Where is your thermostat located in 1990 Nissan Maxima?
Usually in a removable housing at the engine end of the upper radiator hose
I believe it is on the left hand side of the engine, on the firewall side. It is soewhat difficult to get to. You need small hands and patience. This is alwo where the thermostat is located (3 bolts). I believe there is an answer on this site for the thermostat and the sensor is adjacent to it. I considered changing the thermostat, but since it's somehwat complicated, I will leave it to a garage…
The thermostat is located on the RH side (passenger side) of the engine, toward the front. A radiator hose goes into a housing. Looking down from the top, there are (normally) two sensors going into the housing. The thermostat is below these and the cover is held onto the block with three bolts. If you replace it, go with an original Nissan brand. It's worth any extra cost.
The 1990 Nissan Maxima came equipped with the 3.0L V6 and the RE4F02A automatic transmission. This type of transmission requires a professional tear down and overhaul to remove and replace the transmission filter assembly. It is not a standard pan filter with relatively easy access. The internal Maxima filter for the RE4F02A transmission is ATP part #B157.
Maxima Running Rough Try replacing the Idle Air control valve! I am having the same problem, and cannot find anyone who knows where it's located so I can take it off. UPDATE: The Idle Air Control valve is located underneath the throttle body. Remove the throttle body, turn it over, and voila! There she is.