good evening. after I lost some coolant I attached my iCarsoft tool to read the Coolant Temp while driving and was anxious as it went up to 135 C.. so I took the thermostat out.. did another test run.. and again.. it went up to 110 C on the interstate and the gauge showed half warm... stalled the car and waited.. it went up to normal on the gauge.. 135 on the Diagnostic.. then the fan kicked in.. and at 138 it dropped to 130 when the fan stopped..

Where is the mistake ? thnx

 

operating is 90C-100C. gauge moves off the middle at 110C = shut the car down

 

Thnx. So you mean the diagnostic tool must be totally off?

again: when the tool shows 110C the Gauge shows only half warm... when the tool quotes 135C the gauge shows normal... at 138C the fans kick in and cool down to 130C on the tool.. then the fans stop

 

This a big NO-NO. Never run the engine without the thermostat. When the thermostat opens, it closes the by-pass route for the coolant so that all of it flows through the radiator (to be cooled). Without the thermostat, the by-pass path is always open so, roughly, 50% of the hot coolant flows through the radiator and the other 50% of hot coolant is sent back into the engine block without being cooled.

You were probably lucky if the weather where you live is currently cool. otherwise running without thermostat can cause severe engine overheating.

 

OK. Thanks for the input. It was recommended to me. I dont see where the thermostat "closes" another circuit when it opens up?.. all it does is.. it opens?.. this bypass route is within the thermostat housing?
​​​​​​
Do you have any idea to the main question of the 135C can be correct reading if all the above mentioned car functions like gauge positions and fan speeds work fine?

 

The by-pass route/opening is at the bottom of the thermostat housing. The "foot" of the thermostat closes this opening as the thermostat opens.

If your cooling fans (with the AC off) come on and then go off after some time, then the cooling system should be fine. With the AC off, the fans should operate as follows:

Low Speed - ON at 90C, OFF at 86C
High Speed - ON at 97.5C, OFF at 93.5C

So, if the fans a cycling ON-OFF, it means they can maintain the engine temp within the specified range so everything is fine. I wouldn't trust these readings by the OBD scanners, have seen them showing other funny temp readings. If you want to verify what temp your engine is running at, get a good quality digital thermometer with a probe which you can "calibrate" in a pan of boiling water and be sure about its accuracy.

You can also install one permanently. My installation:

Temp Gauge (now replaced with one having a bit bigger display)


The probe (inserted into an adapter at the back of one head; not in contact with the coolant but actually sensing the cylinder head temperature)

 

Thank you for the great support. Totally makes sense.
The question then remains what made the collant to overflow if the car obviously never overheated...