1998 Chevrolet Monte Carlo Thermostat

Replacing the thermostat is not going to solve a leak. That needs to be identified first, then repaired.

The Security system can not be bypassed. They are very effective at keeping owners out of their cars. No abnormal engine noise is a good thing or something to be ignored. As for the battery running down, there could be a draw on it that needs to be identified, or the generator could be not fully charging the battery while you're driving. A professional load test will show that. Since the '87 model year when GM redesigned their generators, they have had a real big problem with them. It is common to go through four to six replacements in the life of the car. To reduce the number of repeat failures, replace the battery at the same time, which you've already done, unless it's less than about two years old. As they age, they lose their ability to dampen and absorb the huge voltage spikes these generators develop. The load test will show maximum current output and "ripple" voltage, two indicators of a defective diode. It's common for those voltage spikes to destroy the internal diodes. When one of the six fails, you will lose exactly two thirds of the unit's output current capacity, and 30 amps from the common 90 amp generator is not enough to run the entire electrical system under all conditions. The battery will have to make up the difference until it slowly runs down.

My opinion is to repair what you have, especially if the alternative is to buy something newer. Your car is still fairly easy for anyone to repair. GM is the leader in making cars now that have to go to the dealer to have new computers installed and programmed to the car. You can't use good used computers from a salvage yard, and they can't sell those computers to anyone. The only one who benefits from this is GM. Unfortunately whatever new thing the people at GM dream up to make money off unsuspecting owners, other manufacturers copy a few years later. Until that business mentality changes, I refuse to buy another new car.