Bad thermostat? Did you buy oem?

 

Factory gauge?
First I would try and get hold of a infrared temp and check actual temperatures.
Then look at an after market gauge.

Cheers Steve

 

I did not buy OEM. I went cheap, which was stupid but at the time made sense. Would a thermostat make the temp. fluctuate that much though? Yes it is a factory gauge. I have an infrared at work, I will have to check actual temps Monday.

 

Yes, thermostats can cause a fluctuation.
Usually seen more in cooler weather were the coolant in the radiator is a lot colder than the engine and dumps the cold coolant into the engine when the thermostat opens. One of the reasons an lot of late model engines have the thermostat on the inlet to the engine, keeps the engine coolant at a steady temperature and reduces thermal shock to the engine.

Have seen many threads where it is said the genuine Cummins thermostat works better than after market, they maybe a little slower opening and don't dump a lot of cold coolant in quickly.
Cheers Steve

 

Mine was doing the same thing 190-225, threw a thermostat in (Napa) and fixed. Like stated already use a thermo gun to make sure it’s accurate, and also check your fan clutch

 

What exactly should I be looking for when checking the fan clutch?

 

Obvious metal sounds, loose/wobbly fan, locked up all the time.

Then with a scan tool/Edge CTS/Smarty Touch, monitor fan speed.

NOTE: The following test may take up to 15 minutes
to perform.
The engine should be at normal operating temperature.
1. Set the parking brake and verify the transmission
is in park or neutral.
2. Set air conditioner (if equipped) and blower fan to
OFF.
3. Start and allow engine to reach normal operating
temperatures.
4. Stop engine, connect the scan tool and select
appropriate model year and engine option.
5. Check for and correct existing DTC’s
6. Connect pin 1 of the electronically controlled viscous fan drive connector, located at the lower
fan shroud to battery ground.
7. Using the scan tool, verify that DTC 0480 set.
8. Start the engine.
9. Go to the SENSOR screen and observe the fan speed.
10. Run the engine at 2500 rpm.
NOTE: It maybe take 15 minutes before fan speed increases.
11. The fan speed should increase according to the table below.
12. If fan speed does not increase, replace the electronically control viscous fan drive.
ELECTRONICALLY CONTROLLED VISCOUS FAN DRIVE SPEEDS
ENGINE RPM/ FAN RPM (Minumum)
500 /490
1000 / 950
1500 /1420
2000 /1850
2500 /2230
3000 /2440
13. If the fan speed does increase and there is still a concern, refer to the appropriate

 

Ok. I didn’t want to pop open the radiator while it was hot so I figured the next best thing would be to temp the radiator itself. It was at 150 so it seems to be operating just fine. I checked the fan clutch and it is operating normally as well. I’m going to be ordering a thermostat ASAP to see if that will fix the problem. Will update once installed.
Thanks guys!

 

Ok. Went with a Napa part instead of Cummins. Read many reviews and both oem and Napa seemed to be the best. Installed yesterday and still have some fluctuation in temp, but no where near as much. Next step is going to be cleaning the radiator.

 

fan clutches are a big deal on these trucks im on my 4th one is 3 years.... keeps eating wires no matter what you do. so far the last one has lasted but well see.

 

Cruising unloaded on flat roads with temp fluctuations? Most likely is the thermostat, offbrand non Cummins are unreliable. Engine fan will not come on during flat road empty cruise, enough air being rammed through the radiator unless the radiator is plugged by bent fins, bug guts, combination of road dust mud oil grease melted asphalt. Pull the radiator and wash all the coolers with power washer on low pressure and a grease cutter solution.
Can also be corrosion deposits buildup in the radiator and block, most often on the tube ends on outflow side. Rod out with a brush and strong cleaner

 

Yes. I replaced the thermostat this past week and the temp. still fluctuates. I plan to clean the radiator and check for corrosion in the coming weeks (work schedule is a little hectic right now). Also, I have checked fan clutch and it seems to be operating correctly. I want to check the coolant temp after running with my infrared but am unsure how to properly check it without the coolant exploding all over me from opening the radiator while it’s hot and pressurized?? Any ideas?? This will tell me whether or not I have a bad gauge.

 

If it is a infra red gauge you just check the top and bottom tanks from the outside, no need to check the actual coolant.
By fluctuate is the temperature close to the thermostat operating temperature? May just be the thermostat working.
Cheers Steve

 

I just had a head job done brand new fleece head with arp studs etc. my trucks coolant temp is still fluctuating way to much to be normal. I pull off from a redlight and it will reach 197 or so instantly and cools back to 188-191 within 2 seconds before I even let off the peddle. I believe my heater core is bad I’m curious if this could cause the issue or what else it may be? Spend ALOT of money on HG and are really
Bummed I’m still having any type of issue.

 

Just where is the temperature gauge sender installed?