Ok, first, I have searched the board.

I have a 305 engine, using the stock spec radiator and a single electric fan. I have no real work done to the car yet.

The problem is, my car at idle will skyrocket up to about 240-250 degrees. However, once I start driving (even going like 25 mph) the temp will drop back down to about 180 - where it usually is. This problem just started. The electric fan does come on - I hear it spinning if I shut the engine off and turn the key on, and I have checked it visually. I'm not sure if I have all the pieces of the airdam, but I know I haven't lost any of them recently if I don't - and, as I said, it just started happening last week.

Any ideas? I think maybe the fan itself is starting to die and it only comes on sometimes, but before I just start throwing money into it, I thought I might want to get a 2nd opinion.

 

sounds like the thermostat.

 

Thermostat or an air pocket has developed.

 

The engine was replaced less then 3000 miles ago, the thermostat was replaced with it.

I haven't touched the cooling system since replacing the engine. I'll try replacing the thermostat anyway (its a cheap enough part), but how would an air pocket develop if the system is still sealed?

 

could be the water pump impeller too, sometimes they rust away or come loose on the shaft.

 

You said it yourself, the fan comes on sometimes. I would check the fan first.

Then check to see if your water pump is in good condition.

Your airdam is fine since you begin to cool down once you start driving. Air dams dont do jack when you're standing.

I'd have to say that its the fan. So far it sounds like a typical third gen problem.

Also, whenever you have problems with the cooling system you should check these things.
Cap, hoses, check for leaks, and flush the system.

Get the fan going, or two if you have the two fan set up. See if that helps, I am sure it will.

 

Ok, I found and fixed the problem. One of the tempature leads had come off the engine and was resting on the manifold, which happened to have a small leak. I've replaced the manifold and reconnected the lead - No more "overheating"!