Why does man. trans. f150 engine idle rev up and down until clutch is engaged?

This could be a simple vacuum line leak or a sticking idle air control valve issue. If you care to bring it in we can usually diagnose this in less than a hour or so. Please call and we will get you right in. Sterling Car Care 586-939-0230

check for vaccum leak at pcv valve hose,common problem with these.hose is very thin and will split and collapse,replace hose and pcv valve,or clean the old valve,if it rattles good after cleaning use old one,check all hoses for vaccum leaks,including line to brake booster.if all vaccum lines good,next would be remove iac valve and clean the white worm gear inside with throttle body cleaner,let dry and reinstall.most of the time you can reuse old gasket.unhook battery first before removing iac valve.neg. first then pos last.when hooking back up,pos. first,neg. last