Why does my 305 overheat after startup, but runs cool afterward?

Question

I replaced the head gaskets on 1990 Chev 305. When starting first thing in morning, temperature rises to 240 or 250 before thermostat opens. After thermostat opens temperature runs fine, does not overheat. I replaced thermostat with a 160 degree. Temperature now rises to 220 to 230 when first started, when thermostat opens engine cools to around 160. No overheating occurs after thermostat opens.

The 160 thermostat was put in because initial starting allows the temperature to only heat to 220. I put in the 195 and the car overheats - temperature guage goes into the red! I stop, pull over,and wait 3 to 5 minutes. Restart the engine and (if the thermostat is open) the engine cools down. Also with the 195 in, the temperature did cycle back to overheat (250+). Stopping again it finally cooled to 195 and stayed there. Coolant is new, radiator is new, thermostat is new, water pump is fairly new (1 year). Coolant flow through heater core looks good at startup and after thermostat is open. Water fountains out if started with radiator cap open and engine runs for a while (indication of temperature overheat?).

More info: Temperature sensor is new. Could catalytic converter be clogged, holding higher temperature in the cylinder head? Yet not clogged enough to cause running problems?

Coolant level is full, air is normal, coolant lines are not full of air - the new 195 thermostats have a open area that allows air in the manifold to pass before the thermostat opens.

Answer 1

Stick the 190 Deg thermostat back in it.
Your computer will not stay in "closed loop" at those lower temperatures.

Do you have electric fans? (not working right)
Did you put the thermostat in upside down? (duh..)
Is your coolant low? (Full of air/steam)

Answer 2

Boss My question is this? Why are you useing a different thermostat? Also your vehicle is computerized and with the setup you need the proper thermostat for your vehicle the temperature is 195 deg.Cooler thermostats cause problems,and the computer controlled system has to compensate and readjust to the temperature that the engine is most efficient at,cooler thermostats are for cars that are smog exempt before the year 1977.One final thing is this your cooling system needs attention everywhere,backflushing and cleaning the radiator and flushing the heater will benefit your vehicle and makes the performance a lot better for you.Hope I helped you out and let me know about how this turned out.Chevrolet car owner and engine enthusiast buff.

Answer 3

This to me simply sounds like an issue of outside temperature and your car's ability to know its own temperature. This problem could be happening because in the morning it is cooler outside so it could give the thermostat a false low temp causing it not to open right off the bat. The other thing I wonder is if you are using a cold air intake because that would be an obvious reason for the temp to go down once your driving.

Answer 4

I would replace the thermostat. Cheap and easy to do.