Strange Over Heating Issue

[Lextreme II]

I have been driving my Lexus for weeks now without any issue. As of yesterday, on the freeway, my car got really hot. The gauge shown almost high. I quickly exit the freeway and stop for a while. This morning I did some testing and this is what I found:

The car runs in high temp after 10-12 minutes. However, when I turn on the heater the air blow is at room temperature. That is what I found to be strange. The gauge show high but heat air is not hot.

Possibility:
* Faulty Water Temp Sensor
* Other Over heating related problems.

What do u think?

 

[cribbj]

David, I'd suspect the temp sensor first. IIRC, it's the smallest of the three on the front water bridge, and it has only one wire going to it. The sensor isn't that robust, and it could be shorting out internally, which would give you a very high temperature reading.

At a summer ambient temperature of 80 degrees F, the thermistor should measure around 2K ohms, and that figure should drop to around 500 ohms when the engine is warmed up to 160 degrees. Much less than 500 ohms would indicate a dodgy sensor, or a serious overheating problem.

I'd try swapping in another sensor first, then if you get the same thing, it's time to have a closer look at the motor, water pump, radiator, etc.

Here's a curve of the thermistor's resistance vs temperature:

 

[stevechumo]

David,
If you turn on the heater and if it doesn't get hot, then perhaps the heater hoses aren't connected correctly. If the coolant got strapped and not being circulated, it'll also cause overheating.

 

[Lextreme II]

Another strange thing is that the temp goes up and down very fast..... So i suspect sensor problem... There is no other sign of overheating.

 

[stevechumo]

I think it's more of the sensor under the front water bridge. It's right behind the right ignition coil on the passenger side. Also, another radiator sensor at the bottom of the radiator could be bad as well.

 

[1uzvl]

Air lock,bleed it up and see how you go.Have you done anything with the cooling system of late??Not losing water at all??

A Air lock will kill the heaters as they are quite high in the system.Feel the pipes leading in/out of your fire wall, will be cold if theres air in it.