Simple things first.

Coolant level?
Fan belt tension?
Faulty thermostat?

steve

No it's not normal, but a $350 car on a maiden voyage going 90 mph--what's wrong with this picture?

To save the car, rad obviously needs work (or replacement if you're really going to keep it for a while). Probably needs flame trap cleaning, a new water pump, seals, tensioner, T belt too (based on price--people don't give away maintained bricks). Air box thermostat could be stuck, thus threatening MAF.

To save motorists, yourself included, tires couldn't be in great shape. Even if tread looks OK, if over 5 years old becomes a safety concern. Blowouts uncommon, but separation not--especially if sitting for a while. Wheel bearings, steering gear, shocks, trailing arm bushings, control arm bushings, tie rods, brakes, all need inspection/attention. Rust-through also a major concern in many parts of the country, thus allowing CO to enter cabin.

While I'm uncertain of your reasoning for driving an unknown vehicle at 90 MPH in the first place, I think that you probably should have the radiator cleaned, or do it yourself. You are asking for a lot of output from that old heat pump, and it is responding. The idea is to exchange the heat so that you don't burn the pump (engine) up. Have a look at the condensor as well, and try back flushing the radiator and condensor with a good garden hose. You might be surprised as to the yucky stuff that will come out of there.

Before you try another Banzai blast, I would look at the suspension, steering, tires and brakes to make sure that you don't become a flying projectile aimed at someone who didn't expect to see a flying brick.

Bill