Temperature gauge reading low around 100

Tiny
CHASE9
  • MEMBER
  • 1998 CHEVROLET VENTURE
  • 109,239 MILES
As titled. My temperature gauge is reading low. It used to run in the normal range. And now about %90 of the time it will show between 100 and 125. I had a bad water pump. I replaced that. And the thermostat at the same time. Took it to have the radiator flush. I know the gauge is working correctly. And not stuck. Because I have seen where it is normally at and seen it move after this low reading issue stuff started. Also did a gauge test just to rule that out. I have flow going through the radiator. Did a few test to confirm that. Such as inferred laser to track heat temperatures in different areas off the radiator. Squeezed hoses got all the normal stuff that should be happening with a unclogged radiator. So I am pretty sure that is okay. No leaks anywhere. So not losing coolant. Fans work. I have seen them come on. Without turning on the A/C. Problem is they will not come on often because the temperature is reading low. So the computer is telling the fans it is okay do not turn on. No symptoms of failing or blown head or intake gasket. No power loss. No stalling. No misfire. There is a very slight rough idle while in park or sitting at lights. But second the throttle is pushed it goes away and runs very smooth. Part of this I suspect is because there is a dirty intake I took off and cleaned the throttle body. But still the slight rough idle. I did see a ton of carbon inside the intake. So my plan was Toby new plugs and wires. Do a Seafoam intake cleaning. Then change my plugs and wires. So I know that needs cleaned. But the low reading thing has me stumped. I have tried include as much information as I could. To maybe get some insight as to pin point this issue. I have seemed to look at the coolant temperature sensor. Because I have most of the symptoms of a failing. One. So really my question is. Has anyone really heard of a failing sensor giving a low reading? My car. Only has one sensor not two. So it controls everything including the gauge. However, I am reading that they will thrown a check engine light and or cause the car to overheat. Mine has not done either. I live in Phoenix Arizona and it winter now. But it is still seventy degrees outside and I have done freeway driving. Long drives. And still no overheated. No fluid loss. If I go for a long drive. The gauge will still read low. But when I get where I am going. I will leave the van running and pop the hood and the fans will be on. And it do not smell hot or like it was even starting to get to hot or overheat. So again. I am stumped. I cannot let it go. It will be 110 or more degrees outside soon. Gauge has to work correctly. When I get paid I am going to replace the stuff I outlined in my post. Someone, anyone, help. Insight please.
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Monday, January 22nd, 2018 AT 8:20 PM

4 Replies

Tiny
HEYMAN1104
  • EXPERT
You are way overthinking this. You got a dud of a thermostat. It happens all the time. It is stuck open, allowing coolant to constantly flow at full speed, and keeping the engine from warming up. Replace it again and you should be good to go.

Keep us posted.
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Tuesday, January 23rd, 2018 AT 5:24 AM
Tiny
CHASE9
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Thank you so much for taking the time to respond. However. I am pretty sure that is not my issue. Due to the fact that when I replaced the thermostat the first time. It was fine. I took it out tested it. And it opened and closed when it should have. I replaced it anyways. But these issues started when the old working thermostat was still in it. Also, even if there is a bad thermostat. Those most of the time will not affect a low reading temperature gauge. And if they do. They will go to hot. If it was stuck closed. If it was open. Once the van got to temperature the coolant passing the coolant temperature sensor would still give me a reading as to the temperature. Not read 100. Not sure about that. It is reading low. That is my issue.
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Tuesday, January 23rd, 2018 AT 5:54 AM
Tiny
HEYMAN1104
  • EXPERT
I gotcha. I am out right now but will check all data again when I get home and see if there is something we are missing. Usually the simplest answer is the right one so that is why I was thinking thermostat. And if the thermostat is opening too early or sticking open, the car will never get to any measurable temperature on the gauge, especially not in the winter. In the middle of summer it would, eventually.
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Tuesday, January 23rd, 2018 AT 7:20 AM
Tiny
HEYMAN1104
  • EXPERT
Well I reread your question and do think that your coolant temperature sensor has to be the problem. I would ask though, how is the heat in your van? Is it blazing hot? Lukewarm? Or non-existent? Hot heat indicates a working thermostat, and tells us that the problem likely electrical (sensor, wiring, or gauge). Non-existent heat indicates a thermostat that is stuck open (or possibly an unrelated HVAC problem, but we will assume that that system works fine for our purposes). Lukewarm heat likely indicates that the thermostat is opening too early, but is opening and shutting in a decent manner.

A coolant temperature sensor is a thermistor. Basically, as it warms up, it conducts electricity with lower resistance. The PCM supplies a base voltage, and then judges coolant temperature based on the amount of resistance in the sensor (as measured by the lower signal voltage being returned to the PCM by the sensor). So, using a multi-meter and removing the sensor, you can heat it up and then measure the resistance in it to see if it is accurate. I am attaching a list of resistances as they translate to coolant temperatures.
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Tuesday, January 23rd, 2018 AT 7:15 PM

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