I can't get the idle (cell 4) BLM's to come above 108 when in closed loop, When in open loop with the BLM's locked at 128, the A/F on the wideband reads 14.0-14.5 or so. As soon as the car goes into closed loop the BLM's shoot to 108, yet the wideband reads lean (17.0-19.9 YIKES) and the car stumbles and gets knock retard. Even when i lower the VE table so that i get a open loop AFR of 17.0 on the wideband the BLM's still shoot to 108 when it switches to closed loop. Once you start driving and it gets out of cell 4 it runs good. This is on a '730 SD 11:1 383 with AFR 195's Comp Cams 236/242 cam, SLP 1 3/4 shorties. Any suggestions as to why the BLM's would shoot to 108 when the car is running lean? I just don't undestand why the ECM (O2 sensor?) would read that the mixture is rich when it is super lean. I replaced the O2 sensor recently with a Delco sensor just to rule that out. Is this large cam causing the problem? Would a heated O2 sensor work better? Can't seem to get past this problem. Any suggestions greatly appreciated.

 

What size/brand injectors & fuel pressure? When I had a 108 BLM at idle I had to lower my FP drastically and retune the other areas.

 

I had the exact same problem using a LPE74211 cam and stock L98 exhaust manifolds. Moved the o2 sensor (heated type) from the stock location to a spot on the y-pipe just before the actual Y. The super lean (~20:1) idle issue was resolved.

My theory was that raw fuel was getting past the exhaust valve just before it closed and triggered a false rich signal. My Wb-02 sensor, mounted just before the cat, didn't see the "rich" condition since the fuel would burn in the pipe between the factory 02 sensor and the WB unit.

Unfortunately the eng would surge at idle. Even with the IAC disconnected. WB-02 showed mixture going way rich, then lean over and over again. Theory on that was I moved the factory sensor is too far down the pipe, it will be one step behind at idle due to low exhaust gas velocity.

Once I installed my Hooker 2055s and used their O2 bung; lean idle and surging issues both disappeared. Life is good now!

 

After reviewing my Datamaster logs, I think that some of the self-EGR from this cam is cooling the O2. When driving (watching Datamaster) the O2 crosscounts produce a nice sawtooth pattern. When I slow to a stop, the crosscounts slow down and the pattern looks streaches out (does that make sense?). As soon as i start driving again the crosscounts speed back up. Looks like one of my projects for the winter will be a heated O2. Thanks for the replies.