There are a number of common problem areas on the turbocharged (and some supercharged) M50 based motors.
With the OBD1 throttle body, there is a port on the bottom for the purge valve. It is generally best to disconnect this from the purge valve solenoid and cap the port on the throttle body, or install a check valve between the port and the solenoid.
On the M50 manifold, the ICV connector that connects the “turkey neck” to the intake manifold is held in with one clip. Under pressure, the connector can rock on its mount and compromise the o-ring seal. This can be especially difficult to find/troubleshoot as it often will snap back in place when the engine returns to vacuum based operation. The two common solutions are to either glue the fitting into the intake manifold, or to use zip ties to hold the fitting in the manifold.
In some cases the crankcase ventilation system will have been left connected to the intake manifold. This should not be the case. The crankcase must be vented to a non-pressurized location. On early M50 based applications, there is a small hose that, in stock form, connects the valve cover to the throttle body boot. That must be vented elsewhere. (We recommend connecting to a catch can, and then to a crankcase evac kit tied into the exhaust.) There is also a small hose that connects the valve cover to the ICV connector mentioned above. That should be plugged on the valve cover side, and plugged on the ICV connector side as well.