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Coolant guage/temp issue

gt350pilot

New CEG'er
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Oct 11, 2013
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Getting the CSVT ready for a family vacation so thought it would be a good time to take care of a few things before we drove a bunch of miles. So put in a new water pump (metal impeller), hoses and thermostat. While I was doing that I apparently hit the temp sending unit and broke the insulator causing it to ground out because the temp gauge was at full hot as soon as the key was on. Checked it by removing the lead and turning on the ignition to confirm and it was now cold.
So got a new sending unit and put it in. Now after running the system the gauge shows much warmer than previously. It now reads at the very end of the normal scale, pointing at the L (AKA Hot) where it previously never ran warm. This led me to thinking that I got a bad sending unit, so I replaced it with a different brand, only to have the same issue.
I don't feel that I have a thermal pocket as I have burped the system including jacking up the passenger side and squeezing the coolant tubes to force out air a couple of times with the same result on the gauge.
So in frustation, after driving the car and getting the system nice and warm (according to the gauge), I pulled back into the garage and pulled the cap off of the coolant reservoir, dropped in a thermometer and it only read 140 degrees. Now I am completely confused. Is this a symptom of a stuck thermostat? Do I have a newly neurotic gauge? Any other suggestions? Is it possible that I am still dealing with a thermal pocket somehow? I am at a loss.

Thanks for any help.
 
The coolant reservoir is not going to reflect an accurate system coolant temp, as it does not circulate much thru the overflow reservoir. Do your cooling fans turn on if you let it idle for 10 min or so? The fans are controlled by a different sensor (2 wire), and it should click on the fans at a temp of (not sure) 210F? What is the official programmed temp?

An IR temp gun is handy for this kind of troubleshooting.
 
Dash coolant temp gauges are not instruments, they are little more than idiot lights with pointers, and aged (dash) coolant temp sensors are known to read low, so replacing with a new sensor might make the gauge read upscale from what you had been used to.

If you fill the system through the recovery reservoir, they are 95% idiot proof full, and these engines are pretty good at burping air bubbles due to the two steam lines from the top of the radiator and the upper hose that return air to the recovery reservoir.

If "after driving the car and getting the system nice and warm" you removed the recovery reservoir cap and didn't get a geyser of flash boiled coolant, then I might suspect that the system isn't running pressurized when hot, or your new thermostat isn't closing and allowing the engine to attain the right temperature. A new reservoir cap might be in order, and I would borrow a cooling system pressure tester from an auto parts store (most, like Auto Zone with rent them and then return the deposit) and see if the system can hold pressure. With the engine up to temp (about 200F), the upper radiator hose should feel firm and pressurized.

It used to be that thermostats were easy to install backwards, this usually resulted in an overheated engine, but the V6 thermostat is difficult, if not impossible, to install backwards, due to the water pump passage bypass disc on the engine side. The right V6 thermostat is a Stant 14139 IIRC.

You could also borrow a scan tool that would let you read the engine coolant temp sensor (different from the dash temp sensor) through the ECU.
 
Thank you both for the feedback. I have borrowed the pressure tester and will work through both of suggestions. I will let you know what I find.
 
My '95 2.5L V6 MTX coolant system isn't pressurized, it has an overflow tube sticking out the firewall corner of the coolant jug. Did they change the design over the years?
 
FYI -- I took 5 temperature sensors (dash gauge senders) from Napa stock, and ohm'd all 5 on the counter. The ohm values were all over the place. This will result in the dash temperature gauge reading differently for each of these Niehoff. Quality control not what I expected/desired.
 
My '95 2.5L V6 MTX coolant system isn't pressurized, it has an overflow tube sticking out the firewall corner of the coolant jug. Did they change the design over the years?

As stated all coolant systems are pressurized. The system is only under about 16 psi, which helps prevent the coolant from boiling.

Also your coolant tank has been changed out. The pre-98 tanks did not have an overflow hose, it was a solid tank.

My coolant res has an overflow tube, it can't be pressurized.

In the Post-98 tank setup the overflow hose is outside the seal of the cap to the tank. The cap is the pressure relief valve ... when it opens the coolant goes pas the seal and into the overflow hose.

If you look at the tank cap it will have a pressure rating on it.
 
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FYI -- I took 5 temperature sensors (dash gauge senders) from Napa stock, and ohm'd all 5 on the counter. The ohm values were all over the place. This will result in the dash temperature gauge reading differently for each of these Niehoff. Quality control not what I expected/desired.

I am not surprised by this finding. I have tried 2 and had different, but higher readings than the stock sending unit I replaced.

After checking the pressure in the system, flow rate of the replacement water pump, and the operation of my cooling system with an IR gun, and switching temp for my fans, I believe the sending units are my problem. I ran through a bunch of tests yesterday including verifying the operation of my thermostat and have just come to the conclusion that the sensors are causing the gauge to indicate higher temps. My gauge was reading past the middle of the normal range with the water temp at the housing at the sensor location with the IR gun was only 155 degrees. By the time that the thermostat opened the gauge read at the "A" of normal. The sensor issue is somewhat validated by the fact that the needle on the gauge will move at start-up into the warming up area of the gauge.

All this being said, I will replace the sending unit with an OEM when I get a chance.

Thanks everybody for your help. I will report back if I find anything else.
 
Using the IR thermometer is a good way to determine the actual coolant temperature. Additionally the temperature can be viewed from a OBDII scanner/data logger, always a good way to confirm what is going on.

As I have noted in the past the temperature ranges exactly as it should if the fans are operational. I see 190 to 212 on my temperature gauge.
 
About 190 to 212F. Thanks BrApple - that's what I wasn't sure of, the computer fan on/off switch points.
 
About 190 to 212F. Thanks BrApple - that's what I wasn't sure of, the computer fan on/off switch points.

Other have reported differently but in my car the fans kick on to the low speed at 212 degrees and they run until the temperature drops to the thermostat temperature of 190 degrees. I believe that the fans turn on to the high speed at ~218 degrees.
 
Using the IR thermometer is a good way to determine the actual coolant temperature. Additionally the temperature can be viewed from a OBDII scanner/data logger, always a good way to confirm what is going on.

As I have noted in the past the temperature ranges exactly as it should if the fans are operational. I see 190 to 212 on my temperature gauge.

Except in my '95 Tour....OBD1 - no temp logger.
 
Just resurrecting this old thread, I was just datalogging and my cooling fans kick on low speed around 216-218F. They turn off at 204F. The temperature gauge on the dash gets to just under the high line when at 216F, and is half way when at 204F. I'd like the gauge to be a little less sensitive when everything is working well, I might look into a new OEM temp sender for the dash gauge...
 
OK, a new motorcraft sender behaves the same way (wasted $15 bucks), and it seems to be a common problem as I search. The thermistor resistance drops as the temps increase, so I'm going to wire in a resistor (not sure of the ohms needed yet) and drive the gauge back to the middle of the sweep when hot.
 
Added a 40 ohm resistor in series with the sender. Brings the gauge down to mid way when the fans kick on at 216F. Just to be clear, this does not mess with the ECT sensor. Just the stupid factory engine temp gauge...

I found a chart that is close to the readings I measured off the factory sender:

40731131341_6507b824fb_o.jpg
 
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