O2 sensor and oil burning question

[threeputtpar]

So, I've been noticing lately that my gas mileage has been on a steady decline for the past couple of years and it's now just starting to bug me. I know that the primary O2 sensors are the main piece of hardware that controls the A/F mixture when in closed loop, so my question is would an oil burning incident from 3 or 4 years ago damage the sensors enough that they have not been functioning correctly all this time and I'm just taking notice?

The PCV system in the Audi is prone to freezing in really cold weather, and one winter probably 4 years ago it froze solid and in the 6 miles between my house and my friend's where the car was parked I went through 1.5 liters of oil. Had the flashing CEL for massive misfires, the big cloud of smoke behind the car, and reduced power. Once the car sat in the garage, it defrosted and ran just fine after a new set of plugs. I just don't know if the episode killed the O2s right then and there, or did it start the slow death of them? The car probably had 90k miles on it at the time, and it's at 150k now. Maybe it's just time to replace them anyhow at this mileage?

I've watched them with my scanner, and they seem to be functioning as they bounce around in the voltage range from about .05v to .75v. I don't know what the correct readings should be.

 

[tomcat27]

You should see periodic readings up around .95 - it looks like yours is a little lazy

 

[itguy08]

Originally Posted By: threeputtpar

I've watched them with my scanner, and they seem to be functioning as they bounce around in the voltage range from about .05v to .75v. I don't know what the correct readings should be.


That's what they want to do - bounce around. You want it to average around .45 volts. The computer will adjust the bouncing to get it to an average of .45. Good reference article:
http://www.autodiagnosticsandpublishing.com/feature/o2-sensor-testing.htm

What you need to look at is short term and long term fuel trim. That is the computer adjusting the mixture based on what the O2 sensors say. + some number means more fuel (from the built in maps) and - some number means less fuel. Ideally they should be 0. + means it's dumping more fuel (usually a vacuum leak), - means it's dumping less (leaky injector, too high fuel pressure).

 

[JHZR2]

FWIW, here was a recent video of the O2 sensors in RT on my Autel scanner, on my 04 Saab 9-3 which has around 69k miles.



http://www.bobistheoilguy.com/forums/ubbthreads.php?ubb=showflat&Number=2964289

 

[Clevy]

To answer your question yes oil contamination can reduce or o2 sensor function. So if your mileage is on a steady decline the front o2 sensors before the cat may be shot.
They are cheap and easily replaced. If your mileage is bad enough new sensors may pay for themselves rather quickly in fuel savings.
The o2 sensors after the cat only monitor catalytic converter function. I've got those ones turned off since I don't have cats and it will throw a cel.
As far as o2 sensors go I've found that oem work best. When 0.04v matter why risk using a part that may not work as designed and possibly negate any fuel savings.
When o2 sensors fail the ecu goes into safe mode and richness the fuel to eliminate any possible lean condition. This also means more fuel gets into the oil and cylinder wash,so having that part of the system operating as designed is pretty important.

 

[itguy08]

Originally Posted By: Clevy
So if your mileage is on a steady decline the front o2 sensors before the cat may be shot.
They are cheap and easily replaced. If your mileage is bad enough new sensors may pay for themselves rather quickly in fuel savings.


They can only help of LTFT (Long Term Fuel Trim) is a large positive. That would mean the computer is dumping more fuel in long term. If it's a small # or negative, you're not going to notice any savings as the computer is already leaning things out.

Here are a few more good links:

http://www.obd-codes.com/faq/fuel-trims.php
http://www.easterncatalytic.com/education/tech-tips/fuel-trim-can-be-a-valuable-diagnostic-tool/

 

[supton]

I know when the turbo on mine died, the O2 sensor died like a couple days after getting the car back. It was trailing blue smoke when the turbo died, but it wasn't dumping oil out the exhaust--wasn't low on oil in the sump.

At least on mine, the sensor is heated. I'm guessing it doesn't take much oil on it to damage it, once that heater kicks on.

 

[TFB1]

A brief period of excessive oil usage isn't going to harm the o2 sensor(s), once the problem was corrected the oil burns off and normal operation was restored...

At 150K mi they are no doubt lazy and at least a contributing factor to your loss of MPG...

 

[threeputtpar]

Originally Posted By: TFB1
A brief period of excessive oil usage isn't going to harm the o2 sensor(s), once the problem was corrected the oil burns off and normal operation was restored...

At 150K mi they are no doubt lazy and at least a contributing factor to your loss of MPG...


This was my hunch, and it they were more easily accessable I'd just pull one out and inspect. Bosch says that their life is typically 100k miles, but they don't say what to look for to know they are going out.

 

[Virtuoso]

We had a Saturn that was driven for around 90,000 miles that burned (when we bought it) a quart every 700 miles to (when we sold it, at 220k) a quart every 2-300 miles.

No o2 codes or reduced fuel mileage. It would leave clouds of smoke during accel after decel; and probably some that I couldn't see during normal acceleration too.

Just my experience with an oil burner.

 

[TFB1]

Originally Posted By: threeputtpar
Originally Posted By: TFB1
A brief period of excessive oil usage isn't going to harm the o2 sensor(s), once the problem was corrected the oil burns off and normal operation was restored...

At 150K mi they are no doubt lazy and at least a contributing factor to your loss of MPG...


This was my hunch, and it they were more easily accessable I'd just pull one out and inspect. Bosch says that their life is typically 100k miles, but they don't say what to look for to know they are going out.


Well Bosch is no doubt interested in selling parts so why not make the 100K mi recommendation? Couldn't hurt... Unless they are damaged(which would surely set a code), a 25K mi sensor isn't likely to look any different than one with 125K mi...

When o2 sensors age they become "lazy" or basically slow to respond to A/F changes... Correct A/F ratio is maintained by the PCM continually monitoring response from the sensors... Sooo if it's reporting lean, the PCM enriches mixture and expects to receive a signal showing the A/F ratio changed... A slow sensor may not report in the expected time, so the PCM enriches again, this time getting a over rich condition so the PCM cuts fuel and not receiving the expected response, cuts it again... Basically the instead of maintaining a close tolerance on A/F ratio, it swings widely from rich to lean which does reduce MPG and may cause some drivability issues... At steady cruse, the condition may not be severe enough to make a difference but in town, hilly terrain, etc a fresh set can yield a noticeable improvement in MPG...

Will they help your milage? Dunno but at 150K mi I'd think they should... YMMV

...

 

[partspro]

Visual inspection of O2 sensors with over 100k is just a waste of time. As previously posted, they get slow to switch. Looking at both short term, and long term fuel trim can provide useful data. The CEL only lights when fuel trim is off by +/- 20% so slow toggling can cause pretty substantial increases in fuel.