Most Mercedes engines are fine to wash but the 104 isn't one of them. Moisture gets down near the spark plugs and carbon tracks the outside of the ceramic insulator and shorts the plug. You should remove the 3 spark plugs under the coils and check for carbon tracking even though this isn't likely the ASR problem. If you find any carbon tracked plugs you should replace the connector on the coil also. An ASR issue after getting the engine wet points to a throttle actuator problem. Whether it's the actuator, it's harness, or the connecting harness will have to be diagnosed. If you search "throttle actuator" here you will probably get more than you want to read.

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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