1999 Suzuki Vitara 2.0L, overheat at idle

1999 Suzuki Vitara, 2.0L, 95.000 miles, overheat at idle.

When I stop the car at idle for some time after a trip or when there are traffic jams and it remains idle for some minutes the temp gauge start to reading higher than normal until it reach the red zone.

If I accelerate the car between 2500 and 3000 RPM the temp gauge start down until it reach the normal position, but after some minutes at idle the temp start climbing again.

When the temperature is higher than normal the radiator's upper hose is very hot but the under hose is cold. After I accelerate the car between 2500 and 3000 RPM, and the temp reach the normal level, the under hose starts to be warm and then hot after a minute.

I replace the thermostat with a new one, the radiator was checked and it is OK. The problem rise only when the car is idle for a while after it has been running.

Climb a mountain does not rise the temperature higher than normal, but stop the car or began to lower the hill at idle will do it.

If I remove the thermostat the temperature is always below the normal level. It reaches 1/3 as max when the car climbs a hill but then it decreases to the low position when the car lower the hill.

When the car run without thermostat both radiator hoses are hot, I think there is no problem with the water pump because without thermostat the temperature is always very low.

Some times, particularly on the night, when the car is at idle and the temperature is higher than normal, the calefaction's hot air is cold. After I accelerate the car to lower the temperature the hot air starts to be hot, very hot.