OXYGEN SENSOR PLACEMENT

[bitemedoughboy.com]

how sensitive to exhaust leaks are the o2 sensors in our pathfinders? currently im running the stock o2 sensor in the original location (after the y-pipe). i have several leaks in the exhaust upstream from the o2 sensor, 1 at the drivers side manifold flange and one in the crossover pipe where it has been cut and re-attached. im too cheap to fix the damn exhaust because im holding out for a set of headers. my question is can those small leaks siphon air in and cause a false lean reading? im not running the stock TBI, i replaced it with MPI from a 300zx with the upper plenum from a 200sx and i just got a new o2 sensor (the one for the 300zx donor car) and i'm thinking about sticking it in the downpipe on the passengers side so that its in a hotter spot on the exhaust and also should be a more accurate reading because i have no leaks on that side.

[Simon]

It has to be after the y-pipe, or the readings will be way off.

 

And yes, exhaust leaks can throw off the readings, though, if it's running okay, it shouldn't be too bad, other than using a little extra fuel to compensate for the lean reading.

[bitemedoughboy.com]

It has to be after the y-pipe, or the readings will be way off.

 

And yes, exhaust leaks can throw off the readings, though, if it's running okay, it shouldn't be too bad, other than using a little extra fuel to compensate for the lean reading.

im not using the TBI, so it doesnt have to be after the y-pipe. the 300zx had it mounted a few inches below the flange on the drivers side.

[Simon]

I don't know. I would think the readings would be different if you've got it on one side as opposed to reading from both sides.

[bitemedoughboy.com]

I don't know. I would think the readings would be different if you've got it on one side as opposed to reading from both sides.

the TBI engines are known to run lean on one side and rich on the other thats why they put the o2 sensor after the y-pipe to get an "average" reading from the 2 sides of the engine. and thats also why the TBI engines made less HP than the MPI engines.

 

i just wanted to know weather or not a few small leaks could throw off the computer enough to cause a rich condition all the time....

[87pathy]

why would it run lean on one side and rich on the other? its fed from the same source.

 

I can only see this happening if you had an intake manifold leak.

Edited November 14, 2008 by 87pathy

[mws]

Air flow differences because of differences in port lengths and geometry. It's impossible to get them identical. The longer the port, the more fuel settles out/condenses from atomized state to to tiny droplets - which do not burn as quickly or completely. Every turn or bump in a port also leads to velocity/pressure variations as well. All contribute to less atomization.

The differences in combustion efficiency from cylinder to cylinder in a carb'd or TBI's engine are shockingly big.

 

That is why engineers have been striving to get the injectors closer and closer to the back of the valves. Ideally, we would want them injecting directly into the combustion chamber.

Edited November 14, 2008 by mws

[mws]

Oh, and to answer the original question. Yes, exhaust leaks, when significant enough, will upset readings.

 

Although after the Y is best if the system uses a single sensor, in your case, sampling from only one bank should be "less bad" than sampling where you know it is iffy. I would try it!

 

And the further upstream you go, the faster it will get up to temp. That is always good as it lets the FI go into closed loop mode asap.