On my Y2K OBW, the mileage has dropped consistently. I was getting 22-23 mpg. for around the City street/freeway driving. The last 3 tanks, it's been 20.

The CEL has never come on, except like it's supposed to at start-up. With 82500 on the odometer, could the mileage drop be the sign of an O2 sensor going bad?

Other than changing them out and running for a month or so, is there any way to tell if one or both are getting weak?

 

Bradford

With 82k miles, the O2 sensor(s) are probably lazy.

 

They practically have to be flatline dead before they'll set a code.

I read somewhere that they really only operate at peak performance for around 30k miles.

 

You need a scan tool with data stream monitoring in order to view their performance in real time.

Their voltage should fluctuate fairly rapidly between .2 and .9 volts, I think.

Most volt meters aren't suitable for reading rapid fluctuations in O2 sensor voltage.

Treeman,

 

With the age of your ride I would first look for the obvious things that can kill mileage, dragging brake caliper, soft tires, plugged air filter, old spark plugs, valve toerances off, etc. You may be surprised at what you do, or do not find. Just a thought.

Thanks for the replies. Yes, I did check everything you suggested, and more. I have been very concious of maintanence since day one. Except the valve clearances..

 

I guess I have no idea how to do this. I thought that 2.5 engines were hydraulic lifters and required no periodic adjustments. Where do I look, and what do I check?

 

B

Treeman,

 

With the age of your ride I would first look for the obvious things that can kill mileage, dragging brake caliper, soft tires, plugged air filter, old spark plugs, valve toerances off, etc. You may be surprised at what you do, or do not find. Just a thought.

You are getting to the age where the O2 probably is "lazy" as Scoobaroo indicated. At around 6 or 7 years, I changed mine out. Mileage was not really off, but I was becoming convined that it was causing a carbon issue with my engine (97 OB). Never had a CEL. The tech told me that it was cycling about 13 times/minute. They are use to seeing around 18. I changed it out and my carbon issue cleared up. As these things age, they do cause the mixture to run towards rich.

 

There was significant discussion a couple years back on O2 sensors. Several people with cars around the 5-8 yr mark indicated that a new sensor made an appreciable improvement.

 

Commuter

Treeman,

 

I can't help you with the valve clearance issue, since I'm not sure if they're hydraulic, adjustable with guages, or adjustable with shims. Here's hoping someone with that knowledge jumps in on the thread!