I think you are confusing the lambda signal with the O2 signal. When lambda is over 50% the engine is running lean and the ecu is enrichening the mixture. Reading under 50% the engine is running rich and the ECU is pulling fuel out. Reading ~90% lambda indicates the engine is very lean either due to the adjustment being way out or possibly a vacuum leak inadvertantly caused during the previous repairs. After ruling out vacuum leaks you'll be fine to enrichen it to approx 45-55% lambda.

__________________

90 300TE 4-M
Turbo 103, T3/T04E 50 trim
T04B cover .60 AR
Stage 3 turbine .63 AR
A2W I/C, 40 LB/HR
MS2E, 60-2 Direct Coil Control
3" Exh, AEM W/B O2
Underdrive Alt. and P/S Pulleys,
Vented Rear Discs, .034 Booster.
3.07 diffs 1st Gear Start

90 300CE 104.980
Milled & ported head, 10.3:1 compression
197° intake cam w/20° advancer
Tuned CIS ECU
4° ignition advance
PCS TCM2000, built 722.6
600W networked suction fan
Sportline sway bars
V8 rear subframe, Quaife ATB 3.06 diff