seagates5

I have a 95 ES300, I jus recently replaced the sensor for the gauge on the dash and it started back working. The only thing now is that the gauge when it starts up automatically climbs to halfway within 4 seconds from a cold start. Then while driving it slow climbs to "overheating". I know its not over heating because i hooked it up to me scantool and I used a infrared temp scanner and they both read 178 F when it got to the point of normal operating temp and it was holding at that temp. The fan is spinning but not super fast. What I did notice that if I turn the A/C the temp starts to drop down immediately on the gauge and once the A/C is off it climbs back up. When Im on the freeway the temp stays close to a normal temp range without the A/C being on. Also I had jus recently replaced the gauge in the dash for the temp because I thougth that was the problem at first and there is a leak in the power steering somewhere near the power steering pump if that makes a difference. At this point I am completely lost and I dont know what else to do, someone please help me out!!!

CUMan

The leak in the power steering could be a factor. Yours is one of the ES models in which the radiator fan is operated by fluid pumped from the PS pump. Low PS fluid could reduce the hydraulic pressure going to the fan, thereby reducing its speed.

You mention that the car does not overheat when at freeway speed. This is because the air flow across the radiator is adequate to cool the car down without depending on the radiator fan.

Proper fan operation is critical when the car is idling or moving at slow speeds. As more cooling is needed, the fan should spin faster. There is a fan control unit underneath the glove compartment which adjusts the fan speed as needed. This unit is connected to a sensor which monitors the coolant temperature. A wiring harness connects this control unit to a solenoid located on the top of the power steering pump. Proper operation of this solenoid is critical to proper fan operation.

There have been numerous postings on this forum about folks finding that this wiring harness has been disconnected from the PS pump. When this occurs, the fan speed does not increase enough; and the car overheats when idling or moving slowly. Look for a wiring harness coming out of the firewall and make sure that its connector is inserted into the top of the PS pump.

I would surmise that mechanics working on these cars often disconnect this harness when working on PS pumps or perhaps when replacing belts. Then, without realizing how important this connector is, they fail to put it back in place. The result is overheating when the weather gets warm.

LexusFool

(I'm assuming your scantool reads 178 while your guage shows overheating in the red? you should testdrive with the OBDII scanner and compare the two temps, there will be a little lag time in temp readings between the two.)
If the scan tool and your temp gun says 178 F but the dash guage shows overheating in the red, then your coolant temp sender is bad, it's a single wire sensor under where the radiator cap is.
The scantool reads from a different 2nd temp sensor. The coolant guage is just a dummy guage for the driver, but ours actually moves like a real temp guage, most other cars just just have a few positions on the guage, middle for normal or high in the red overheating which is too late already. The Toyota online manual will give you ohms (resistance) parameters for the coolan temp sender unit which you can check with a volt/ohms meter. The sender could be a defective part, there is a bad ground, or the wire is corroded since coolant from the radiator cap spills on it.

-Part to check is the thermostat, that can give erratic temp fluctuations.
-Another part is the water pump, see if there are any leaks or coolant loss and while the engine is warm see if the radiator hoses feel firm, the top radiator should feel hot also on both sides. I've had a water pump failure far away from home (hoses was not firm at all and the top of the radiator was warm to the touch) and had to drive fast on the highway just to keep it below 200 F, when off the highway it would climb to 212F and the fan would come on cycle constantly on about 15 secs and then off 5 secs. After I replaced the water pump (also replace timing belt, idlers, belts, and cam/crank seals if leaking) the temps were back to normal.

I think your fan fluid pump pressure is good because you say with the A/C on it does cool down.