Truck won't stay at operating temp

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HawkeyeScott

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I've installed a new 195 degree thermostat and temp sender/sensor on my 1997 GM 5.7 w/ 167K, trying to correct a fluctuating temp gauge. Upon startup, it slowly warms up to 210 degrees, which I understand to be the normal temp. But after about 1 minute there, it drops very quickly -- over 10 seconds or so -- to about 155. This morning, the gauge spent the last 2/3 of my commute dancing between about 160 and 170. It never gets back to 210.

The coolant is full and the system should be properly burped. Based on the gauge starting on the Cold mark and hitting 210 initially, I'm led to believe it is accurate.

I'm wondering if a fan is kicking on and then staying on. Or if there's something else happening that's triggered by the system hitting 210.

This is a new one for me. It's not a huge deal, but I do wonder if there's any long term issue with running the engine cool. A higher chance of sludge from lack of moisture burn off, for instance.
 
If you have one of those infra-red temperature guns, use it to confirm the temp on the thermostat housing. If you find the temperature is not actually jumping around, it's the sensor or the dash gage. The sensor is not expensive and is mounted on the driver's side above the exhaust manifold. There is another temp sensor on the intake manifold but that one is for the computer.
 
Originally Posted by Snagglefoot
If you have one of those infra-red temperature guns, use it to confirm the temp on the thermostat housing. If you find the temperature is not actually jumping around, it's the sensor or the dash gage. The sensor is not expensive and is mounted on the driver's side above the exhaust manifold. There is another temp sensor on the intake manifold but that one is for the computer.


+1 but I have had some bad new thermostats.
 
Yes. The fact that the Tstat is new does not guarantee success!

But the infra red thermometer is a piece of cake, works well for this diagnosis.
 
Originally Posted by HawkeyeScott
I've installed a new 195 degree thermostat and temp sender/sensor on my 1997 GM 5.7 w/ 167K, trying to correct a fluctuating temp gauge. Upon startup, it slowly warms up to 210 degrees, which I understand to be the normal temp. But after about 1 minute there, it drops very quickly -- over 10 seconds or so -- to about 155. This morning, the gauge spent the last 2/3 of my commute dancing between about 160 and 170. It never gets back to 210.

The coolant is full and the system should be properly burped. Based on the gauge starting on the Cold mark and hitting 210 initially, I'm led to believe it is accurate.

I'm wondering if a fan is kicking on and then staying on. Or if there's something else happening that's triggered by the system hitting 210.

This is a new one for me. It's not a huge deal, but I do wonder if there's any long term issue with running the engine cool. A higher chance of sludge from lack of moisture burn off, for instance.


First check the temp of the coolant, such as by using an infrared temp gun. Verify that it isn't up to temp and the gauge isn't faulty. My brother has a 97 K1500 and his temp gauge never goes above the 1/4 mark. Gets up to temp just fine but the gauge doesn't work correctly. His tach also reads about 5K RPM anytime he starts driving along with his speedo being about 5 MPH off, so he definitely needs a new cluster. He just doesn't want to pay for one and really doesn't care. The other option would be to hook up a data logging tool to verify the info from the PCM is correct. That would help verify you have a faulty gauge.

If the gauge is accurate, does your truck have an electric or mechanical fan? Most of these trucks came with mechanical, which would most likely be the fan clutch. If you have electric then you most likely have a faulty sensor, which is causing the PCM to command the fan on all the time. Either that or the A/C is commanding the electric fan on and keeping it in, although this is less likely than a faulty sensor. Again, this can be checked with a data logging tool with the info from the PCM.
 
Originally Posted by Rand
Did you use OEM t-stat?

You beat me to it! I only use OE thermostats. I've had more than one aftermarket thermostat prove to be defective. Even though a company like Stant might make OE thermostats the stuff they sell for the aftermarket can differ slightly from OE.
 
