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boiling coolant

14K views 18 replies 12 participants last post by  Steve in Chicago  
#1 ·
I had the water pump replaced 6 months or so ago and all has been fine. Just recently the collant was boiling in the reservoir and the mechnaic that I had look at it said it was the water pump or the thermostat. Note that the temperature is a steady 190, never over.

I took it back to the shop who did the water pump and they said tha t the pump was working and they replaced the coolant cap. The coolant is still boiling in the tank.

I am taking it back to them, but I am wondering what it could be.

ANy thoughts would be appreciated.
 
#5 ·
try putting more coolant in there, you may have over diluted your mixture.

Why is it that people think that dilluting the coolant will cause this??? I don't care if you run straight water in your cooling system, as long as the radiator/coolant tank cap is good and of the correct pressure rating it don't matter. The cap will raise the boiling point to somewhere around 245 degrees farenheit, (a rough guess casue I'm to lazy to do the math) if at this point if whatever is in the cooling system starts to boil you have got issues that aren't going to be resolved by correcting the mixture of antifreeze to water.

Does it boil right after you shut the car off or is it boiling while the car is running??
 
#3 ·
keep in mind that the temp gauge unless aftermarket is going to read 190 whether its between like 90 and 200+. Its not a good measure of actual temp, just cold, warm and over heated. Thats why it always sticks to 190.
 
#6 ·
What car do you have? What indication do you have that the coolant is "boiling". Do you see coolant squirting into the coolant reservoir from the small hose connected to the top or it? Most passats have electric after run coolant pumps that circulate coolant. The circulating coolant in the coolant reservoir can look like boiling coolant after the engine is shut down.

Also, the temp gauge has a deadband from about 167*F to about 225*F where the gauge will read a rock solid 190*F. So assuming your temp sensor is working okay, your actual coolant temp is likely less than 225*F and like,|1tech says even straight water with a proper pressurized system will not boil at 225*F.
 
#10 ·
I am sorry to hijack the thread, but this raises a question in my mind. The "dead zone" which causes the gague to read 190, regardless of the actual temp, is that in the Gauge or in the sending unit.

If someone were to change out the sending unit would they get a more accurate reading at the dash? My preference would be to have the truth in front of me as a driver, rather than an modified idiot light.
 
#17 ·
Here's about the range of values when reading from the curve in Bentley's.

*C__*F_ Min. to Max. Ohms
__0___32___5.0k-7.0k
_10___50___3.2k-4.3k
_20___68___2.2k-3.0k
_30___86___1.5k-2.0k
_40__104___1.0k-1.4k
_50__122___700-950
_60__140___530-660
_70__158___390-490
_80__176___280-370
_90__194___200-280
100__212___150-240
ECT = thermostator. variable resistor that has an adverse reaction to heat. (lower temp, higher resistance and vice versa.)
 
#12 ·
As an experiment, I suspended the temp sender from my 1999 1.8t in a cup of boiling water and took ohm readings as the water slowly cooled.

The sender has 4 connection points. It appears to be two separate sensors in a single housing.

results...

water/ Ohms/ Ohms
temp/ pins A-B/ pins C-D

200Âş/ 234/ 126
180Âş/ 310/ 161
160Âş/ 0/ 216
150Âş/ 0/ 258
140Âş/ 0/ 303
130Âş/ 0/ 363
120Âş/ 0/ 0

0 = infinite or "open"
 
#13 ·
Adding antifreeze raises the boiling point of the coolant. A 50/50 water/antifreeze mix will have a freezing point of somewhere around -34Âş and a boiling point of somewhere around 276Âş. Running a pressurized cooling system raises the boiling point even further.

The temp sensor can only monitor the temp of the coolant surrounding it. When the engine is off and the coolant stops circulating, the coolant around the sender won't get any hotter. Meanwhile, coolant in the head or block will continue to absorb heat from the hotter areas of the engine. I have no idea what temps those areas achieve.
 
#14 ·
I am not sure what the temp sender is supposed to ohm out at and and what temp(basically I am to lazy to look it up right now) but in your above readings it would seem as if you have a bad sensor being that the sensor is going open. The sensor is a 2 part sensor, one for the pcm and the other for the temp gauge.
 
#16 ·
Those are readings I took of a new sender that is now installed in the car. After installation, the temp gauge started working properly and the engine smoothed out significantly. I was actually surprised at how much of a positive change it made in engine behavior. Do you think this sender is also bad?

Another thought... I don't know what voltages the car applies to this sender to get it's resistance reading. I did my readings with a Fluke digital meter which probably puts out a very low voltage.
 
#15 ·
Here's about the range of values when reading from the curve in Bentley's.

*C__*F_ Min. to Max. Ohms
__0___32___5.0k-7.0k
_10___50___3.2k-4.3k
_20___68___2.2k-3.0k
_30___86___1.5k-2.0k
_40__104___1.0k-1.4k
_50__122___700-950
_60__140___530-660
_70__158___390-490
_80__176___280-370
_90__194___200-280
100__212___150-240
 
#19 ·
well, dang... thats the third sender put into this car in the last year, and the first one that appears to be working properly (or so i thought).

Chicago Auto Center put one in, DubWerks put one in, then I put this last one in I got from ECS.

I wonder if my meter is broken.