I have a mild 350 in a '68 Camaro with a factory style brass radiator, fan clutch, fan and fan shroud and a high flow aluminum water pump. I also have a Vintage Air system in the car. If I run a bypass hose from the water pump to the coolant connection on the intake manifold, it runs right at the 180 degree setting of the thermostat under all conditions. However, if I hook up the VA heater core to the water pump and the instake manifold coolant connection, it appears to overheat in local traffic conditions (not just idling in heavy traffic) with rapid and seemingly random fluctuations in temperature (210 to 180 degrees in less than a minute). Although I live in sunny SoCal, it wold be nice to have heat on some of those cool mornings. Any ideas?

Make sure the inlet hose to the core is connected to the intake nipple and the return is connected to the water pump.

Anytime there is fast rapid oscillations of temperature it's air that's trapped. It's impossible to be anything else.

Lift the front end and purge the air out first. Then see what happens.

Also........throw some WaterWetter in....it will help with purging air.

can you tell me what lifting the car will do ? I'm not sure I understand that ?

just get a flush kit and install on the upper heater hose ( upper line coming out of the heater core to burp the system)

I just installed a VA system and they have a coolant shut off valve in the system, ensure that the valve is open, Should be able to blow air through the hose and heater core. Best way to bleed air from the engine is leave the intake maifold hose off as you fill the radiator, this allows the air to escape from the block

Trapped air will migrate to the front and top position while you run it. This aids purging air from the system. My 69 was difficult to get all the air out.....and lifting the front end while the thermostat was open and cap off helped "burp" the air out.

Just a thought but you definitely have air affecting your system.

Best way to bleed air from the engine is leave the intake maifold hose off as you fill the radiator, this allows the air to escape from the block

That's how we used to fill them in assembly plants before we installed the automatic evacuate-and-fill systems that do it all through the radiator filler neck opening. :)

Thanks for all the suggestions. I think there are two things. I've long thought that there was air in the system, but didn't know how to get it out. Now I do. Second, call me a dumba$$, but I think I had the inlet to the heater core coming from the water pump rather than the intake manifold.

I developed an oil leak last weekend so I haven't tried any of the fixes. I need to decide whether I'm going to find and fix the oil leak, or take this (this is really a lame rationale) as an opportunity to drop in the 383 I've had sitting in my shop for the last five months or so. I'll let you know how it goes. Again, thanks for the help.