My 1991 had a tough time going through VA emissions testing in September. The car ran very hot throughout the test (this test runs the car drive wheels over some rollers). The mechanic checked the car and found that the temperature sensor was not working. The temperature sensor sits to the front of the engine at dead center before the FI unit. It has a two contact connector (versus a temperature sending unit on the block which has one). I installed a borg warner aftermarket unit and swapped out the thermostat one at a time. The new sensor is working but because the electric fan wasn't kicking in until the temp was well past half way on the guage, we swapped out the thermostat. No change. Here are some of the quirky things that we found:
Fan will not turn on until well past mid mark on gauge except when you turn the car off and back on again.
Gauge will not EVER go past mid mark if the AC is on.
Have only seen the gauge past the mid mark when the car is idling in park.
Mechanic said it may be the sending unit. As I indicated, the fan is turning on, only it appears to be turning on later than I would like.
It could be that this is somewhat normal operation. I would like to know if anyone has had a similar experience. Should I have gone with the factory temperature sensor versus the Borg-Warner?
Tom

 

My actual guage was reading approx 10-15 deg. over actual temp. I used a infa-red temp measure gun. Used at differents location in eng. compartment. Temp was really 190+ deg. and not 215+ deg at guage.My mechanic used his and then I used mine later. Replaced guage and it is still off a bit. I have a '91 coupe.

 

I'm assuming you know the cooant temp sensor is working because you used a scanner which translated the temperature it was conveying to the ECM. Your gage uses a different sensor threaded into the right hand cylinder head. You may need a new one, or you may need a new display. Disconnect the gage sender and needle should peg at 0 or LO. Ground the wire to the block and it should indicate max temp. If so, the gage and the wiring are at least working and you can try a new sender.

For fan control, the PCM is going to ground the main fan somewhere around 217 based on the signal from the Coolant Temp Sensor you replaced. The secondary about 226 (I may be off on these #'s). In any event, with the a/c off and a scanner hooked up, see what temps the fans are coming on at. With the a/c on, both fans should be cranking - the PCM signal for a/c operation is from the Pressure Switch on the high pressure line and not the Coolant Temp Sensor. You can see what that high side pressure # is with a scanner too. Emissions testing is always with the a/c off as all foreign and domestic production are certified for non a/c operation only (they have to be, as good a/c operation mandates lower coolant temps and richer mixtures which won't pass current standards).