Engine Cooling problem
1978 Datsun 510 4 cyl Two Wheel Drive Automatic 128000 miles
My Datsun has a tendency to have heat spikes while passing on the freeway. It runs a little hot all of the time as well. It has plenty of coolant, and the bottom hose is hot, so I know the thermostat is working. It also has a cracked exhaust manifold, which I am in the process of replacing. Could the exhaust manifold be causing the spikes and the high running temperature, and if not, what could it be?