Heat comes and goes....?
 

Great ever since I got all the work done on the MC the heat is been on and off, blower motor is working fine it's blowing at full capacity and the coolant system is been bleed over and over to release the air trapped but ever since my heat comes and goes, like for 20 seconds it work and then it's blows cool air for another 20 seconds then switches back to hot air. Is it the heater core saying its last good byes?

That sounds to me like a low coolant issue...or.. you have air still in the system... Did you bleed it with the motor off?
If you didnt.. then that's the problem it has an air pocket still in the system. That's causing the hot cold issue...
Also you may need to bleed it multiple times. Sometimes as many as 10 times. Be sure to open the bleeder in small increments. .. Being careful to not burn yourself..but be sure the vehicle is at operating temp..before you bleed the system.

The heater core leading to and from are hot as hell to the touch so are the radiator hoses. In idle (warming up) the car has heat the entire time it doesn't blow cool air unless I'm in motion.

I'll wait for a few minutes for the car to cool down but this is gonna be attempt number 50 bleeding the system

Going at attempt # 55 lol I'm determined to find a resolution to this issue

I know a few things that can cause what you describe.
- Tstat not opening properly
- leak in the cooling system, causing the air pocket (grabbing a radiator pressure tester can help diagnose that issue).
- Clog in the cooling system (but I would think you would have heat when you're RPMs are up and lose heat when you are stopped).
- Failed head gasket.

In the 3100/3400 cars, you can have a failed head gasket causing compression gases to to get into the cooling system and also over pressurize the cooling system (which is really bad long term).
What does your temp gauge read when your blower is blowing cold after you just had warm/hot air?
I just wrote a novel about what I know regarding failed head gaskets on the 3100/3400. The link is below:
https://montecarloforum.com/forum/ne...66/#post586956

I had a similar issue. A tech at my job put it up on a lift and noticed that I was leaking coolant from the water pump. I got a new water pump for $20 at Advanced Auto and had it put in, and I never had the problem again. Not to mention that the system was also bled about 5 times.

you didn't say what work you had done but sounds like you've got a heater control issue in the heater box itself. there is a door in the box that controls heat and a.c. parts of the box.sounds like the control of this door has broken or has come disengaged. that is why i was asking what kind of work. on early models this was all controled by vacume and allways gave trouble or the control leaver for the door would brake and the door would free float cold to hot like you expresed. i'd pull the glovebox and check the heater mech. or check the service manual at you auto zone or advance parts store.

@Rumpfox - The actuator for the flapper for the blend door is a good thought. But I'm going to say that is probably not it considering the OP is still bleeding air out of the cooling system. That should get resolved before ripping into the HVAC module.

Ok so I did a rad cap test & it doesn't leak any air, I did a compression test on all 6 cylinders and all are above 100 psi. Coolant not mixed with antifreeze, no white smoke! Odds of having a blown head gasket is slim to none right now.

What I got done as far as the cooling system goes is water pump, tstat, hoses, and new coolant.

I bleed the system many times with car on and off,

The temp stays at 180ish and air blows cool (not cold) and then switches back to hot air, the hoses leading to and from the heater core, one is very hot and the other one is just barely warm

Its recommended,
To only bleed off the air "without" the motor running!
But at operating temp before you Bleed it without the motor running...

To properly bleed a system I have always been taught to run the engine with the radiator cap off, heat set to maximum but blower on lowest setting, let the coolant burp itself while it warms up a little and fills. Then put the cap back on, and when it's up to full temp / pressure to open the bleed valve over the thermostat.
Once cooled down check coolant levels fill as needed then again repeat the bleeder screw process till it's a stream coming out.

correct me if it's wrong?

@Habbibie - At this point sounds like you might be doing nothing but chasing an air pocket.

Below are the steps from the GM shop book for the 3400 to properly bleed the air from the system (note, the car is to be flat/level, not raised up).

1. - Car off, open the cooling system bleeder screws.
Important: Use a 50/50 mixture of DEX-COOL antifreeze and deionized water.
2. - Slowly fill the cooling system with a 50/50 coolant mixture.
3. - Close the cooling system bleeder screws.
4. - Install the coolant pressure cap.
5. - Start the engine.
6.- Run the engine at 2,000 - 2,500 RPM until the engine reaches normal operating temperature.
7. - Allow the engine to idle for 3 minutes.
8. - Shut the engine off.
9. - Allow the engine to cool.
10. - Top off the coolant as necessary.
11. - Inspect the concentration of the engine coolant. Using the J 23688 .
12. - Rinse away any excess coolant from the engine and the engine compartment.
13. - Inspect the cooling system for leaks.