Question about thermostats.

[Fett]

Can a thermostat "bleed" a little hot water?

Better yet, let me explain my question.

A few weeks ago my Taurus fan started giving me problems, it wouldn't come on from time to time. Found out it developed a dead spot and just wouldn't come on at times, causing the car to run hot. Never over heated it, just ran it hot a few times. Fixed the problem by replacing the fan with another Taurus fan and running it on the low speed, now the car is cooling perfectly. BUT one problem, I am noticing the fan is coming on too early....as in before the thermostat even opens.

My thermo switch is a VW switch that is set to turn the fan on at 198 and turn it off at 189, and is drilled right into thetop of the thermostat housing. It worked flawlessly before my last issue. The initial fan start up would happen a little while after my thermostat (180*) opened up, my stock gauge would rise to just above half way, then the fan would come on, and the gauge would stay just below half way the rest of the time. The fan would switch on and off as it should.

Now the fan comes on when the engine gets warm, but the thermostat is not opened...well before it gets to half way. I felt the hoses and the bottom one was staying cold while the top one was heating up, but not to the point where it was obvious that the thermostat was open. Then it would get really hot like it should after the thermostat opens up, and the car never gets above half way.

My theory is that the thermostat is sticking slightly open, ever since the running hot problems I was having, and it is allowing hot coolant from the engine to pass through the thermostat, just enough to trigger the sensor and turning on the fan.

So my question is, can a thermostat bleed or leak through slightly? Just enough to trip a thermo sensor located just past the thermostat?

 

[NIKwoaC]

Thermostats are cheap dude. Replace it. Its either failing or your temperature switch is. Replace the T-stat and if that doesn't work replace the switch.

 

[vristang]

Normally there is a 'check ball' valve at the top of the stat...
If this valve were positioned properly (in the highest point), then yes, it could lead to localized temp rises in the coolant....

This would be the first I have heard of this happening, but I can see how it may be possible...


Maybe try moving the fan sensor down stream from the stat???

Consider me subscribed,
jason

 

[LiquidGT]

Is it a Mr gasket thermo, I had one of those and it would bleed open early, then fully open at its advertised temp.

Are you sure you got the air pockets out, and are you using the stock gauge?

 

[HISSIN50]

Good ideas here. Jason is right-on with the jiggle valve. They can indeed pass enough coolant to affect a reading. And as LGT noted, the balanced stats often start opening about 10*F before their advertised rating.

Are you able to get a resistance (or voltage) reading from your VW sensor? It might be interesting to try to relate a reading, the fan's status and how warm the upper hose feels.

Good luck Chris.

 

[Fett]

Appreciate the input. The more I think about it though, it can't be the thermostat. The thermostat is opening at the proper time, so even if it is leaking through it still shouldn't trigger the switch.

The switch is designed to open at 198*, so any coolant that may be passing through the closed thermostat is going to be cooler than 180*, as that is what the thermostat is. So even if warm coolant is getting past the thermostat, it shouldn't be hot enough to trigger the switch.

Actually the switch shouldn't open even when the thermostat opens, as the thermostat is 180* and the switch shouldn't open until 198*. It should open a few minutes after the thermostat opens, which is how it was working before.

So I am thinking the switch is bad. As I understand it, the switch is a relay style as opposed to a resistor style.

I'm going to throw a new switch in it and see what happens. I mean, it can only be the thermostat or the switch, and theoretically speaking it can't be the thermostat.