I've got a problem with misfires. The spark plugs and wires have been changed, but still misfiring. When I logged the misfires, I noticed the O2 sensor voltage appear to be all over the place. They start out fine, then go crazy. Is this normal? Does it mean I need new O2's? Here is my config and log file. Can you guys look at it and tell me which O2's I need to replace if any? Thanks.
Anyone?
I forgot to mention this is a '99 GMC Sierra (5.3L) that is completely stock. Tune is stock also.
Surely SOMEONE has some input on this.
Is there a way to look at the scan and tell which o2 sensor is bad?
Ok, let me give you some more background. Apparently, thats what I'm not doing right.
In closed loop, I get misfires on #6 and #8 in bank 2. And this is only at idle. When I am cruising under a load, there are no misfires. I then forced the PCM into staying in open loop. #6 and #8 misfires went away, but #4 starts misfiring. Again, even in open loop, the #4 misfire goes away when I am driving. It only misfires at idle and when decelerating. After drving a few days in open loop, I pull the plug out of #4. It looks very white. I had looked at this plug before going to open loop and it looked fine (although alittle dark). So I then loaded the original tune back in (closed loop) and the misfires on #6 and #8 came back.
I've changed all of the plugs to new ngk 55 plugs. I switched plug wires (these are fairly new wires) around from the cylinders that were not misfiring to the ones that were. No difference.
The only thing I can think of is bad o2. Trouble is, which one?
Also, I thought about the possiblity of one of the injectors being bad--#4. IF #4 was bad, and not giving the cylinder enough fuel, it could be making the o2's for bank 2 read lean. Then could be adding too much fuel to compensate making #6 and #8 misfire. Then when I run open loop, #4 cylinder is not getting enough fuel. The o2 sensor doesn't care because its on vacation. #6 and #8 are happy. That could explain the lean looking condition of #4 plug.
What do you guys think? The problem is that I'm not very well versed on reading o2 sensor voltages because my other car has been running in open loop all this time. I have no idea what I am looking at when I read the o2 voltages. Please help!!
Your O2s should switch back and forth like crazy, that is normal operation. The ECM is actually making the vehicle run lean, then rich, then lean, then rich ... over and over again as fast as possible (during closed loop) to maintain the perfect "AFR" for emissions.
What mods does your truck have?
No mods, completely stock. Tune is stock also.Originally Posted by 12secSS
Your O2s should switch back and forth like crazy, that is normal operation. The ECM is actually making the vehicle run lean, then rich, then lean, then rich ... over and over again as fast as possible (during closed loop) to maintain the perfect "AFR" for emissions.
What mods does your truck have?
*edit* - The truck has over 106,000 miles on it.
Last edited by 7420NW7TH; 09-10-2007 at 04:14 PM.
When's the last time the plugs and wires were replaced? That could be it.What are your fuel trims like?
I just replaced the plugs w/ ngk 55's. The wires are fairly new according to the previous owner who I know pretty well.When's the last time the plugs and wires were replaced? That could be it.
See the thing is its just the passenger side bank that is having misfires. #6 and #8 and sometimes #4.
One thing I tried is moving the passenger side O2 sensors to the drivers side and vice versa to see if the problem moves to the driver side bank. Didn't change anything, still misfiring on #6 and #8.
And the misfires occur just at idle and when declerating. I'm starting wonder if my fuel pump or filter needs replacing, or just dirty injectors?
I might try some seafoam first to clean out everything, and go ahead and replace the filter.
Any suggestions would be appreciated, this problem is driving me
To answer your question about the fuel trims, the passenger side trims are like -10%. Does this mean its taking fuel out? I noticed it pretty much stays there close to -10%. The drivers side stays right around 0% +/- 1%.
Yup, with a -10% (yes, it is pulling fuel) on the passenger's side I would suspect a problem. Possibly injectors. Seafoam it, give it new filters and plugs (seafoam will contaminate them), and if possible have the injectors cleaned/checked. Good luck!
Thanks for your help!
I just thought I would update y'all. It was a bad fuel pressure regulator. I picked one up at autozone and installed it. No misfires. Runs great!
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