I've got a strange problem:

The car is a stealth ram equipped 350 with a moderate cam, I had it running reasonably ok with stock 350 tables and just changed the fuel injector constant and disabled egr. It was running a little rich (blms of low teens across the board) so I bumped up fuel pressure constant by a pound. Now what's happening Is that as soon as the ecm goes into closed loop the blms climb up to 160 and sit there. However, in open loop, the car is running pretty well, a little lean if anything (judging by smell) and as the blms climb to 160 (takes abot 30 seconds) there is an increasing ammount of black fuel smoke. It seems like the ecm thinks its running lean, but it actually isnt. Theres a brand new heated o2 sensor in there. The o2 millivolts fluctuate between 100ish and 600ish at idle, so I dont think it is a sensor problem. Also, I *think* this problem developed before the new injector constant, because right before the new chip, the car fouled a set of new plugs to the point that it wouldnt run in like 5 miles. The set of plugs before that ran 800 miles before starting to foul (why i initially leaned out the chip). What could cause the ecm to go full rich like this?

 

I had the same problem, look for an exhaust leak near the O2 sensor.

 

Ok so I did find an exhaust leak about 6" downstream of the O2 sensor, I'm going to fix that tonight. If that doesnt work does anyone else have any suggestions?

 

On my setup (350, 24# injectors, stock TPI) if I bump the injector constant by .2 pounds, it'll give me a 10 BLM change. My opinion is you changed your injector constant by too far and are really going lean. Try lowering it by a half pound. On my setup, a 21.3 injector constant gives me 128 from idle to 2000 rpm, then it starts to get rich, except when the engine is hot. This is with stock AUJP (speed density) tables.

 

I agree, fix the exhaust leak. Even downstream ones can adversely affect driveability.