Hmmm...I not 100% sure about this but if I recall right the decel switch will only affect your ecu to cut of fuel when engine rpm are above certain amount of rpm. Then when rpm drops below this rpms it will pressurize system for normal use again.
My point is...wheater the switch is working or not it shouldn't affect idle. But I hope someone with more knowledge back this up...I am not sure.
'87 MBenz 190E 2,3 8vlv