Are the secondary (downstream) O2 sensors really by-passable?
03-29-2007, 10:58 PM
#1
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Are the secondary (downstream) O2 sensors really by-passable?
I have a 99 CA Spec Max. It has 4 O2 sensors as most of you know. Two go before the pre-cats, and the other two after the pre-cats. I installed Cattman headers and simulated (bypassed) both secondary (downstream) 02's using the O2 simulator I purchased from Cattman. The O2 simulator box blinks like it should, and I have no CEL's. Car idles and drives just fine.
I'm posting because I have somehow managed to lose 2 psi of boost and have been unable to get it back. I've spent hours and hours checking everything over with a fine tooth comb, and can't find anything wrong with my work. So, I got to thinking.... what have I changed..... and my mind keeps coming back to the two secondary 02's I simulated. I read the FSM, and it appears that the two downstream 02's don't communicate with the ECU about A/F, only "other" stuff which I don't quit understand.
So, what do you know about the downstream O2's? Do they need to be in my exhaust stream and "talk" to the ECU, and then maybe I'll get my 2 psi back?
I'm posting because I have somehow managed to lose 2 psi of boost and have been unable to get it back. I've spent hours and hours checking everything over with a fine tooth comb, and can't find anything wrong with my work. So, I got to thinking.... what have I changed..... and my mind keeps coming back to the two secondary 02's I simulated. I read the FSM, and it appears that the two downstream 02's don't communicate with the ECU about A/F, only "other" stuff which I don't quit understand.
So, what do you know about the downstream O2's? Do they need to be in my exhaust stream and "talk" to the ECU, and then maybe I'll get my 2 psi back?
03-30-2007, 07:26 AM
#2
The rear O2s ONLY check to see if the precats are functioning. they perform no other function. basically, they're only there to cause a CEL when things aren't working.
It will not change your issue.
When you lost the boost, was this before/after you bypassed the sensors, or before/after you installed the headers AND bypassed the sensors.
I would most likely say you're 'losing' boost because the exhaust flows freer now, causing better cylinder scavenging and more engine efficiency. so as the engine sucks more air out of the intake each revolution, the supercharger shows lower pressure, but is still pumping out jsut as much air.
That's my theory anyway. It would make sense to me that an SC would drop a bit of pressure because you're freeing up the blockage between the pump pushing air in, and the far end of the exhaust.
It will not change your issue.
When you lost the boost, was this before/after you bypassed the sensors, or before/after you installed the headers AND bypassed the sensors.
I would most likely say you're 'losing' boost because the exhaust flows freer now, causing better cylinder scavenging and more engine efficiency. so as the engine sucks more air out of the intake each revolution, the supercharger shows lower pressure, but is still pumping out jsut as much air.
That's my theory anyway. It would make sense to me that an SC would drop a bit of pressure because you're freeing up the blockage between the pump pushing air in, and the far end of the exhaust.
03-30-2007, 08:16 AM
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Originally Posted by Matt93SE
The rear O2s ONLY check to see if the precats are functioning. they perform no other function. basically, they're only there to cause a CEL when things aren't working.
It will not change your issue.
When you lost the boost, was this before/after you bypassed the sensors, or before/after you installed the headers AND bypassed the sensors.
I would most likely say you're 'losing' boost because the exhaust flows freer now, causing better cylinder scavenging and more engine efficiency. so as the engine sucks more air out of the intake each revolution, the supercharger shows lower pressure, but is still pumping out jsut as much air.
That's my theory anyway. It would make sense to me that an SC would drop a bit of pressure because you're freeing up the blockage between the pump pushing air in, and the far end of the exhaust.
It will not change your issue.
When you lost the boost, was this before/after you bypassed the sensors, or before/after you installed the headers AND bypassed the sensors.
I would most likely say you're 'losing' boost because the exhaust flows freer now, causing better cylinder scavenging and more engine efficiency. so as the engine sucks more air out of the intake each revolution, the supercharger shows lower pressure, but is still pumping out jsut as much air.
That's my theory anyway. It would make sense to me that an SC would drop a bit of pressure because you're freeing up the blockage between the pump pushing air in, and the far end of the exhaust.
I seem to have lost the boost a while back when I installed a UR Ultra SC (oem size) crank pulley. Its actually a little more complicated than that. If you have time, please read the following link for a complete timeline of events. It explains everything.
http://forums.maxima.org/showthread....17#post5609917
Also, I thought of your theory as well, but said, "Nah, can't be" but now you got me thinking again about it.
I also didn't mention that before I installed the Cattman Headers, I sent them out to HPC and had them fully ceramic coated (inside and out) with their Extreme 2000 series. This was done to help the scavenging of the exhaust and to drop my engine bay temps (think heatsoak of aftercooler right above front exhaust manifold)
Again, thanks for your input.
03-30-2007, 09:49 AM
#7
Hmmm.. I vaguely remember seeing that post a while back. scratched my head and went on.
depending on the pulley diameter, it may or may not actually have changed.
But as Nismology said (holy ****, I'm agreeing with him!
), dyno and track times should be the true test. If you got faster, then it's all good. 
depending on the pulley diameter, it may or may not actually have changed.
But as Nismology said (holy ****, I'm agreeing with him!
), dyno and track times should be the true test. If you got faster, then it's all good.
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