Okay I have an 88GT 5 speed with a 3.8SC swap it has a fast fieros modified original water neck thermostat holder thingie, Just put in a champion 3 core aluminum rad

I know I'm supposed to use a non vented cap ....I have a NAPA 31528 is this the right one?

The level in my coolant catch tank under the hood never moves ? because of cap?

The radiator and hoses in front don't seem to get hot gauge usually reads low till pink totally hot?

new 160 thermostat..... the level of the coolant seems to go down under the engine cap ....SEEMS TO ME that coolant doesn't flow to the rad then and coolant sensor sees air it runs funky limp mode kinda thing

Found the rad was leaking so replaced it with aluminum one.
The engine coolant cap was leaking or sucking air so got another new one seems better
today it gave me a PO 117 engine coolant temp circuit low input way too low? on coolant

I fill it in the back at the engine till coolant comes out the rad then I cap it then fill it till its almost full at the back put the t-stat in and cap it ....Am i doing this wrong? how do you bleed air out seems like the heater circuit doesn't get full of coolant blows cool too

And yes I have done the Lower intake manifold gasket already and no no white smoke runs good
happy

maybe crushed tubes underneath I straightened them how do you check flow? seems to get hot idling good when driving though

any ideas? hope you can follow my train of thought need someone knowledeable

First off fix all of the leaks (if not already). Sounds like you’re filling it correctly, slowly fill it with the radiator cap off (slowly is important), then put the radiator cap on, start the car and continue to fill via the thermostat cap. With the engine running (and thermostat out) you should see coolant flowing.
To check the running temp, compare the ECM data to the gauge. They are two different systems so if they agree odds are you’re getting accurate data (as accurate as factory senders will give you). Keep in mind that I don’t know details about the 3800 ECM system but I’m pretty sure it’s coolant sensor is different than the one for the gauge.