E85 = Check with manufacturer.
Idle speed and duty cycle % are inter related.....unless duty % is 50% and ranging + or - 8% the idle will not set to target.
(this is suposing the system on your car is the electronic idle system...must have a idle control valve just ahead of the CIS-e fuel distributor...has a two pin electrical connector.)
http://www.***********************/apw_parts/mercedes~300e~idle_control_valve~vmr-mmp~apw43bc687193231.html?apwcid=V1092232004W411a2344ecd29&apwkwd=Mercedes+300E+Idle+Control+Valve
Hi-test, premium fuel will usually burn clean in a warm healthy engine.
however a 89 might perform better as it will burn better or more complete reducing t/p emissions.....not always the case.
Igniton lock problem is co-incedental.
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