My gas mileage on my 230K 850 turbo dropped to 12.5 miles per gallon with black smoke out the exhaust. With 3$ gas prices and with what our dollars fund when we buy gas (terrorists), I absolutely loathe stopping at the gas pump, so this problem was driving me crazy.

The ODBII code I got was 172 (system too rich, bank 1). I figured that if the upstream oxygen sensor was the problem I would get a code indicating an oxygen sensor was the problem, right - WRONG!!!

First, I replaced all the pcv hoses and made sure that nipple on the turbo intake was squeaky clean. Then I checked the fuel pressure regulator with a fuel pressure gauge hooked up to the shraeder valve on the end of the fuel rail and then removing the little white vacuum hose to the regulator, while the car was running at idle (make sure the connection to the pressure gauge is tight so you don't get fuel around the running engine) - the pressure shot up 5 pounds so it passed the test. Then I checked the fuel injectors with a mechanic's stethoscope. Each of the five clicked away just fine. Ok. Then I checked the fuel injectors for leaks by removing the injection rail and injectors from their ports and turning the ignition key to postion II, with a cool engine (BUT DO NOT TURN ANY FURTHER OR YOU MAY CAUSE AN EXPLOSION AND FIRE). No drips, no leaks. Still 12.5 miles per gallon.

Today, partly as a result of reading posts on Brickboard I installed a new upstream oxygen sensor. WHALA! Now getting 28 miles per gallon on highway as opposed to 13.5. Better than a 100% increase in gas mileage. In a month of driving I will cover the cost of the new o2 sensor 100$ (free shipping and discount from fcp groton).

Also, I am thinking that odbII will pinpoint the o2 sensor for some conditions, but not necessarily when the o2 sensor allows for the burning of too much fuel - by signaling the injectors to stay open too long. I am wondering if anyone knows more about this?

Anyway, thanks for the posts by ClausC and others and the Brickboard for all the helpful info. And still find it hard to believe that the little o2 sensor means so much to mileage - at least in my case.

VolvoWalt

This happens more than you think. Small air leaks into the intake manifold will cause the mixture to enrich which causes soot to collect over the O2 sensors. The O2 sensors will no longer be able to respond to changes fast enough to fluctuate the mixture and then get coated with more carbon. Eventually, the carbon hides the mixture from the O2 sensor and your mpg goes down hill rapidly.

The O2 sensor is still 'reading' temperature changes and reporting these to the ECU, so no fault codes will appear. The O2 sensor is essentially dead at this point. There is a way to 'clean' the sensor, but I suggest to replace it.

I am glad you replaced and cleaned the vacuum lines. 14 years is a long time for rubberized parts.

While Viola is the correct spelling, walla is the correct pronunciation :) The French never could spell correctly anyway. "oui" is "wee"???

Klaus

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Cynthia Amendt, RIP

Thanks Claus, I did have the intake off a couple of times in the past two weeks without replacing the gasket: I didn't say that I first got a code for a knock sensor and I checked the wiring and took the sensors off and put them back on with 15 pounds of torque and with the 3 and 5 o'clock orientation, after which the knock sensor code went away. I will spray some carb cleaner around the intake to see if I have any leaks. Maybe I can get her up to twenty nine or thirty on the highway.

This has been terrific. Not only do I learn more about my volvo, I also learn to improve my vocabulary.

Claus, I am sorry for your loss.

Au revoir for now my friends.

Before you go completely crazy, check all of the intercooler hoses that you loosened. Especially the throttle body connection, when mine was loose it would idle fine, just couldn't give it gas. If the turbo tries to spool up, there is no pressure.

Klaus
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Cynthia Amendt, RIP

Great story - thanks for the report.

BTW I think 'WHALA!' should be 'VOILA!' - It's a french word (maybe I only know that because I'm Canadian?).
http://en.wiktionary.org/wiki/voil%C3%A0
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1998 V70 AWD->FWD Turbo 200k+

I don't want to attack anyone, so don't read it like that, but is 13-17 mpg expected by many drivers? My 850 sits between 28-31 on the highway. Well, sat.. Now its dropped to 25-26 and seems rich when it starts (first couple of minutes cold, but less when if its warm). So the other day I bought a new O2 sensor, still waiting for it to get here...

The way I see it, is those 2 mpg I lost fixing now saves me $50 in gas than waiting 6k miles to see what happens...

Before I spend money on an O2 sensor, I would check for vacuum leaks or a clogged air filter, etc. Basically free stuff.

Oh, did you check the air pressure in the tires? Have you ever checked the spare tire pressure? :)

Klaus
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Cynthia Amendt, RIP

Yep checked air pressure. Checked for vacuum leaks with a propane torch (off and let the gas go, listen for the engine to pick up a little). Cleaned MAF, Checked air filter. Went through records and found the O2 sensor has never been replaced. Probably due every 225k miles or so, right?

Checked it via [url=http://www.brickboard.com/AWD/index.htm?id=1323596&show_all=1] this thread[/url] and it seemed to adjust slower than I would think it should. So I figured it'd be worth the $100...

Good. I changed my last one at 170K miles, but the PCV system needed to be changed out. Crudded the O2 sensor beyond cleaning.

Klaus
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Cynthia Amendt, RIP

There's nothing worse than having a flat spare tire. About a year ago, I had my first flat tire ever (it actually blew UP entirely when I hit a piece of shrapnel in the road), and got the spare out and after fighting with the stupid Volvo tire iron which is awful BTW, finally got the spare on only to realize it was nearly as flat as the shredded tire.

Well, it got me home anyway.

A can of carb cleaner will help you easily detect vacuum leaks...spray around and and all vacuum lines you can find and listen for a change in your engine speed.