i bought my 1987 2.5 base coupe about six months ago.ive been driving it for short trips while trying to fix various issues.none of the guages worked other than the speedo.i bought a new to me dash cluster.i 'checked'the temp guage using the rodney dickman 'kit'.the guage checked close to right.i bought a WVE sender from rockauto.the guage now says the car gets to 260 degrees after about ten minutes.i have a temp gun that says its only 145 degrees but i don't know how accurate it is.i used teflon tape on the sender per my chilton book.i have a cheepy 2 inch guage from amazon coming today to verify the temp.id rather use the dash guage.

Fuel, Engine Temp or Oil Pressure gauges have problems? by theogre
https://www.fiero.nl/forum/Forum2/HTML/146784.html

Dash Temp & others has senders that Ground to the engine by their threads. Don't use Teflon Tape for them. Some get lucky & work that way but many don't often because too much tape.

Location of sender/sensor matters...
Dash senders goes into head or block.
ECM coolant temp is in the T-stat tube.

See above link too.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

it turned out the new sender was bad.i replaced with one from my parts car and now the guage works right.