292 water temp problem.

[factoryorange3]

I have a 79 Chevy k10 that I swapped a mid 60's 4.8 292 motor into. Have been driving truck for about a year just recently put a rebuilt head onto motor (new water pump, thermostat) I am experiencing inconsistencies in water temperature. I will start truck in morning to let warm up .(temps been below zero lately) I watch temp gauge slowly rise and keep rising almost to red before gauge slowly drops again to operating temp. If I let truck idle gauge will go up again dangerously high until I throttle engine up in order for temp to go down. The heater in the truck blows good heat when at operating temp but when gauge rises I lose heat in cab until I throtle motor up and gauge drops. When I did the head work I had radiator cleaned and flows great. Yesterday put another new thermalstat in truck but did the same thing this am. Any ideas of what might be happening would be great!! Thank you.

[cooperhw]

I would lean towards pulling and checking the thermostat. Sounds like it is a little slow opening. I put them in a pan of water and when it opens check the temp of the water. I have seen new ones that are bad. I also had a similar problem on an LS-2 motor in my Chevelle. Had to jack the car's nose up as high as possible and open both the resivoir and the radiator to allow water to flow into motor and force the trapped air out to the front. I don't ever remember having that kind of problem on the old 6 bangers, but,

[Kimjongthrill]

Sounds to me like you lost a water pump impeller

[factoryorange3]

Thanks cooperhw. Thermostat seems like the problem I changed the thing yesterday and filled filled block right through thermostat housing with coolant.don't think there would be a problem with air. I am still kind of confused as to why when ideling at running temp my heat goes out. (Blowing cold air) but shortly after reving engine heat returns.