O2 Sensor keeps failing?

Hey Guys....been awhile since I checked in...but I have a 1999 GMC 4X4 Suburban with a 5.7 Vortec in it that is driving me nuts trying to nail down this problem....

Last year the truck started stumbling at idle and started running terrible at all speeds....lots of black-ish smoke under throttle and the smell of cats gone....
Upon inspection I found the left bank running fine, while the right bank plugs were totally fouled with carbon....not too wet...just caked on black carbon...
I replaced the pre-cat O2 sensor on the bad bank and all was well for about 6 months....
It started happening again so I swapped the "new" sensor with a known good one out of my friend's 5.7 Vortec and all seemed good until a few weeks later and I have the same problem....
I could use any input you might have on where to look next....as I think the O2 sensors are the only things that would screw up an entire bank by itself...
The distributor appears fine, wires are new-ish and test fine, plugs are fine, occaisionally run some Seafoam or Lucas through the fuel system, etc....the truck has 260K kilometers on it and is in excellent shape besides the current BS I am experiencing...
The problem does not immediatly occur until the engine starts to warm up and it is only evident about 60% of the time during a full days' worth of driving.
I am also suffering from a "vibrating" fuel level needle which I thought was a stepper motor in the guage cluster,, however I was recently told that these models do not have stepper motors and that the cause was a failing fuel pump/sending unit....but this would not affect only ONE side of my 5.7 (would it?)...
Basically...I am thinking that the failed O2 sensors are a RESULT of the problem and not the ACTUAL culprits....but again....the injectors are not likely to fail/stick on one entire bank only....are they?

Ideas?...Thanks in advance....Phil.

Hey Guys....been awhile since I checked in...but I have a 1999 GMC 4X4 Suburban with a 5.7 Vortec in it that is driving me nuts trying to nail down this problem....

Last year the truck started stumbling at idle and started running terrible at all speeds....lots of black-ish smoke under throttle and the smell of cats gone....
Upon inspection I found the left bank running fine, while the right bank plugs were totally fouled with carbon....not too wet...just caked on black carbon...
I replaced the pre-cat O2 sensor on the bad bank and all was well for about 6 months....
It started happening again so I swapped the "new" sensor with a known good one out of my friend's 5.7 Vortec and all seemed good until a few weeks later and I have the same problem....
I could use any input you might have on where to look next....as I think the O2 sensors are the only things that would screw up an entire bank by itself...
The distributor appears fine, wires are new-ish and test fine, plugs are fine, occaisionally run some Seafoam or Lucas through the fuel system, etc....the truck has 260K kilometers on it and is in excellent shape besides the current BS I am experiencing...
The problem does not immediatly occur until the engine starts to warm up and it is only evident about 60% of the time during a full days' worth of driving.
I am also suffering from a "vibrating" fuel level needle which I thought was a stepper motor in the guage cluster,, however I was recently told that these models do not have stepper motors and that the cause was a failing fuel pump/sending unit....but this would not affect only ONE side of my 5.7 (would it?)...
Basically...I am thinking that the failed O2 sensors are a RESULT of the problem and not the ACTUAL culprits....but again....the injectors are not likely to fail/stick on one entire bank only....are they?

Ideas?...Thanks in advance....Phil.

the question really is why is the passenger side cylinders running too rich ?

I would get a fuel pressure guage and do the fuel system test..could be rapid leak down and the regulator is leaking..

Thanks for the suggestion....
But do you know if there is a way to accurately measure injector duty cycles on this motor?.....
The reason I am asking is the fact that ALL 4 cylinders on the right bank (Bank 2) seem to be fouling....(over-rich).

When I scan the data I get the Upstream O2 reading in the 400's....and yet the Downstream one seems to be switching correctly....
The left bank sensors seem to be calling for a lean mixture to compensate...

I will need to buy yet another Upstream sensor to better daignose this issue...but killing four O2 sensors is not going to make me a happy guy:mad:.

It definitely appears to be a simple case of a failed Upstream O2 sensor...I know that...but three sensors in the same location over 6 months is definitely not the normal rate of failure on this type of motor.

Any other ideas?
Thanks...Phil.

Thanks for the suggestion....
But do you know if there is a way to accurately measure injector duty cycles on this motor?.....
The reason I am asking is the fact that ALL 4 cylinders on the right bank (Bank 2) seem to be fouling....(over-rich).

When I scan the data I get the Upstream O2 reading in the 400's....and yet the Downstream one seems to be switching correctly....
The left bank sensors seem to be calling for a lean mixture to compensate...

I will need to buy yet another Upstream sensor to better daignose this issue...but killing four O2 sensors is not going to make me a happy guy:mad:.

It definitely appears to be a simple case of a failed Upstream O2 sensor...I know that...but three sensors in the same location over 6 months is definitely not the normal rate of failure on this type of motor.

Any other ideas?
Thanks...Phil.

so why do you ignore my suggestions on checking the fuel system for proper operation ? the fuel system is controlled by the computer ..the injectors are all on one side defective but the other side good ? no ..

fuel is somehow getting into the intake and fouling up the combustion chambers with excess fuel , this will damage the O2 sensors..

the injectors open when they are grounded ...the injectors are normally closed..time of the grounding depends on the O2 sensor output to the computer..

the other issue that could cause this would be coolant leaking into the exhaust damaging the O2 sensor..