1995 gmc k1500 4wd does not heat replaced thermostat but still no heat

Thinking water pump. The vanes, often plastic, will wear shorter and sounds like the pump is working but at a greatly reduced capacity. The lukewarm one will be the core feed, and it sounds like it's flowing because the other does come out cold, just not enough flow.....bringing us back to pump issue.

After replacing the thermostat did you get all the air out of the cooling system? An air pocket in cooling system can also cause the gauge to jump around and also lead to no heat when heater is turned on. Just a thought.

Thank you, that's what I was thinking. Cause where the bypass line coming off of the heater core that runs to the pump, where it goes into pump the has been some leakage on the connector. How can I test the water pump?

I'm with Dandyoun. You are air locked for sure. And it sounds like the coolant level is dangerously low in the engine. Don't run it until you fix it. (FYI: 20% of all new thermostats that I by are not 100% functional. They are cheap I always buy extra and test them in water before I install them. I've cooked 1 good motor through over heating so leason learned. My procedure: 1.) Inspect, there should be no light visible through the sealing surfaces. 2.) Place the thermostat on a level suface and ensure the seal holds water. 3.) Put it in a pot and warm water until it opens (you can verify temp too). 4.) pull it out and watch it close. 5.) It should close completely, if it sticks or does not close throw it away, it's not worth damaging your motor. ) On any motor you should ensure that the engine is full of coolant up to the thermosatat, even if there is a bleed valve, before installing the thermostat. With your motor the heater hose should leave the engine block and circulate through the core back to the rad (The warm hose is probably being heated from the engine and the air lock not fluid). You should be able to fill your rad (not the external reservoir) and run your engine for short periods of time to break the air lock and re fill the motor with coolant. Keep checking the level IN THE RAD every time you shut it off and top it up until it remains full. Once full and the level doesn't drop, put the rad cap back on and run the engine up to operating temp, the thermostat should open and purge the remaining air out of the block. Let the rad cool, open the cap and top it up. Now that most of the air is out the system should operate correctly and draw additional fluid from the reservoir as needed. NOTE: If the second hose returns back to the motor, not the rad you will have to bleed the air out of the top of the motor either by pulling a heater hose off or pulling the thermostat back out.

Was writing that long reply while you posted your reply to the other comments. Check the coolant level in the rad first before you look at the pump. It cannot pump if there is no fluid. The return from the heater will be the line entering the water pump and the hot supply should be towards the back of the block. The water pump pushes fluid into the block forcing the coolant out of the top to the rad or the back to the heater, then back to the suction side of the pump. If the engine is air locked there will be no fluid circulating through the heater hoses.

Hey guys . I need to flush sys,change therm,and water pump. I'm assuming I'll need to change orfus tube also. It ran hot cold repeatedly.Almost home it got over 260, would not cool so I parked to cool down.Question is if ok to change oil first or last?? Thanks