Heater/coolant system issues

[76K20CrewCab]

Hello All,

New here and have a conundrum that maybe someone here has expierenced and has insight!
I have a 1976 K20 3+3. When i purchased it 2 years ago it was pretty rough but slowly I've been tinkering and making it more reliable. Now that I have it road worthy I decided to get the heat working properly. Recently I replaced the radiator and hoses as the old radiator leaked. It seemed to be running pretty cool so i replaced the thermostat with a new 195°.
Still not getting much heat ( I have front covered and truck is at operating temp). I noticed low flow from heater core return hose to the radiator when I had rad cap off so I changed the heater core.... return flow to radiator increased substanially and I am getting more heat through the vents but still not as warm as most squarebodies Ive owned. When I was running it after heater core install I ran with radiator cap off to hopefully purge the air from the system. I noticed after it was up to temp drastic surges in coolant level and as I further investigated I can feel surging in the pressure side heater hose (5/8" hose from intake to bottom of heater core) cant really feel the surging on the return (3/4" top of core to radiator).
What the helk is causing the surging? I never noticed the coolant surge before installing the new core. Am I not getting all of the air out of the coolant system?
I was worrying about a headgasket but that seems unlikely as there are no other headgasket signs....very frustrating lol

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[DoubleDingo]

I am no expert, but I would highly recommend topping off radiator and overflow bottle, installing the radiator cap, turn the heater temperature lever to hot, the heater lever to heat/vent/or defrost, and let the cooling system burp itself with running the engine, then cooling down.

I only leave the cap off after first fill, and with the engine running, and once the thermostat opens, and it's confirmed that the radiator is full, there is no need to introduce air into the system with the cap off.

The surging and fluctuations is air in the system. Steam is not what you want, air in the system gives you steam, steam is hotter than boiling water.

 

[fast 99]

After heat cycling engine a couple of times air should be out.

With a new core things to check,

do heater case doors work and seal properly.
what is coolant temperature, using heat gun on stat housing?
inlet and outlet core temps with heat gun
cheap stats are worthless, use AC, Delphi, Delco or higher priced NAPA

Inexpensive heat guns are close enough for this diagnosis. They really should be in every tool box today. Lots of uses. $20

Edit, after any work requiring draining of the cooling system some air will almost always be present after refill. If the radiator cap and overflow bottle are working properly, when the engine cools coolant will be sucked back into the radiator. Refill the overflow bottle as needed.

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[AuroraGirl]

I am no expert, but I would highly recommend topping off radiator and overflow bottle, installing the radiator cap, turn the heater temperature lever to hot, the heater lever to heat/vent/or defrost, and let the cooling system burp itself with running the engine, then cooling down.

I only leave the cap off after first fill, and with the engine running, and once the thermostat opens, and it's confirmed that the radiator is full, there is no need to introduce air into the system with the cap off.

The surging and fluctuations is air in the system. Steam is not what you want, air in the system gives you steam, steam is hotter than boiling water.

also accelerate the engine while bleeding if possible, hold it high for at least 20 seconds this pushes more volume and air out than just idle