Coolant temp gauge starting to hunt around during cold weather

Alex05

Alex05

TJ Enthusiast
Joined
Jun 18, 2020
Messages
128
Location
Madison, WI
Maybe I'm just eagle-eyeing it now, but I have been noticing my coolant temp gauge hunt around on either side of the '210' dot. Once at operating temp, the gauge can't seem to decide where it wants to land. It will move from just below the 210 dot to just above it during normal driving. Actual temp seems fine; There are no obvious physical signs of overheating.
Of course, this gauge is mostly a "OK/Not-OK" gauge. Before I put a scan tool in my passenger seat and watch temps while driving, has anyone else encountered this issue? Was it actually anything to do with your thermostat?
 
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I don’t think a thermostat would cause your issue. They usually fail stuck open or closed. Maybe the viscous fan drive is nearing its demise?
 
Do a
Alex05 said:
Maybe I'm just eagle-eyeing it now, but I have been noticing my coolant temp gauge hunt around on either side of the '210' dot. Once at operating temp, the gauge can't seem to decide where it wants to land. It will move from just below the 210 dot to just above it during normal driving. Actual temp seems fine; There are no obvious physical signs of overheating.
Of course, this gauge is mostly a "OK/Not-OK" gauge. Before I put a scan tool in my passenger seat and watch temps while driving, has anyone else encountered this issue? Was it actually anything to do with your thermostat?
Click to expand...
After you are finished investigating and ruled out/confirmed the problem is with the thermostat or radiator cap; you may need to move onto electrical problems.
Do any of the other gauges fluctuate in a similar manner when the temperature gauge is fluctuating ?
Are any other electrical items affected in the cab ?
Check the grounding stud connections (oxidation and tightness) near the oil filter and the grounds that are attached to the screws for the front speaker mounts and beneath the dash panel at the side kick panels.
 
Richard49 said:
I don’t think a thermostat would cause your issue. They usually fail stuck open or closed. Maybe the viscous fan drive is nearing its demise?
Click to expand...
What is a viscous fan drive? Edit: Sorry about the double reply! Not sure what happened there.
 
Ive had a few vehicles that constantly cycled temp wise in cold weather only. The 3, 1st gen Ram diesels we burned thru were worst. Normal or good, probably not. it cycled with big enough changes at times I figure any day now Id be replacing a cracked head or block. Stayed that way for years in cold cold weather only. I attribute it to crap thermostats as I wasnt gonna spend $100 for the Cummins one. Lesson learned.
 
that's completely normal. You're just seeing the operation of your thermostat.

Your engine gets warm (needle ticks just above the 210 mark), thermostat opens, a bunch of cold water that had been sitting in your radiator works its way through the engine and hits the thermostat (and sensor) before reaching 210...thermostat closes, coolant warms, thermostat opens, and the cycle continues. It's more pronounced in colder temps because what's sitting in the radiator is much colder now than it would be if it was 85 outside.
 
CharlesHS said:
Do a

After you are finished investigating and ruled out/confirmed the problem is with the thermostat or radiator cap; you may need to move onto electrical problems.
Do any of the other gauges fluctuate in a similar manner when the temperature gauge is fluctuating ?
Are any other electrical items affected in the cab ?
Check the grounding stud connections (oxidation and tightness) near the oil filter and the grounds that are attached to the screws for the front speaker mounts and beneath the dash panel at the side kick panels.
Click to expand...
Definitely familiar with where the ground strap is on the block, but not at all with the grounds near the speaker mounts. Can you basically not miss them once you take the dash trim pieces off?
I don't notice any egregious electrical oddities. The only thing I notice is when I am at a stop and turn on the dome light, the light output has a minor fluctuation. This does not seem abnormal to me, though.
 
Once the speaker grills are removed; the ground wires are under the screws fastening the speakers to the dash.
There are also grounds beneath the dash attached to the side kick panels.
 
If the cooling system isn't exactly up to par the thermostat will be mostly open. The thermostat operates very smooth when it's in the normal range. Put one in boiling water and watch. it also is sitting in coolant so it responds quickly to temperature change. What doesn't is the fan clutch. The radiator has to start dumping heat for it to operate. After the radiator gets very hot that air starts to spin the fan faster and the radiator cools down. The fan clutch has quite a bit of lag compared to the thermostat. Long story short if you experience slow temp swings it often points to a general reduction in cooling capacity due to rust and clogs and other crap leaving the thermostat mostly open and the fan controlling the temperature. Just food for thought. Some people like to dispute just about everything because it's the internet, if you disagree I don't care, please don't quote me.
 
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Thanks Jerry! I'm at least going to follow-up with some actual temp data from a scan tool. Obviously there's some short term memory bias here but I cannot remember the temp gauge doing this dance in prior cold Wisconsin winters.
Related, I miss the days when gauges were not mostly idiot lights. That's a whole other ball of wax conversation, though it would certainly make troubleshooting and investigating a bit more direct. And this does make me want to finally invest in BlueDriver.
 
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