problem.
After replacing the spark plugs and the 02 sensor, my car is running smooth and I ran it for some 50 miles on a 40-50 mph zone and it gave me a mpg of 35.
Hopefully this is the end of the problem. However I am still planning to get the oil test done just in case. Thanks a lot fishexpo for your help.
People like you help poor foreign grad students like us to scrape a decent life here in the US.

[Nil]

So everything seems to be running fine, but out of curiosity I checked under the hood, and the coolant level seemed pretty low. So I took it to the Jiffy lube shop where I get my oil change done and they topped off the coolant . However they put some green coolant in it and I insisted that the coolant in the radiator is reddish in color.
I am worried if this intermixing of coolants of different types is a bad thing and I should get a radiator flush done to prevent in future problems.