Wish I'd found this forum earlier it's fantastic and I've found solutions to many of my 155 problems
Anyway, to the current problem.
My 155 1.8 16v ts has a tickover problem. Basically it starts fine, runs fine but just cuts out if you come off the throttle. This has been an ongoing problem for a while now. Alfa dealers keep cleaning? something and it is OK for a couple of months before it starts again, usually quite suddenly on startup. There are no associated warning lights either.
so you take your foot off the accelerator and the revs drop like a stone???
Idle valve, lurks in the air intake pipe after the airbox, air temp sensor and airflow meter and before the throttlebody, simply undo two torques bolts (least i think they're torques ) remove Idle valve being careful of gasket, spray liberally inside with carb cleaner, empty sh1tty stuff out, liberally spray with wd40 to lubricate and .....refit is a reverse of removal