1998 Buick Century losses significant power at lower speeds

the cheapest thing to try is a fuel filter

check whether is your crank or cam sensor. unless is your fuel pump drops the sending pressure. Use a scanner to check if any fault code is coming on

If the engine died you would loose power assist steering and power assist brakes. I have to think it was an electrical power failure somewhere.
Will it start up and idle now ?

The oil light coming on when stopped at a stop sign tells me your engine was already damaged before you had the power loss problem. I would not waste the 650 to put in an oil pump. You would be further ahead to just replace the engine.

Your PS pump is fine.

This vehicle has:

Variable-assist power steering: A power steering system that uses valves and speed sensors to vary the amount of steering assist according to engine or road speed. At slow speeds more steering assist is delivered and steering the wheels is easier; necessary for parking, etc.. At higher speeds, steering assist is reduced and more steering effort is required to steer the car, giving the driver greater feel of the road. Also known as Speed-sensitive power steering.

Loss of current to the magnetic coils would cause a loss of power assist at low speed. Coil resistance can be checked with an ohmmeter, and should read about two ohms. An infinite (open) reading indicates a bad coil (requires replacing the rack since the coils are not serviceable). Checking for shorts between both sides of the coil assembly and rack housing is also recommended.

Best wishes

you need to install a lower temp thermostat. and check the timing too far advanced will also cause this problem

cam shaft sensor

This could be the speed sensor in the transmission. I believe it is electric on the 98 takes about 15min to change and is located on the long end of the trans.

try adding it via the reservoir. make sure you have a 50/50 mix of water and antifreeze. If you want to cheat, cheat on the side of more antifreeze than water. Look for a leak in the hoses and water pump to find out if she has a larger problem that is causing the loss of coolant. look for drips under the car.

Buy a new car!