Overheating I have idea what else it could be?

If you have changed all of those but it still overheats check that the water pump is circulating coolant. While not common I have seen the impellers fail on them. When you say it overheats do you mean it gets hot enough that it boils over or is it just hot on the gauges?

I forgot to mention I did change the water pump also, And it comes out of the reservoir cap and also the temperature gauge goes past the middle but when I turn the heater on, it goes back to the middle and stays there, but twice the heater didn’t help so I shut the car off, but like I said that only happened twice.

I also bled the cooling system by pouring coolant into the block, then putting the thermostat on with the GM engineer recommended hole that I drilled into the thermostat flange and then filled the radiator and so on and so forth.

I was thinking maybe the heater core but there’s no coolant on the passenger side, floorboard and the heater blows hot air.

The heater core isn't really part of the cooling system in that respect. The only time it will help is if you have air blowing through it that can carry the heat away and that only happens if you have the heater turned on, so mid summer 100 degrees out you are not likely to want hot air pouring into the cabin so the core isn't really there for cooling the engine. Did you change out all of these parts because it was getting hot or did you change them out and then have an issue? The coolant coming out the reservoir cap sounds like it was over full. Are the cooling fans turning on?
The normal filling method on these is to start with a cold engine. Fill the system until it reaches the cold line. Then start the engine and rev it a couple times and let it warm up. Now shut it off. Once cool you top it back up to cold and repeat the process until it doesn't take any extra coolant when cold. No other steps needed.
Your description sounds like it has air in it.

Because I changed the thermostat and the water pump, the block was empty so I filled the block put the thermostat in then filled the radiator and then bled as normal with the bleeder screw on the thermostat housing and no it was getting hot so I did all of this. It did not happen because I did all of this and no there is no air in the system because when I cracked the bleeder screw no more bubbles come out.

And no, it was not overfilled. It was directly at the line and still is directly at the line.

If there was a leak, that would allow air in, right? But would a steady stream come out of the bleeder screw like it is? Or is it possible to have a steady stream but still have air in the system?

A leak won't let air in unless it is above the level of the coolant. Plus, as soon as the system pressurized a leak would show up by leaking more. That it doesn't overheat every time, and the coolant was staying in the correct level, suggests either a bad thermostat that is sticking or the fans are not coming on and the coolant is overheating. However, the cooling fans would be noticeable. What was the vehicle doing when it got hot? Just driving or ?

It does it when it’s driving and when it’s idling.

I will use my power probe today to verify the fans are working and then I’ll test the relays. I’ve already checked all of the fuses.

Simple test for the fans, just unplug the coolant temperature sensor. That should trigger them on.

They work.

And I’ve used aftermarket thermostats and Gm ones because that’s what I thought it was, too. It happens no matter which thermostat i use.

The intermittent part of it is what doesn't make sense. I would expect it to overheat every time, not just once or twice. Try this, grab an IR thermometer or a thermal camera if you can find one as a loaner. Use that to inspect the system when it gets hot. See if you can find something like an area that isn't moving coolant. If the hoses are getting hot and the rad is evenly heating then check the actual temperature and see if the fans come on starting at 202 degrees, then to high speed when it hits 220 degrees.

They don’t come on at 202. They don’t come on until it’s after the halfway mark. And when I say overheating, I mean over the halfway mark. I never let it actually overheat. It used to stay below the halfway mark until it didn’t which is when I changed all that stuff.

On the temperature gauge on the dash, I mean.

And now the coolant level is dropping. Could it be because I need motor mounts so it’s causing the engine to shift too much? Because I noticed the clip that holds the coolant reservoir is gone so maybe the vibrations were too much and it bounced out or something and the vibrations make the coolant come out of the reservoir? Because the cap isn’t a screw on it literally just pops on and off just like a windshield wiper fluid reservoir cap.

Okay, the dash gauge isn't very accurate, you need to see what the actual coolant temperature is. With coolant in them they run well over the boiling temperature of plain water.
Overheating normally would be in a red area on a gauge or when it actually starts dumping coolant. I would measure the actual temperatures it's running at.

Turns out it was just my forgetfulness. I didn’t have the reservoir clipped down so I’m assuming because the motor mount is bad, the vibrations caused the coolant to overflow out of the top. I clipped it in, and it stopped overheating, although I do still smell a slight coolant smell, and it idles a little rough sometimes. But it hasn’t gone past the middle since I secured the reservoir in place.

Nevermind it overheats when it’s hot outside. It just went to 3/4 between c and h and only went down when I turned heater on full blast.

