The function of the EGR is checked by watching the engine vacuum during certain decel conditions. The MAP sensor reads engine vacuum but in a lot of models in that time period (including yours, I think) use a switchover valve to switch between reading engine vacuum and fuel tank vapor pressure. A failure of the MAP sensor or the switchover valve that selects it's signal source can cause EGR faults in an other wise functioning system.

This of course assumes a functioning EGR system, so you need to get that switchover valve working before diagnosing further.

Discounting an EGR valve that is sticking open, it should have no influence in idle quality.

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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