the car runs. but shakes violently when turned on.

The KA24DE engine in your 240 has a four cylinder engine if one cylinder is not contributing to the engines performance the engines performance will suffer by 25%. Putting sound that sounds like a big exhaust leak that increases with engine RPM could be a spark plug that has blown out of the cylinder head. I have sen the intake manifold fail but it sounds like your car has something more critical wrong like a failed fuel injector, or spark plug, perhaps something more serious.

i had the same promblem in my 92 240sx ka24de. I first started by geting timing done, then i changed out maf and last but not least, i checked for any shorts. For some odd reason the ealier nissan motors then to lag when a short is present. For more info, contact me @ Roksoldier@gmail.com