1998 Chevy Blazer
4.3L Votec
Automatic Transmission
4x4

Awhile back my gas mileage started to get really bad, I noticed this when I took a 200 mile round trip, and was getting around 13mpg highway. It got worse from there.

I changed my air filter, which was past due, as well as my oil and my spark plugs. The spark plugs were worn out as well. The gas mileage jumped way up immediately, I figured I was good to go.

However just recently I have noticed my gas mileage is TERRIBLE once again. I just filled up a few days ago and today I am on empty. I got under 200 miles to a tank. I believe it is in the area of 130-150 miles on that last tank, which took around 15 gallons to fill.. It was mostly 50-55mph with cruise control on. So i'm getting ROUGHLY 10mpg. I have an engine code for a bad o2 sensor. I will change it when money permits but I did not think that the it would drop it THAT much.

Could a bad o2 sensor bring my gas mileage down to 10mpg?

Like I said before, after the tune-up it was getting good mileage again. The car runs great. It starts up without a problem, and does not miss. It does not lack power or anything like that.

Are there any suggestions to correcting this?
Thanks in advance.

 

Yes, a bad O2 sensor can cause bad milage providing it is one before the cat. Change out the sensor and let us know how things work out.

 

Okay, thanks. I will follow up after I change it.

 

So I noticed that my check engine light turns off randomly. It shut off yesterday and has not been on since. But, again randomly.. it will come on. The engine light is on for one of the O2 sensors.

Does anyone have any idea's what might cause the light to turn on and off?

 

Cat might be plugged. Does it have the same amount of power as usual?

Ignition coil is also possible.

 

I have had a cat start to plug without any noticeable power loss. But it did effect the mileage.

 

I might have mentioned that. My cat was falling apart inside, rattling around a lot. So I punched it. Also that there has been no power loss, and the engine code did this before I punched the cat as well as after.

 

if there is a sensor after (or in the middle of) the cat and youve gutted it, that will cause issues. dont know any way around it besides putting another cat on.

 

Also make sure that the engine is running at 195 degrees. A stuck open thermostat will kill your mileage.

 

Temperature is normal. Would the missing CAT cause the O2 sensor code to come on and go off randomly?