the first thing to look for with idling problems is air leaks, on the m50 motors one of the most common leak points is the two lines comming off the valve cover rot off, the big one goes to the air boot sometimes going through a small tube atached to one of the supports for the manifold before it hooks to the boot, the small ones goes to the fitting that the idle control valve ataches to under the intake about dead center with the nipple facing the front of the car
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