1985 745t

Went out and started the car this morning before going to the office. I let it idle while I finished up in the house, then went out to get going. However, when I got in I saw that the needle for the temp gauge was up JUST below the red zone, say a few cm off or so.

I sat in the car for a couple minutes and moved the car a litle bit on the driveway, but the needle just sat way up there. I opened the hood and felt the radiator and it was fairly cold to the touch. I felt the valve cover and it was hot. I felt the hose from the radiator to the t-stat housing and it was warmish at the radiator side, hot at the t-stat side.

I decided to start driving and see what would happen. Lo and behold as I started driving the temperature gauge dropped, eventually dropping all the way down to its normal "initial" operating positing of a couple cm off the bottom of the gauge. However, I wasn't going nearly fast enough (10-20mph) for any noticeable difference regarding airflow over the radiator. So I figured it was just a wierd fluke or something, but when I drove home I stopped in the driveway and the same thing happened in reverse! The gauge slowly started climbing up until it was around the halfway mark. At the same time the heat coming out of the blower/heater disappeared, and I started getting only cold air inside.

Now I know my cooling system needs work and can be funky. Generally when I start the car the coolant temperature will rise to 2/3 and then drop when the t-stat opens. However, this problem just kind of has me boggled. My car is an early model brick, so it doesn't have an aux fan requirement to run or anything. It's also still cold outside (like 30f today), so I wouldn't expect this kind of overheating to occur. Finally I would normally guess a faulty heater valve for the disappearing heat at teh dash, but the ABRUPT change in temperature occured in like five seconds, and that seems to quick to lose all the heat in the core.

So what should I look at? Is it possible I have a faulty valve, that I'm losing vacuum at idle and that the valve is allowing coolant to quickly evacuate the core but not to refill it? Why would that cause my coolant to start heating up at idle though? Shouldn't the brick be able to handle normal idle heat without the surface area of the core?

Sorry for the long post but I wanted to be thorough. Thanks in advance for any help.

Best,
Rick