I just purchased a second truck to tinker around with. Just like my 78, this 77 has an 4.9. The only difference b/n the two is that this one gets hot 235-250 degress when I get it out on the highway in 4th gear (1700 rpm). Compression good (130+/-) vacuum good (17), new water pump, flushed radiator, pressure tested the radiator and cap (good), flow appears good, new plugs rotor and cap, no exchange of fluids (water in oil and vise versa). The only thing that is odd is the timing is set to 13btc as opposed to 10btc. Could this be the cause? Could to lean of a fuel mixture cause this as well? The truck runs and idles great...Has approx 85k on rebuilt by previous owner. What gives? Did I mention that I already removed the thermostat and it still runs the same temp...In the city (stop and go) and only in third, the temp stays around 195

 

Timing can cause it, and a lean condition will to, so check your spark plugs and see if the are a golden brown or white, if they are golden colored then set the timing.

Also it could be the fan clutch.

 

i would change the timing to 10.

 

New plugs (150 miles) are lightly and evenly carboned. Not wet, gummy or fouled. Would 3 degree advanced timing cause this?