Ever since it'* started hitting 30' and all the way to -10', my car can ever seem to get any warmer then like 168 degrees.

One of the only older things in this car is the radiator- but why would that keep it from warming up? Or, could it be the t-stat?

Do you have a stock t-stat?

not sure...

before the engine swap, I had thrown in a lower temp (185'/190'?) tstat from AutoZone, because it was running way hot.

The engine had misfire and knocking issues, then finally blew the UIM, so it was swapped with an 02 3800 motor.

So, I am not sure if the tstat I put in the other motor went to the trash, or if it was put in this one.

I'd go thru the following.

1 - t-stat (may be sticking)
2 - coolant flush (dexcool sucks)
3 - Replace/Rot out Radiator (could be clogged)
4 - Heater box - (could have a clog in it as well)

I would watch the scan tool and not pay attention to the gauge in the dash.

You really want it to be above 190 - if not 200-ish for good fuel mileage and performance.

I have changed thermostats this winter in three vehicles that were not coming up to temperature. These all failed by opening early or not completely closing. New thermostats raised the operating temperature about 20 degrees in each case. One of the new ones opened early, too. That one took two tries.

Radiator and heater core can be skipped. Those would typically not cause you to run cold.

This points to a T stat. Depending on how accurate of a reading you are seeing, it'* probably time to change.

Only reason why I mentioned radiator & heater core was if he had it all apart anyways and depending on how the heat in the car was when it was below 0. I clicked submit before finishing my post, my apologizes.

The first thing is to watch the scan tool as the accuracy of that gauge is questionable on GM cars.

Autotap read it as never going above 168', maybe 170...