Perhaps one of the least understood components on the E30 M20 is the EVAP Fuel tank breather valve. If the valve is stuck open or if the hose has a tear, it will cause a significant vacuum leak.
The green hose connects to a brass barb on the underside of the throttle body. It all may look like one piece, but the valve in yellow is separate; the green hose just slips over it.
The actuation of the valve is controlled by the connector in orange.
The opposite side of the valve connects via a 8X13MM fuel hose to the charcoal canister, which then goes to the fuel tank.
Without the green hose this is what the valve looks like (it can be actuated/tested by simply applying 12v and ground across the two pins):
The green circled elephant looking hose has been superseded by a regular fuel hose; part number 13311288284:
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Does this exist on the M20B20 also? First time I’ve ever heard of it, I suspect mine may be missing…