Coolant temperature going up and down

Tiny
EDDIEM23
  • MEMBER
  • 2017 RAM 1500
  • 3.6L
  • 6 CYL
  • 2WD
  • AUTOMATIC
  • 81,000 MILES
I recently had to replace my radiator and thermostat; I am having an issue when driving on the freeway, the temperature goes up from 230 degrees to 247 then back down and back up, continues this cycle whether the A/C is on or not. I turn on the heat and the temperature goes down and stays constant around 219, does not do this in city driving only on freeway. I bled out the air in the system, cannot find help anywhere online so far.
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Friday, June 24th, 2022 AT 8:20 PM

7 Replies

Tiny
KEN L
  • ADMIN
Did you use an OEM thermostat? Also is the radiator the same size and thickness? Also, when it is running hot can you hear the clutch fan kick in?

Hear is the test you should do to see if the viscous fan clutch is bad:

Diagnosis And Testing - Radiator Viscous Fan

Noise

Note:

It is normal for fan noise to be louder (roaring) when:

The under-hood temperature is above the engagement point for the viscous drive coupling. This may occur when the ambient (outside air temperature) is very high.
Engine loads and temperatures are high such as when towing a trailer.
Cool silicone fluid within the fan drive unit is being redistributed back to its normal disengaged (warm) position. This can occur during the first 15 seconds to one minute after engine start-up on a cold engine.

LEAKS
Viscous fan drive operation is not affected by small oil stains near the drive bearing. If leakage appears excessive, replace the fan drive unit.

VISCOUS DRIVE
For the following test, the cooling system must be in good condition. It also will ensure against excessively high coolant temperature.

WARNING:
Be sure that there is adequate fan blade clearance before drilling.

Drill a 3.18-mm (1/8-in) diameter hole in the top center of the fan shroud.
Obtain a dial thermometer with an 8-inch stem (or equivalent). It should have a range of -18 -to-105 C (0 -to-220 F). Insert thermometer through the hole in the shroud. Be sure that there is adequate clearance from the fan blades.
Connect a tachometer and an engine ignition timing light. The timing light is to be used as a strobe light.
Block the air flow through the radiator. Secure a sheet of plastic in front of the radiator. Use tape at the top to secure the plastic and be sure that the air flow is blocked.
Be sure that the air conditioner (if equipped) and blower fan is turned off.

WARNING:
Use extreme caution when the engine is operating. Do not stand in a direct line with the fan. Do not put your hands near the pulleys, belts or fan. Do not wear loose clothing.

Start the engine and operate at 2400 rpm. Within ten minutes the air temperature (indicated on the dial thermometer) should be up to 88 C (190 F). Fan drive engagement should start to occur at/between:
5.7L Automatic Standard Cool - 94 C - 99 C (201 F - 211 F)
5.7L Manual - 93 C - 100 C (199 F - 212 F)
5.7L Heavy Duty Snowplow - 76 C - 83 C (169 F - 181 F)
Engagement is distinguishable by a definite increase in fan flow noise (roaring). The timing light also will indicate an increase in the speed of the fan.
When viscous drive engagement is verified, remove the plastic sheet. Fan drive disengagement should start to occur at or between:
5.7L Automatic Standard Cool - 69 C - 74 C (156 F - 166 F)
5.7L Manual - 71 C - 76 C (160 F - 170 F)
5.7L Heavy Duty Snowplow - 55 C - 60 C (168 F - 178 F)
A definite decrease of fan flow noise (roaring) should be noticed. If not, replace the defective viscous fan drive unit.

CAUTION:
Some engines equipped with serpentine drive belts have reverse rotating fans and viscous fan drives. Installation of the wrong fan or viscous fan drive can result in engine overheating.

CAUTION:
If the viscous fan drive is replaced because of mechanical damage, the cooling fan blades should also be inspected. Inspect for fatigue cracks, loose blades, or loose rivets that could have resulted from excessive vibration. Replace fan blade assembly if any of these conditions are found. Also inspect water pump bearing and shaft assembly for any related damage due to a viscous fan drive malfunction.

This guide can help as well

https://www.2carpros.com/articles/engine-overheating-or-running-hot

Please run down this guide and report back. Let us know what happens and please upload pictures or videos of the problem.
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Saturday, June 25th, 2022 AT 11:54 AM
Tiny
EDDIEM23
  • MEMBER
The radiator and thermostat were purchased at AutoZone, the fan on the radiator does turn on, it gets loud when the temperature goes up.
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Saturday, June 25th, 2022 AT 12:07 PM
Tiny
KEN L
  • ADMIN
Okay, so the heating problem happens right after the repair, correct? I would say start with the thermostat and get a OEM unit or try installing the old one back in. Also, do you still have the old radiator? I would compare the old one with the new one to see if the amount of cooling tubes are the same.
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Saturday, June 25th, 2022 AT 5:25 PM
Tiny
EDDIEM23
  • MEMBER
The old radiator had a crack in it, so I replaced it and it matched the old one, but the heater didn't work after that, and the temperature was erratic like it is now. I put the new thermostat in, and the heater worked after, but the temperature still acted odd on the freeway. I forgot to mention that I tried to use blue devil for the radiator leak but that did not work. Do you think the AutoZone thermostat is faulty?
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Saturday, June 25th, 2022 AT 6:57 PM
Tiny
KEN L
  • ADMIN
The blue devil is wild stuff, and it can plug things up or coat things internally. I would disassemble the system to remove as much of the blue devil as you can, (you will see it). Please remove the coolant sensor as well to make sure nothing is blocking its ability to read correctly. Here is the location, also replace the thermostat with an OEM unit. Check out the diagrams (below). Please let us know what happens.
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Monday, June 27th, 2022 AT 1:15 PM
Tiny
EDDIEM23
  • MEMBER
Sorry took long to get back to you, I replaced the sensor from a mechanic because it was difficult to get to for me. I put in an oem thermostat and had to put in a new water pump from autozone, the dealer here is back ordered. The Temps are running fine when around town, on the freeway it is going up and down when I have the ac on the coldest setting not max, works fine if I go slightly warmer on the temp setting, one bar above the coldest setting on knob. Really stumped on this and not sure what else to do?
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Monday, July 25th, 2022 AT 5:49 PM
Tiny
KEN L
  • ADMIN
Is the blue devil still in the system, it could have clogged up the new radiator? Also, you may have air pockets in the cooling system, sometimes you can jack the front and then the rear of the truck up to relieve them. I would use a flashlight and look inside the radiator and coolant reservoir to make sure there are no floaters of blue devil.
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Wednesday, July 27th, 2022 AT 11:36 AM

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