Bonneville running rough check engine light flashing low on coolant

SOURCE: Coolant fill procedure

NEVER open the radiator when the car (or any other engine) is running!! The radiator is part of the pressurized cooling system (under that cap is coolant at 15psi)

Correct way to check the coolant in the radiator, is wait until the engine is COLD, then remove the radiator cap and re-fill.

SOURCE: low coolant light keeps flashing

Replace the coolant level sensor.

SOURCE: Dexcool vs Green coolant

Dexcool is an extended life coolant-you cannot mix the 2 or you you will have serious issues. If you are going to switch over the the dexcool from the green you need to flush the entire cooling system.

SOURCE: Low Coolant light flashing on 1996 Buick Regal 3.8

The low coolant sensor should be located in the passenger side radiator tank, You need to take it out and clean the sludge off of it for it to work right. Or you can just unplug it and forget about it, the light will go out and it won't cause any problems.

SOURCE: my 1994 pontiac bonneville has a very rough idle,

rough idle sounds like it could be your TPS system going bad.... as for the coolant light, it may be a faulty sensor, or you may have a tiny leak and the system detects that the pressure is lower than it should be.

As for the battery itself when tested, it should read 12-12.5Volts across the leads when the car is off. When the car is running, the battery should be reading 13-14Volts across the leads, anything lower means the alternator is not charging your battery, anything higher means the alternator is over charging and damaging your batter

Testimonial: "Appreciate the advice, I was told the same thing about the coolant light by another mechanic. Good info about the battery and the rough idle."