1999 Grand Prix coolant temp fluctuates from 190-240 at idle!

Hi,

I have a 1999 Grand Prix that has the coolant temp fluctuating from 190 to 240 degrees at idle. It takes about 1 to 2 minutes for the temp to go from 190 to 240 and if I accelerate the rpms to 1500, the temp will go back down to 190 in about 15 seconds. This is with the car in park and the outside temp at 80-85 degrees.

The car has 119K on it and just don't know if the coolant temp sensor is shot or if the water pump just is not doing anything at idle? The fans turn on at 220 degrees but the temp stays at 240 degrees until I push the rpms to 1500 or more.

Any thoughts?!

First, check coolant level in the radiator when the car has cooled down. If the radiator level is fine, you may have a bad t-stat. If the coolant level is down, it could be a water pump (they will leak externally via the weep hole in the casting) or a bad intake plenum/gasket (they will leak internally, or around the throttle body). Either way, get it looked at, before you blow a head gasket or hydrolock the engine.

The coolant level is normal and there are no leaks under the car. I have the 3.1L engine with the aluminum plenum. I hear the 3.8L engines with the plastic plenum are very problem prone!

I will change the thermostat and see if that rectifies the problem!