The oxygen sensor is usually mounted in the exhaust manifold pretty close to where the exhaust pipe bolts on.
The crankshaft sensor is located on the lower part of the engine closest to the firewall, on the passenger side of the car.
It is installed on the top of the back side exhaust manifold.
MAP sensor
If it has a 3.1L engine, it'll be in the exhaust pipe just after the exhaust manifold. Rotate the engine forward, look down between the engine and firewall, it'll be pointing up at you, with a wire coming out the top. Cheap/easy to replace, will better your mpg's!
There is no cam sensor on a 90 3.1. Only a crank sensor.
sensor is on the left side right above where the heater hose connection goes into the engine. It is a 2 wire plug
It is screwed into the exhaust manifold on the back of the engine.
My 1990 Lumina 3.1 had the same problem and it was caused by the crank sensor.
in front of the air filter.
It is in the exhaust pipe where it comes off of the engine on the passanger side.
The 1990 Buick LeSabre oxygen sensor is located on the outside of the air cleaner housing. The oxygen sensor will be on the back side of the housing.