After car has reached operating temp, if I don?t move for a while the temp. will keep rising. I notice that the fans have not been coming on, but they do turn on when I turn on the A/C. Where is the problem coming from?? This is in a 1996 Chevrolet Impala SS.

170 F is 76.6 C which is around 10 C lower than is required for diesel engine operation
Cummins have thermostats at various opening temperatures and it is in this area that you should be concentrating your search
How it works
coolant in an engine is recirculated until engine operating temp is reach ( around the 85C mark) at which point it starts to open
That allows water to pass to the radiator and the air cools the coolant by air passing through the fins The coolant flow is low and so the temp keeps rising until the thermostat is fully opened and full flow is through the radiator
Now if there is insufficient air flow to lower the coolant temp as in slow moving / heavy load and pulls , then the fan clutch cuts in to assist with the air flow
so back to you statement , what you need is a thermostat with and opening temp or 85 C(185 F and fully opened at 95C (203F)

possible and an accredited radiator shop will be able to do a flow test to check that
unfortunately , it indicates a gasket or cracked problem so have a compression test done to eliminate that
if that test proves good then have the thermostat removed and checked/replaced if the engine runs a viscous fan hub then check that as well
viscous fan hub failure is most noticeable when stopped at lights ( temp gauge goes up but comes back down when moving along the road) temp rises in slow moving traffic , using high engine rpm as in up hill or any condition where the air is not moving through the radiator unless the vehicle has speed
if you have electric fans then the problem may be in the coolant SENSOR unit that sends the temp to the ECM to switch on the fans

The coolant fan should kick on when temp reaches 192 degrees. Have you changed your thermostat? The 2nd fan is for the a/c. If car is running hot, it's probably thermostat or cooling fan

It's very common for the temperature sensor to go bad on these cars. A quick fix is to pull the connector to the sensor. It is a single wire, the sensor is under the distributor. With the wire pulled, the fan will come on and stay on all the time. This way you won't overheat. Replace the sensor and you've fixed the problem...Chris, owner of 5 of these Australian Capris...

could b emssion control filter may be blocked and need replacing as 4 the fan leave car idling at normal opperating temp after 5min or so fans should kick in if it dosent and temp starts rising u may have a faulty thermo switch

check the fuses to the fans or they may just need to electric fan motor replaced

the air from moving thru the radiator keeps it cool, check to see if the fan is coming on

The thermostat may be wearing out. A thermostat controls the engine temperature to keep it at a consistent temperature, The other fan will probably turn on if you turn on your Air Conditioner.

Don't run the car without a thermostat. It helps the car to reach operating temps quickly. also helps burn off blow by gasses by keeping the temps up and steady. check your radiator level once driving a few miles. Their may have been some air pockets that purged and need to be replaced with additional fluid.

The temp sensor for the fan is along the radiator hose. You should see wires coming out from it. If you pull out the sensor from its plug, the radiator fan should turn on and if it does not then the fan could be faulty. Or the fan may be weak in blowing air in