1995 GT - O2 Sensor indicates lean

[doctorj357]

So I been chasing down a vac leak for past few weeks. Would have a idle surge issue after car warmed up, hanging around 1500rpms and then would slowly fall. I chased down the IAC - replaced that, EGR - hooked that up correctly, TPS - verified voltage was at .99v, cleaned MAF, cleaned TB, replaced distributor cap and rotor, and replaced broken gaskets on the TB, spacer, EGR, and the car would still have the surge issue. So this obviously had to be vac related.

One thing I tried tonight, I put the stock airbox back on, and actually it seemed to tame the surging idle a little better, but was still there. Then I got to thinking, maybe its the intake. So I took the upper off and discovered that on the lower, the bolts holding it in were finger tight, if even that much . Long story short, the person that had the car before me, apparently reassembled the intake without torquing any of it. So I followed the torquing sequence, followed by the upper intake, and re-assembled the spacer, TB......etc. Put it all back together.

Test drive - car seemed to run great. My surging idle was gone, and overall just seemed to feel a little snappier. Could just be a placebo effect I know but the car did feel better. Anyway, got it on the highway and started feeling some bucking/detonation and then the CEL comes on. Pulled the codes and I am getting two codes I never got before

172 - o2 Sensor circuit indicates system lean (right side)
176 - o2 Sensor circuit indicates system lean (left side)

Now, list of mods: Edelbrock Performer intake & 190 heads, E303 cam, FRPP shorties, O/R H pipe, stock MAF, BBK 65mm TB, BBK CAI, and a SCT chip with bama tune for all this. I'm wondering that if by going back to the stock airbox I caused a lean condition. Now knowing that my vac leak was likely due to the intake bolts being so loose, I should probably go back to the bbk cai. In case I'm wrong, or its not because of that, what else causes the 172/176 code?

 

[jozsefsz]

Going back to the stock airbox is very unlikely to blame - if it was all installed correctly and there aren't any rips in the tubing past the MAF and you're using the same MAF which wasn't damaged when you switched it from the CAI to the stock box (which it never hurts to clean - a dirty MAF will sense less airflow than is actually flowing and give you less fuel). Double-check all of your vacuum connections - after removing the upper intake, you had to disturb most of them. Also verify the condition of your upper intake gasket - if it's a paper one it may have needed to be replaced if you didn't. On the stock intake there's a big vacuum line for the PCV system under the upper intake, that one's easy to knock loose (or forget about) and causes a huge leak if you forgot to re-connect it. The PCV system is also completely sealed - the hose from the air intake to the oil-filler-neck has to be there or you'll also have a large vacuum leak. I'd check all that stuff first.