S70 1998 non-turbo: replace before/after/both O2 sensors

kscomp0 · Post by **kscomp0** » 30 May 2007, 08:02

I am trying to address a noticeable decrease in gas mileage that has occurred over the past few months. The mpg is around 15 now, and the car has 118,000 miles on it. The O2 sensors are original.
I was going to replace the sensors, but I saw in another post that the rear sensor is monitoring the condition of the catalytic converter.

Should both sensors be changed at the same time, or should I just change the before sensor to try and address the decreased gas mileage?

sixdown · Post by **sixdown** » 30 May 2007, 09:48

I've also been getting ridiculously low gas mileage, and the OBDII computer says that its an EVAP vacuum leak. The sensors also are often the problem, but there are hundreds of other possibilities.

If I were you, I'd replace all the sensors at once. Makes things easier for the future and helps reliability.

Post by **Ozark Lee** » 30 May 2007, 10:33

I doubt you are going to get much if any improvement in mileage by changing out the O2 sensors if you are not throwing codes that point to them.

Have you checked your airbox thermostat? Shortly after I picked my car up I noticed that the heated air hose had come loose from the airbox. Being attentive to details I promptly hooked it back and immediately suffered a decrease in mileage from 29 to 30 mpg to under 20 mpg. Since then I jammed the thermostat open and the mileage went back to its former level.

...Lee

kscomp0 · Post by **kscomp0** » 30 May 2007, 12:03

Thanks Lee. I just read some other threads on the airbox thermostat thanks to your post; I'll have to take a look at that.
No codes on the O2 sensor; I was just hoping that was a common cause of decreased mpg.

io · Post by io » 31 May 2007, 10:02

sixdown wrote:I've also been getting ridiculously low gas mileage, and the OBDII computer says that its an EVAP vacuum leak. The sensors also are often the problem, but there are hundreds of other possibilities.

I just fixed this same problem in my '98 S70 T5. There is a black retangular box up under the car, near the rear right tire. I believe this box is the EVAP charcoal canister. Check all of the rubber lines coming into and out of this box. I noticed that the ends of my lines were splitting and cracking, causing a vacuum leak. I cut a small amount off the end of each line to get rid of the cracks and found that they were still long enough to connect and clamp with new hose clamps. I now have over 1000 km on the car after the repair and the code hasn't reappeared.

Also my fuel economy before this fix was about 14.7 MPG. Now I'm getting 18.1 MPG.

remermn · Post by **remermn** » 31 May 2007, 22:49

If the vacuum line fix doesn't work you might try replacing the oxygen sensor.
My understanding is that the forward sensor controls the fuel air mixture so if you are going for fuel economy you only need to replace the forward sensor.
I wouldn't worry about replacing the rear one. My Volvo expert says the sensor should be replaced at 100,000 miles. I replaced the oxygen sensor on my 99 suburban about 2 years ago. The vehicle had about 110,000 miles on it at the time and my mileage went from 10mpg to 13 mpg !! Good luck.