You need to measure the actual temperature it is running at. C & H mean nothing really. Now if it pegged at H and coolant started bubbling out of it, then you know it's overheating. However, at the moment it doesn't sound like it is actually getting hot. The temp decrease with the heater on is 100% normal. You are adding a second place for air to pull heat away. That still doesn't tell you the coolant temperature. So, use an actual thermometer and run it, now measure the temperature at the coolant outlet, the thermostat housing, the radiator and see what it actually says. If they are under 200 degrees, it isn't overheating. If it hits 202 and the fans start turning, it's normal, if it hits 220 and they are on full it's normal. What you don't want is for it keep rising.

I’m saying halfway between c and h but it’s because i can see the temperature because my Autel is hooked up to it and I'm watching the live data. When it is halfway between c and h it’s at 213. When it’s almost to 3/4 it’s at 230. The recommended operating temp for the car from what I read is around 200 to 225. Like right now it’s hot outside. I live in Los Angeles County so it’s 80 degrees right now but it feels way hotter. I have the heat on full blast and both fans are working and it’s at 213. And now I turned the heater off at it climbed to 222 and when I accelerate is when it jumps to 3/4 between c and h or 230 degrees. It also smells like burning coolant and exhaust so I’m assuming it’s bubbling over and dripping on the exhaust or something.

Do I still need to measure the temperature at those spots individually with my infrared thermometer or is the Autel reading enough? I can go get my thermometer out of storage if I need to.

You want to measure the temperatures. For instance, you look on the Autel and it says it's 220 degrees, you measure at the top of the radiator tank and it's 150 and 120 at the bottom, you have a flow problem. The other thing to check is how fast does it heat up? You said you changed the gaskets, but you could have a cracked head or block. A quick and dirty test for that is to start with a cold engine, put a balloon on the radiator fill and start the engine. It normally takes coolant a while to expand, and the balloon will just very slowly expand. With any combustion gas leak it will blow up fast, long before the coolant heats up.

Okay, I’ll try that, and I don’t know if it helps to diagnose, but I’ve noticed that the temperature goes up a lot faster when idling and even faster than that when the A/C is on.

Like in the years leading up to the cooling issue, it never went above halfway and when I say the temperature starts to rise when idling or the A/C is on, I mean above halfway. I haven’t had a chance to get the infrared thermometer yet but when I do, I’ll check the temperature when idling and when the A/C is on and let you know. And bottom of the radiator hoses and stuff like that.

If I can jump in here, have we checked for a cracked block? I would do a chemical check to confirm we don't have exhaust gasses in the cooling system, this guide can help:

https://www.2carpros.com/articles/head-gasket-blown-test

Please go over this guide and get back to us.

Top and bottom said 180 something and Autel said 213.

Okay, that is a normal operating temp for that car. If it's dumping coolant at those temperatures I would look at a bad cap or a crack in the neck if that's where the coolant shows up.

The coolant shows up on top of the reservoir and when it does that the car starts to overheat. I replaced the cap when I did the water pump.

And to the post from a few weeks ago about the balloon and asking how long it takes everything to heat up, I finally tried the balloon trick, except I used a plastic bag and some rubber bands because a ballon would not fit on the reservoir cap. It’s way too big. The bag didn’t inflate fast at all, and it takes a while for everything to heat up.

It is still overheating. I’ve done every test, and everything is coming back normal, but coolant is still coming out of the reservoir. Could it be a clogged cat? I stuck my borescope down the exhaust, and this is what I saw. It’s kind of hard to see but hopefully you can see enough to be able to tell.

That doesn't look very nice, almost looks like the substrate is damaged, that could cause it to heat up though. The white looks like it may have gotten very hot and fused the ceramic closed.

There was a hole in my exhaust manifold for a little bit, but I replaced it. I did use the exhaust weld stuff, but it wasn’t very long before I replaced it. Maybe some of that stuff fell in? So that's probably the reason I’m overheating still? That sucks I was hoping it was something way less expensive.

Plugged cat would hold heat in the head and cause it to heat up. You could cheat a bit for testing. Get a chunk of straight pipe and cut it just long enough to bridge the cat opening. Remove the cat and stick the pipe in place. That will quiet it down so you can run it in the driveway without the neighbors showing up all angry. If it stays cold, you have the answer. If you are not in a CARB state like NY or CA, you might be able to get one through Rock Auto for much less. The Walker 52507 is about $250.00 for the federal version.