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Old 10-27-2003, 04:16 PM
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Please Help with OBD II

Hi Folks...I'm having problems getting my engine faults sorted out and would appreciate any help you can provide (my local GM dealer will no longer allow techs to "hand out free advice").

Basic Setup: I have a stone stock 98 GM 5.7 Vortec in a 91 Sonoma. For reasons not (I don't think!) related to this problem I am running it with a 1997 computer.

Problem: It just doesn't seem to run right. Hard starting...what sounds like a misfire but see below...plugs and exhaust pipes look a little carboned...doesn't want to idle...not as much power as I seem to think it should have.

OBD II Data: No trouble codes being set. No misfires being detected. Idles at 675-695.

O2 Sensors: Here's where I think the problem may be, but still with no error codes. This is the data...

>>While the sensors are warming up...
O1 = starts at 0.85V and slowly increases to 0.90V
O2 = Starts at 0.74V and stays there
O3 = Starts at 0.84V and slowly increases to 0.88V
O4 (which my scanner is calling 1B !!) = Starts at 0.45V and stays there

>> Once the sensors heat up I get this ....
O1 = Drops abruptly to about 0.70 V and cycles up and down from about 0.65V to about 0.75V
O2 = Drops abruptly to about 0.25V and cycles there about like O1 does ( plus/minus about 0.5V)
O3 = Still stays very stable at about 0.88V-0.90V
O4 = Jumps from 0.45V up to same as O3 and is very stable.

I wish I could post the graph as I'm not too good at describing what I see.

I swear this thing is running rough but no misfire is getting detected. I also think it is rich, but no error codes are being set.

I'm thoroughly puzzled (and more than a little p****d at the dealer who won't let me just show the graph to their tech guy.

Thanks for any help you can give me!!


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