Hi,

My 1998 3000GT SOHC 5 speed wont keep temperature when at hightway speed when it's under 40 degrees outside.

It will warm up on idle and keep somewhat warm around town. But when I drive on the freeway my temp sensor goes down to near bottom and I only get lukewarm air from the heater. Should I block part of the radiator to decrease cold airflow through it or is this a thermostat issue?

Thanks