I have been having this issue for some time now. The temp gauge always read a little low but I never paid too much attention to it. I have since done a 5.4 swap into the old 4.6's place and the truck is doing the same thing. Let me give you a run down.

fan clutch spins free when cold
replaced degas cap
no coolant loss
replaced thermostat with 195* units twice
heater core replaced last year and 150*+ temps from the vents
radiator flushed and backflushed twice using Prestone cleaner and Tee kit
I believe that the radiator is the only thing not replaced at this point

here are the results of driving it around for 30 minutes in 90*+ ambient temps
X4 shows 148*
IR gun readings-
upper rad hose 161
heater core hoses are 160 and 161
intake manifold crossover 137
thermostat housing 156
lower rad hose 125

I'm stumped

 

Look at the engine temperature the computer sees if it is not around 195 to 200 then you have an issue to solve.
A Scanner is needed to look at this live data.
You can't rely on outside readings.
Good luck.

 

Sorry if it wasn't clear. I used the data from the SCT X4 to show 148* . This was in the original post

 

Here is what I can tell you about your situation if it is accurate.
1.The PCM should have set a code for a cold engine that did not come up to temperature in a certain amount of time above 175. The code likely would be 0125.
2.Your heater temp and hoses will not be up where they should for normal heating use.
3. The fuel injected would be too RICH because the PCM is getting a signal from the CHT that the temp is to low. A normal reaction by the system.
You need to verify these functions with a Scanner and act accordingly.
This is why diagnostic codes and live data are provided to see what is happening.
A cold engine is either uncontrolled coolant flow or a temperature indicator false presentation that could also be air around the CHT instead of coolant.
Good luck.

 

I am a little confused.

CHT aside, why would the temp sender say 148* and the IR gun verify 148* when pointed at that very sensor after driving around town on a hot day?

Not a single spot I checked with the gun was remotely close to 195*

Planning on replacing the thermostat again tomorrow. It's all that really fits the symptoms to me so far. I may just go buy a scanner at some point, too. Frustrated

 

Possibly your IR is not accurate.
Another check you could do is hook up a good sensitive timing light to any coil connector lead so it gets a trigger signal.
Point the gun at the fan.
Watch awhile to see if the fan goes into and out of sync.
This will be your fan clutch locking and unlocking according to the thermostat opening and closing temperature changes.
I would say that in hot summer temps the motor will still get up into the 165/175 range even without any thermoset installed.
The motor has to get rid of heat to the coolant in any event or you get over heating.
Good luck.

 

Replaced the sensors in the crossover and it's perfect now. Feeling pretty foolish but at least I have the peace of mind back. Thank you, Bluegrass!

 

Great to hear.
Good luck.