Over heating issue.
 

Alright, after the fiasko of my front end, I now have an issue with my heating. I just changed my upper tax hose and thermostat. The truck is overheating. It has full fluids, no leaks, fan works fine, and have no heat blowing into the cab if te truck. As soon as I start the truck the temp goes up to the over heating mark.

What could be the issue? Btw its a 95 gmc z71. I can't figure it out.

I'd pull the thermostat and test it or get another new one. You could leave it out for testing the trucks cooling system, but only for testing.

I'll second that.

If I understand the symptoms correctly, you said the gauge went to overheating immediately after start up. I wouldn't think anything could cause it to overheat that quick assuming a cold start. You may want to check the temperature sending unit or the gauge. Good luck.

From this description I'm leaning towards an electrical problem.
You need to confirm through a secondary device the actual coolant temp. It could be a scan tool, mechanical gauge, infared temp gun etc. The ECM and gauge cluster use two different sensors for their readings.

well, i got the overheating issue fixed, just needed a new thermostat and a engine system clean. now the heat doesnt blow out as hot as it should, and my temp guage needle rocks back and forth. im thinking bad temp sending unit. nut i also might need a new heater core. thoughts?

Before changing the heater core make sure the mixing system is OK. Make sure the heater water control valve is OK. If the system uses vaccum make sure there are no leaks. Check and compare the recirculated inside air verus outside air.

As far as the temp gauge make sure the connections on the sending unit are clean and tight.