I have a 1999 ssei supercharged V6- 3800 3.8L. It has 252,000 miles (had 51,000 when I purchased it) but has had an oil leak for a while. Finally find that is is coming from valve cover. The engine light stays on, air conditioner only blow on recirculation and only from windsheild vent, dash monitor bings excessively at times and front tires deflate regularly. With that being said I have had no other problems with my car. However I don't want to get ripped off getting these things fixed and I really need to keep this car. Can anyone tell me reasonable prices for the major repairs. Valve cover first, to stop leak. Rims are old and I did find a way to help fix that problem. Thanks for any suggestions.

Wow sounds like the old girl needs some TLC. I don't see a shop charging you more than 150.00 for the V/C gaskets. As far as the HVAC it could be something as sime as a vaccume leak. My 94 SSEI had a cracked vaccume reservoir that caused the heater to act like that. The wheels might have perossity issues causing them to deflate. Sometimes the tire shop can add green slime to stop that problem.

Valve cover gaskets can be purchased for around $15-20. If its just the front one leaking, its pretty simple to replace yourself. Then take the other one and jam it in the garage somewhere. If its not broken, don't fix it.

Take your car to Autozone and have them scan it. Make sure you write down the codes. We can tell you where to go after that.

the systems monitor is grounded in the troublesome ground bus under the carpet

https://www.gmforum.com/electrical-1...1999-a-303224/

HVAC blowing only from the defroster is the default mode when the HVAC control head fails. Kind of like your transmission going into limp mode and staying in second gear when there is a major failure. You probably need a new HVAC control head.

Take one of the leaky tires off and while slowly rotating it, spray with water and a little bit of dishwasher detergent. Look for bubbles. Make sure you spray the bead area, and the valve stem opening. If you have bead leaks the tires need to be dismounted from the rims and the rims wire brushed on the bead seat area to clean them up.

Here is what I mean by HVAC control head, depending on which type of climate control your car has:

96 99 Pontiac Bonneville Heater AC Control Unit with Rear Window Defrost Tested | eBay

95 96 97 98 99 Pontiac Bonneville Heater AC Climate Temperature Control | eBay

Also, as 75Racer mentioned, you should check out the vacuum control system for the HVAC. Here is a good explanation:

https://www.gmforum.com/mechanical-1...2000-a-279092/