The sensor below the thermostat (which you replaced) is what tells the ECM the coolant temperature (for all purposes). So, this sensor tells the ECM when to turn on the fans (the ECM actually grounds the activation side of the fan relays to turn on the fans for your year).

The sensor in the right head (which you also replaced) is what tells the dash's analog coolant temperature gauge coolant temperature.

FYI, Often the analog coolant temperature gauge is inaccurate and HIGH (or miss-read as high) and does NOT agree with the sensor below the thermostat. This can also be caused by corrosion on the connectors (remember the gauge and ECM reads the 'resistance' and translates it to a temperature).

There are also times where both sensors are accurate but different. The difference between the sensors can be real and be caused by bad thermostats or aftermarket thermostats which do not open enough or at the correct time.

MOST C4's also have the ability to display the ECM's coolant temperature on the digital dash by using the D.I.C. gauges button.

If yours does too, you could see the actual coolant temperature the ECM is seeing and better determine when the fans should come on (primary/driver side fan should come on 228 F (+/-) and secondary/passenger side fan on at 236 F (+/-).

So, in summary, there's a chance by reading your analog gauge you have perceived the engine was hot, but the ECM did not perceive the same thing -- hence why the fans may not have come on.

However, do not just dismiss the 240 F reading -- check the connector and maybe check the OHM reading on that sensor and compare it to the FSM table info to see if the gauge is sensing the temperature correctly.

If it is, suggest replacing the thermostat (many seem to have problems with aftermarket thermostats, suggest using only ACDelco).

Also, if the anti-freeze is older than 2 yrs, suggest also changing it too as it might have lost its ability to prevent spot boiling which can cause the hot spots the analog coolant gauge might be detecting (remember your year DOES NOT use dex-cool) and you will probably want to also avoid 'compatible with all' types too (e.g. Texaco or Zylex).

 

An update:

After reading all the responses I did the following:

- drained the coolant, flushed the system and installed another thermostat - 160 F

- cleaned out the inlet area to the radiator - hoover vacuum cleaners could not have made a better debris catcher than the space between the rad. and the transmission cooler. This area was full of leaves!!!!!!!!!

- re-installed my PCM chip

- removed the Coolant temp. sensor and re - installed with only part of the threads covered with teflon tape****

- filled with coolant

- ran the car and checked the coolant temp. - I have the climate control where you depress the 2 arrows and -00 comes up, scroll to the coolant temp. code ( 16) , press the auto fan button and..............

at 91( I assume the temp is 191 F - or 197 C from the link) the fan came on

VIOLA!!!!!!!!!!!!

Test drove the car tonight for about 2 hrs. - runs great and no overheating.

I have concluded that the overheating was caused by the blockage at the rad. inlet and the fans didn't come on because of the coolant temp. sensor not being able to ground - on my initial install I covered all the threads with the teflon tape.

THANKS TO ALL FOR YOUR INPUT!!!!!

FLATBUSH 862