the Idle Control valve may need to be replaced. High idle is a known problem with these.
What is your VIN number: WDBCAxxxxxxxxxx us a US spec one and WDBxxxxxxxxxxx is the Euro.
I presently own two US spec. Southern California W126 coupes, as seen here at the Jaguar dealer in Ventura last month, the left one is a 1990, bone stock; and the right one is a 1991 has euro lights, wipers and corners, Bosch French spec Euro fog lights, and a Japanese sold only, Mercedes Benz parking assist pole, and it also has some AMG kit, and other go-fast bits on it.
See also the 1998 "Buyers Guide" I wrote at
www.mbcoupes.com for more general maintenance tips for these cars.
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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...
1995 E 420, 170k "The Red Plum" (sold)
2015 BMW 535i xdrive awd
Stage 1 DINAN, 6k, <----364 hp
1967 Mercury Cougar, 49k
2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive

(sold)