My 94 Firebird overheated last week. So far I have replaced the thermostat and the ECT sensor, but I am still having problems. The fans do not sound like they are turning on- when plugged in to Datamaster, it does not look like the relays have been activated. Furthermore, the engine temperature within the display on Datamaster seems to be about 80C~180F, even when the gauge temperature is above 200. I checked the fans and the relays, they both seem to work. Any opinions or possible solutions?

 

There are two coolant temp sensors on a LT1. One for the PCM and another for the gauge in the dash. The PCM one is located on the water pump and the dash one is in driver's side cylinder head between cylinders 1 and 3.

From Shoebox's website....http://shbox.com/1/4th_gen_tech1.html

PCM Coolant sensor:


Dash Gauge Coolant sensor:

 

which sensor did you replace?
the one in the head is for the dash gauge only. the one in the waterpump is for the pcm. if the one for the pcm isnt seeing at least 226f ( i believe thats correct for stock programming) then the fan(s) wont come on.

 

You indicate the fans are not running, even when the gauge indicates temperatures above 200*F. That would be normal. GM designed the engine to run at 210*F. The first fan (or low speed) doesn't avctivate until 226*F, second fan or high speed at 235*F.

Check both temp sensors for resistance vs. temperature. Check the voltage on the circuit to the water pump sensor. Use Shoebox's guide.

 

I replaced the sensor that connects to the PCM- the one in the first picture. However, we took out the old sensor, and tested the resistance cold all the way to 210. I got about 3500 ohms cold, and 220 ohms while hot- the original sensor seems to have been ok.

We were letting the car get a little above the third mark on the gauge- about 235, and neither fan was coming on. I was however getting a reading of 180F from the OBD connection, so there is a mismatch between the temperature at the gauge and the pcm. The gauge was at 240 when the car first overheated, so I am inclined to trust that one. I also reset the PCM BAT fuse, but got the same results after doing so. Any ideas of what to check next?

 

Check the voltage between the two wires on the CTS sensor plug, harness off the sensor. Should be 5V. The PCM bases it's reading on the voltage across the two wires, not the resistance. So if the reference voltage isn't correct, or there is any resistance in the wires, the PCM is not going to report the correct temperature.

 

The voltage checked out to 5V, and the ground wire had an impedance of about 1 ohm. We also hooked up a 178ohm resistor to the connector, and the pcm reported 101C, so that seems to suggest the PCM is ok. We also tried bleeding air out of the radiator, but I still could not get the engine sensor over 82C. Could the water pump be bad?

 

Did you buy the correct thermostat specifically for an LT1? You can't use a generic small block Chevy t'stat, and many parts people don't know that.

Courtesy of Shoebox:

http://shbox.com/1/tstat.jpg

 

Since you get normal temp readings from the OBD connector, and you have checked and replaced the temperature sensor in the water pump, you need to verify that your gauge is reading correctly. I would replace the temp sensor in the head or you could temporarily install a mechanical gauge to make sure the car is actually overheating.