Water temperature sending unit fitted from 1992-1996 and on all JDM MPIs. This sending unit was fitted on all SPI cars. Identified with black plug and mounts on the bottom of the SPI unit upside down. A bad temp sensor can cause multiple issues, from running poorly to incorrect reading of the gauge. 
 
This is a tricky sender to replace, however it can be done fairly easy by gaining acess from underneath the car using a long extension.