To clarify, there is of course the heater flow control valve up against the firewall near the heater core. Check to make sure that it opens and closes as I described above.
Further, you have the two black metal lines that run from the intake manifold back to the heater core. Those metal lines attach to the intake manifold via two metal adapter fittings. One of those fittings has a flow restrictor in it - I believe the restrictor hole is about 3/16" in diameter. If all else fails, make sure your fitting has the retrictor in place. If you don't, way too much coolant will flow to the heater core, especially when the thermostat is closed.
Read Post #14 in this thread:
http://turbobuick.com/forums/threads/what-is-the-thread-pitch.343228/#post-2737540
Before doing any of the above, I would do the following: use your radiator petcock (love saying that word) to drain out about 4 or 5 inches of water from your radiator. Makes sure you can see the top 4 or 5 rows of tubes inside the tank through the radiator cap opening. Of course, do this when the car is cold. Then, start the car and watch the temp. gauge and tubes in the radiator tank.
1. When you start the car cold, you should see no coolant coming through the tubes. If you're not sure, rev the engine - you should still see no coolant coming out of the tubes. As the engine warms, the coolant level in the radiator will begin to rise as it expands. It gets a little scary seeing it inch it's way up the tank while the cap is off. This is normal. That's why it's important to drain off several inches of coolant from the radiator in the first place.
2. When the car reaches thermostat temperature (about 180 degrees, may go a bit higher), the thermostat should open, and you should see some coolant coming out of the tubes. It will be kind of a trickle at idle. If you're not sure, rev the engine - if the thermostat is open, you will see coolant streaming out of the tubes.
3. The thermostat may re-close for a moment when the cold coolant in the radiator rushes into the engine at the time the thermostat opens. This is normal. When this happens, coolant flow through the tubes might stop again for a short time.
4. From this point on - if the coolant temperature is above 180 degrees, you should see coolant flowing out of the tubes. If the cooolant temperature is below 180 degrees (say the 164 that you said above), you should see no coolant flowing out of the tubes.
If all of the above happens the way I wrote it, your thermostat and temp. gauge are working correctly. If your temperature gauge shows 164 degrees, but you see coolant coming out of the tubes, you know for sure that either your gauge is wrong, or the thermostat is not working properly.
If you go through the above and everything is working correctly, then I would start looking at the stuff associated with the heater core (i.e. the heater control valve and the restrictor). No sense wasting your time on the heater core stuff until you verify your temperature gauge and thermostat operation.
Hope this helps...
