I have a 2003 Toyota Camry LE, with 200k miles. Recently my head gasket went out so I worked with my dad on taking the entire engine apart and replacing that, along with replacing the thermostat, water pump, radiator cap, timing chain, and also ended up replacing 3 out of the 4 coils. Now I've run into the issue of the coolant in my overflow reservoir boiling to the point that its overflowing out of the top. This only happens when going 70-80 mph on the interstate after about an hour of driving. Any ideas as to what's going on now?

 

You replaced some of the possible problem areas such as t-stat and radiator cap.

How is the radiator? Did you flush the radiator or use cleaning chemicals?

Any air leaks in the cooling system?

Was the head checked for cracks? Was the head resurfaced?

Suggest doing a "block test," This test checks for exhaust gases in the coolant which means a cracked head, block or problem head gasket.

 

We ran water through the radiator and the flow seemed good, didn't seem to be blocked, and the head was resurfaced and checked for cracks. As far as we can tell there aren't any leaks in the system. We at first thought it was the head gasket again but there aren't any other same symptoms as we had before we replaced it.

 

When the engine cools off, is the reservoir full and the radiator top empty?

If the old radiator cap was OK, swap it out.