There are only 2 temp sensors on the 1991 Chevy 4.3L on the intake manifold just to the left of the thermostat housing and on the passenger side head between the exhaust manifold pipes. the one on top is for the computer and the one on the side is for the temp gauge or warning light!
sould be on the manifold
problem with the coolant sensor check the connection on the sensor reset your OBD I code if that don't work replace the coolant sensor
Here's a link to a schematic and instructions on replacing the coolant sensor (from the Auto Zone website) http://www.autozone.com/shopping/repairGuide.htm?pageId=0900c15280067b92 Good luck!
Green coolant
code 15 coolant temp sensor low temp indicated change cts sensor or test with multi meter
intake manifold vaccum leak
The intake air temperature sensor is built into the MAF sensor on a 1991 Nissan pickup. It is between the air box and the throttle body.
Green coolant (prestone, etc)
By the thermostat
No it does not. Chevy did not use them in that year on there trucks.
The coolant gauge temp sensor is a one wire sensor located on the right rear of the engine block.
It sure will.