kplange

Coincidence or not? I just replaced the intake manifold on my '81 Olds 307 with a 3711 Edelbrock manifold. Reason for replacement was pinhole coolant leaks in the manifold after sitting for six months.
Installation went relatively well. Gasket was installed correctly. Removed EGR valve and replaced with plate. No apparent coolant or vacuum leaks so far. Car runs well, except for an increased idle speed, which I'm still in the process of dealing with.
Vehicle seemed to run fine around town for short trips of five to ten miles. Took it out on the highway today for a forty mile round trip at 65 mph. Five miles from home, after slowing down to city speeds, had the engine temp light come on (first time in fifteen years). Opened the hood to find the coolant reservoir percolating. No loss of coolant.
Had replaced the thermostat at the same time as the manifold with a 180 (down from the original 190). Engine was rebuilt five years ago along with new rad and water pump. Thermostat seems to be functioning as I can see the increased coolant movement when it opens.
After I let the engine cool down for a few hours, I went back and drove the car home for the last three miles. Took the rad cap off back home and noticed that the coolant was pushing out of the rad for the next few minutes, although not boiling.
No idea what is going on. Air in the system? Defective new thermostat? Something messed up with the manifold installation?
Suggestions welcome.

Ozzie

My first thought is to verify the opening of the thermostat at the specified temperature. New doesn't always mean "good". Verify by heating it in a pot of water on a stove using a known good thermometer and be sure that it opens sufficiently at the designated temperature. Also try to verify the actual temperature of the engine when overheating was suspected. I've no experience with that manifold. Others may be able to contribute more.

joe_padavano

Do you have the CCC system operating or not? Did you rejet or otherwise adjust the carb when you removed the EGR? The factory mixture control ASSUMES the EGR is operating and runs the carb lean at part throttle when the EGR would normally dilute the mixture. By removing the EGR, you have likely caused a lean condition.