Trying to find an answer in the FAQ's regarding an exhaust leak I current have where the Header Pipe connects to the exhaust manifold on my turbo B230FT.
I can hear the slight leak at idle and wonder if that causes a lean signal condition on the O2 sensor.
Fuel economy is now generally 14 MPG in city and upwards of 18-19 MPG on the freeway, which is similar to the post from Volvomies on December 2nd regarding his drop in MPG.
Can air be sucked into the exhaust header pipe upstream from this leak and register as a lean condition by the O2 sensor, thus enriching the fuel mixture?

"Can air be sucked into the exhaust header pipe upstream from this leak and register as a lean condition by the O2 sensor, thus enriching the fuel mixture?"

Yes, most definitely!

I had leak at flange three bolt joint and one exhaust manifold gasket was leaking also. It didnt effected to my mpg when i got leaks fixed.




I had also chevy pickup with header to exhaust pipe flange leak and it didnt effected o2 readings or mpg.



These are just my cases but usually leak near o2 sensor is baddest. My friend has leak about 2 inch before catalysator (flange) in his renault and it gived check engine light and emission test failed.



So at my cases leak far before o2 sensor didnt do much. After i welded, chevys co driver side pipe dropped out of y joint, got very loud and bad voices. Then check engine light came couple times at 100miles trip but leak was very bad and y joint pipe eated air under car.





Check by multimeter how o2 sensor voltage reacts at idle and take some vacuum hoses off to make vacuum leak. At emission test leaking exhaust does biggest oxygen and exhaust gases scam at idle, i think at rpm exhaust leak will effect less?

If you firmly press a rag against the exhaust end, do you hear any exhaust gasses escaping (hissing) where they shouldn't?

Suffice to say there is a leak at this location. The flange on the header pipe is somewhat deformed and can't be reshape or tightened any further. The question is if a leak here can actually draw outside air into the pipe upstream from the single Oxygen Sensor.
I suspect this improper lean signal to the O2 sensor is unnecessarily enriching my fuel mixture, resulting in my lower MPG.

That is a good question. Will a leak that far downstream affect the O2 sensor? No question further upstream it will. I think it can because the O2 sensor is reading exhaust gases to make mixture adjustments. I know a leaking exhaust manifold gasket affects the sensor. Do you also have a check engine light? If not, have you checked for any OBD codes that may not be triggering a CEL?

How old is your O2 sensor?
--
Will I buy another Volvo??? We'll see....

Hi,
Maybe I will stab for some answers?

I do not own a 900 but only 240’s but the engines are the same.
I also have never owned a turbo engine either, but this does not exclude me from saying to you, that an oxygen sensor sends out a low voltage signal when it’s not working correctly or at all.
This feature makes the fuel management system thinks its running lean and will run richer by a default program. So yes, the mileage will suffer some!

I would think a no output voltage would trigger a CEL so it must be having fluctuations to fool “mother” ECU!

If you do have an air leak, then you are running lean according to its atmosphere around the sensor.
Any incoming air will be oxygenated or have less or leaner fuel combustibles in it.
How much extra, is anyone’s guess.
Being lean is one thing the fuel management hates to see coming in.
The ECU will start with a richer mixture but it will also adjust the timing to keep combustion temperatures down. This is the reason for a knock sensor.
Things are very fast to compensate so you might not even know it.

I have only seen turbos in a JY and their mostly stripped out.
I suppose that the exhaust vane pump pushes air through an inter cooler system.
I have never thought to study the arrangements of components to where and how the air gets measured by the AMM.

SINCE your milage is so low, while cruising on the highway, you probably have more of an issue going on? You are down about five miles per gallon.

Is the rear end ratio? Is it a lot lower than an MPG or standard sedan model?
Are you running a 92 C degree thermostat? Too cool of one will zing away your mileage and even worse on the short runs if it’s not staying hot enough in between!
Checked it lately?
There’s the Computers, ECT sensor, within the engine block too?
Spark plugs and the tailpipe should be winking at you.

Humor me some, but maybe, a bad AMM or you got some Big Toes clamped over on the top edge of that gas pedal! (:-)
Maybe it’s looking like something out of a Big Daddy Roth’s magazine of the sixties? (:-)

Just kidding tonight and with a turbo driver, this is a plus!
I’m out of my normal realm due to COVID but it raining at this safer, hunkering house!

Phil

You Wrote: I current have where the Header Pipe connects to the exhaust manifold on my turbo B230FT.


So, Is the Leak Between the TWO surfaces????--

1) try snugging-up the nuts...that hold the TWO together.

2) Replace the GASKET between them.

Afer that's done...???? post results

Hi CeeBee1.
There is a header pipe flange deformation where it connects to the manifold 3-stud connection.
I can't reshape the header pipe flange at this time, and the connection does not utilize a gasket of any kind.
I would consider the leak minor and low on my priority list of things to do, except my MPG is low and no other issues are apparent.
Just want to know if an exhaust leak at this location could actually suck in ambient air causing a lean reading from the Oxygen Sensor.
Thanks for any assistance.