thank you all it was the fitting on the intake

thank you for your input and i'll let you know what i find. sorry i took this on, three other shops had it before me.

took heater hose off and ran truck hardly any flow from 3/4 hose

heater box was broken /and it had the prob with heat when it came in so its been a nightmare from day one / dose any one think it might be a intake gasget

3/4 hose gets hot/5/8 hose dose not even get warm/ even clean out heater core/

Anyone have any clue why a '96 Chevy Tahoe with a 350 would lose heat when driving?

Changed t-stat, water pump, heater core, nothing helped. Also changed heater box.