Over heating puzzler

[69stepside4x4]

I have recently had a problem with my '72 K5 temp gauge moving quickly to the hot side of the dial and don't really know why. As some of you may know from former postings, I recently returned after a year-and-a-half living abroad, so my truck was stored for a while.

Although the gauge doesn't move over immediatly to the the hot, it does shoot over after only running 5-10 minutes and then will back down (just a little) when I am at a traffic light?????? I thougt it might be a defective thermostat, so I replaced it today, that's when a bigger problem occurred. After installing the same temp thermo, the temperature shot up quicker than before and went even further past the hot marker (but not as if there was a guage short or anything, it just started moving up as I drove around the neighborhood and was up high within 3-5 minutes). I am stumped. Help please!

Some additional background is that I have just had the TH350 rebuilt and installed by a quality shop, but this problem was noticed before I had this work done, so I don't think it's the tranny.

[NuBomb]

Could the temp wire be hitting the exhaust manifold? Can the water get that hot after just a few minutes? I'm pretty much just asking questions I guess. Probably not much help.

[bouncytruck]

If idle for a while, see if the water is cirulating in the radiator.

[HatGuy]

Try a drive around the neighborhood with the
Radiator Cap off and when the temp is hot check
to see if it ts really that hot. If it's peged it should
be boiling over.

[vtblazer]

Could even be the sending unit itself.

Remove the cap, let idle/warm up and make sure it circulates when the t-stat opens.

Sounds like its more electrical then mechanical.

[Blazer1970]

Have you drained and refilled the coolant recently? It takes a while for all the air to get out of the system, and it is common for air pockets to form around the sender in the head which causes those high spikes on the gauge.

[69stepside4x4]

Thanks guys. I drove home today with the rad cap off and even though the gauge spiked, it never over flowed. I found that the sending unit wire was really close to the exhaust manifold too, so I think my next move is to replace the sending unit and run the wire away from the heat.

[72gmcshorty]

Sorry to jump into your thread...I have a rad cap with one of those pressure relief things on it. Could I drive around with it left open to allow any air pocket to escape? I too am having heating issues.