1994 855T decreasing MPG

azureblue · Post by **azureblue** » 11 Jul 2014, 12:21

I am going to assume the front O2 sensor is at fault, since I can't think of anything else that would make the MPG's decrease over about a year's time. It's down to about 21.5 on the highway. My former mechanic said the rear one was going, but that it had no effect on the MPG's. This will be my first O2 sensor job on an 850 - do I have to use an Volvo sensor, or is there a reliable substitute?

But please correct me if I'm missing something.

Post by **rspi** » 11 Jul 2014, 12:29

There are a few things that can effect your gas mileage. When was the last time the car had an ignition tune up?

Get a Bosch sensor.

azureblue · Post by **azureblue** » 11 Jul 2014, 12:47

Thanks -I'll pull the distro cap - it and the rotor are about 3 years old, and check them out. Wires are about 4 years old and they are Volvo wires. Changed the plugs about 6 months ago, but that's about it. (to NGK plats for turbo gapped down to.025 - the car runs great). I have no idea how old the O2 sensors are - the PO replaced them so they have to be at least 5 years old. Bosch it is, then...

Post by **mecheng** » 11 Jul 2014, 13:08

Clean them with MAP sensor cleaner and put a bit of anti-seize on the threads. 5yrs is not old especially for Bosch sensors

azureblue · Post by **azureblue** » 11 Jul 2014, 16:42

I'm not sure if they are 5 years, but the PO replaced the engine, and had everything updated about 12 years ago. Cleaning them doesn't look like much work, so I will try that. I need to do a MAP clean in another car I own, anyway...

Post by **oragex** » 11 Jul 2014, 21:06

I had the front O2 sensor failed on my car. In city driving as well as in stop and go traffic, the mpg was very low, around 15mpg. On the highway however I was still getting a 30mpg. I believe a failed sensor affects the mpg mostly when the car is accelerating.

azureblue · Post by **azureblue** » 12 Jul 2014, 09:17

That's good to know - my in town MPG is pretty good. I used to average about 24 MPG but that has dropped to 21.

Post by **abscate** » 12 Jul 2014, 12:41

Recent London poster solved his surging problem with a new fuel pump. A side symptom was low mpg like your report. Check fuel pressure at rail.

Post by **osman** » 12 Jul 2014, 15:19

Once had my turbo wagon go all they way down to 11 mpg all kinds of black smoke, I replaced everything starting with the front O2 sensor, FPR, ECT, Spark plugs, wires, cap and rotor, you name it. It finally ended up being the ECU, got one from the junkyard for $20. Car ran like normal.

Post by **mecheng** » 12 Jul 2014, 15:24

I find if the o2 sensor is getting lazy "slow response" codes sometimes all they need is cleaning, mpg is usually down slightly. If the mpg is very low the O2 sensor is shot and needs replacing