Is that possible? It idles just fine in park or Neutral without AC on, but turn on AC(which blows out the correct vents) or shift into OD or R, it drops to 500-600rpms, and starts bucking. When I hit the brakes in P or N, it stays up at 1K, where it should. I know Cyl. 4 is lean, as the primary header tube glows a little bit. It does do the whole snap crackle and pop thng while driving, but never in P or N. Is a leak under load possible? Or could I have Opti issues? I hope this is it, because i have replaced everything but the opti, injectors, and wiring. Any other ideas?

Thanks.

LT4 363ci with CC306 and others.

 

Even though EGR is disabled, there still is the valve here...Could it be the problem? I'll have to go check that...