Mechanic just replaced 2.4 engine. Driving away from shop 11 miles down the highway the car wont go over 40 mph and the rpm goes from 1 to 4 with little power. Shop closed til Monday. Suggestions?

Sounds like good advice. Curious...why was the engine replaced?

blown headgasket

First he said the radiator caused over heating and blown head gasket. Now he stated his exhaust specialist said catalytic converter needs to be replaced and he thinks thats what caused the blown head gasket. Yet he brought it to a different. Would you suppose the catalytic converter went out and got smothered from the blown head gasket?

When a Cat. Converter goes bad it could cause a restriction in the exhaust system which could cause the symptoms you described above. I'm inclined to agree with your mechanic. There is a exhaust system back pressure test which can be performed to confirm the bad CAT. A blown head gasket allows coolant to enter the combustion chamber and consequently allows coolant to enter and damage the CAT. Good Luck