GMC Troubleshooting Questions? Ask a Mechanic for Answers
The 4.3 manifold is blocked at the rear of the engine (no coolant crossover). The front of the intake has a coolant crossover passage, this is also where the thermostat housing is. There are no restrictions in the intake gasket on this engine. There is a coolant bypass hose coming off the front of the manifold to the top of the water pump. You could crimp off this hose with pliers and see if this allows the engine to reach operating temperature. If it helps, you could install a restrictor inside the bypass hose.