If I had to bet on what was wrong with it, I would bet it's still airbound. Water sits in block and gets hot. Thermostat opens and lets the hot water out, no cold water is able circulate, and the temp as seen by the temp switch, drops to 155. After a while the water starts moving and the temp goes up again. AIRBOUND.,,
 
Originally Posted by BigCahuna
If I had to bet on what was wrong with it, I would bet it's still airbound. Water sits in block and gets hot. Thermostat opens and lets the hot water out, no cold water is able circulate, and the temp as seen by the temp switch, drops to 155. After a while the water starts moving and the temp goes up again. AIRBOUND.,,

Possibly, it is certainly something worth checking, although the OP said: " The coolant is full and the system should be properly burped." Should be could mean might not be properly burped. OP I'd check that out.
 
Originally Posted by HawkeyeScott
....... Upon startup, it slowly warms up to 210 degrees, which I understand to be the normal temp. But after about 1 minute there, it drops very quickly -- over 10 seconds or so -- to about 155. This morning, the gauge spent the last 2/3 of my commute dancing between about 160 and 170. It never gets back to 210.

The thermostat is probably initially sticking when it's heated from a dead cold start. My 180F degree thermostat in my 1991 Ford F-150, 5.0 does the same thing. I do not have a numerical temperature gauge. Instead it reads, "C - NORMAL - H".

When I start it dead cold it will rise up to in between the "O" and the "R" in "NORMAL". Then when the thermostat opens, it quickly drops to just below the "N", and stays there regardless of the outside temperature, or how I'm driving.

My old thermostat started doing the same thing, so I replaced with this Stant 180F, which is what is in there now, and it continues to do the same thing. It's not a problem, so I just live with it. My cooling system is well maintained, and the truck has never run hot. Not even pulling an overloaded U-Haul trailer 200 miles in Southwest desert Summer heat. Thermostats can be quirky like that.
 
- eliminate a bad stat by checking it in some (bringing up to) boiling water on your stove. (you should see it open/close)

- if it's not the stat, pull the sending unit and check it's resistance. (there are videos how to do this, it's super easy).

- if it's not your stat and not your sending unit, it's the fans or air trapped in the system. Pull the fan fuse and see if the coolant temp stays in range/rises.

But I would put it up on some ramps and try bleeding it again before pulling things off.
 
Last edited:
Great feedback. Thanks everyone. I've got some testing to do. Think I'll start by driving home from work tonight with the scan tool hooked up.
 
Originally Posted by HawkeyeScott
Great feedback. Thanks everyone. I've got some testing to do. Think I'll start by driving home from work tonight with the scan tool hooked up.

If you can find a shop manual or somebody with access to one, find out when the fans kick on/off. This will help when looking at live data... not totally necessary but it'll help provide a fuller picture of just how the cooling system is working, or not working, right now.

Mine for example kicks on at 200 then off at 190 IIRC. If yours is kicking on and off at the right temps than i would be sceptical that the sending unit is malfunctioning...but who knows, just another piece of data to help properly diagnose the problem.
 
Last edited:
Originally Posted by HawkeyeScott
I've installed a new 195 degree thermostat and temp sender/sensor on my 1997 GM 5.7 w/ 167K, trying to correct a fluctuating temp gauge. Upon startup, it slowly warms up to 210 degrees, which I understand to be the normal temp. But after about 1 minute there, it drops very quickly -- over 10 seconds or so -- to about 155. This morning, the gauge spent the last 2/3 of my commute dancing between about 160 and 170. It never gets back to 210.

The coolant is full and the system should be properly burped. Based on the gauge starting on the Cold mark and hitting 210 initially, I'm led to believe it is accurate.

I'm wondering if a fan is kicking on and then staying on. Or if there's something else happening that's triggered by the system hitting 210.

This is a new one for me. It's not a huge deal, but I do wonder if there's any long term issue with running the engine cool. A higher chance of sludge from lack of moisture burn off, for instance.


Do you still have coolant circulating through the heater core? Just guessing that you live in a warm place and may have bypassed it. The heater core is used as a water bypass and that allows for a steady flow of coolant around the thermostat. Without that bypass flow, you will have the kind of issues you are describing.
 
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