I hope somebody can help. My dad gave me a 1998 chevy truck with a 4.3 V-6 engine. The truck runs great but the heater does not work. He recently got it and gave it to me. The prior owner said the heater core was just replaced. When I got it I noticed the temperture gauge would fluctuate between 100 to 190, but mostly staying around 150. The heater also would only blow luke warm air. I thought maybe it was a bad thermostat. I changed it and the bottom radiator hose and replaced coolant. I immediatly noticed a gurggling sound coming from the heater core and the truck would still not maintain the correct temp and the heater still only blew luke warm. I ideled the truck for a long period of time and got a lot of air out of the system and added more coolant. The heater then blew hot but the temperture gauge never got above 160. Put thermometer in radiator and it read 155. When you let it cool off the heater will not work again. you have to take the radiator cap off and run it for a long period of time to get the heater to get warm. Every now and then you can hear a little gurrgaling in the heater core. The heater hoses are old and are not the correct ones. They are peiced together with different size hoses. Could it be the water pump, head gasket, air in the system, blockage, or what. All of the hoses get hot. Sorry for the long post but I am really frustrated. Any ideas? Thanks.