if this is a true euro 85 500sl then it should have black valve covers and ke injection instead of just k.[remove air cleaner and check for black govenor on rear of fuel distributor with two pin plug going to it] if this is the case your idle problem probably is caused by the flywheel pick up that feeds the ignition module down in the left front fender.lrt me know and i can help and also expain why. david poole european performance dallas texas.