i just recently bought a 95' SSEi bonneville thats been sitting for 6 months in storage...it just turned 94000 miles on it---some things dont work but everything is starting to coem together and work again...but the gas milage is TERRIBLE---i've heard and read that these engines (3.8 supercharged) get about 20 mpg....so i calculated it--this car has an 18 gallon tank...i have 1/4 tank left and almost have 200 miles to this tank...so im getting about 12 mpg...this is TERRIBLE....what'* wrong?...also just today it periodically started this----longer than normal starts...one of my buddies said something about a pressure regulator---he said turn the key to ON....then turn it off....and then start the car up---i did this---and he was right---it fixed the problem...meaning that the problem was this valve or w/e....if anybody knows where this valve is at or anything else about my problem...PLEASE help me...also--when in gear...it does idle a bit low(it vibrates a small bit like any car does when it'd idling too low--give it a tad bit of gas and it smooths out)...this is just my thought from a quick thinking--plugs and/or wires could use a change and possibly at the same time help my fuel milage....but then i also thought--well i could have bad injector(*)....this could possibly cause the longer than normal start too maybe---from leaking or something----but anyways im looking for help because this fuel milage thing is really upsetting--if anybody has any advice or knows anything or has experienced the same issue---please let me know and help me out...


thanks

Firstly, congrats on that car and welcome to the forums!!!!

Now, the second most common cause of bad gas mileage (Next to a 2 ton foot on the gas pedal) is your O2 Sensor. If you peer over the back of your engine, down towards the bottom, you'll see a connector that sorta looks like a big spark plug with a wire running out of it that is plugged into the exhaust. That'* your O2 sensor. When I picked up my 94 at around the same miles you had now, I was getting @#$@# mileage too. I changed that sensor and all is great.

BTW, if you could go into your profile and set up your signature with your Year and trim level as well as any mods you may have made to your car up to this point, it'll help the uber gearheads in the forum give you the best advice quickly.

Once again, welcome to the forum!

if it has been sitting do a full tune up...including the O2 IM betting that its toast...DO NOT put a Bosch sensor in....a AC delco is preferred

What 1993_SLE said. Bosch products don't seem to play nice in our cars. If you visit www.gmpartsdirect.com, you'll find a kick butt resource for buying OEM parts for our car. The AC Delco O2 sensor is reccomended by the resident gearheads. (AC Delco Part number AFS75)

your going to want to clean the Throttle body, i was experiencing hard starts and low idle. i bought some TB cleaner, sprayed it up in there cleaned it with a toothbrush, and it fixed my problemfor the most part. i recently did plugs and wires. my car is running excellent now!

Ohhh yeah, you'll want your TB to look like this when you're done:

I didnt think of the oxygen sensors----btu which one should i change....or is ther only one?...and if it was a bad o2 sensor---wouldnt that kick on the engine light?

most times an O2 will not set a SES

There'* 2, but you don't need to change the second one. That one is behind the cat I believe. One of the gearbrains can chime in on that. The front one wouldn't neccesarily trip the idiot light... Anywho, just change that top one on the exhaust. BTW I want to second and third the sentiment that you should give the car a good tuneup:

O2 Sensor
Plugs + Wires
Clean the TB (Change the TB gasket while you're there. $6 bucks)
Change the oil and oil filter
PCV Valve (2.99)

That basic maintenance should go a long way towards restoring performance and gas mileage.

Mortehl...95 SSEi has one O2 sensor, as does your 94.

Where are you located? I have a perfectly good O2 that I swapped out of my 95 SSEi because I was unsure it is was a problem.

On the throttle body, being supercharged you DO NOT want to spray cleaner on the epoxy on your supercharger rotors. I learned the hard way.

I would start with a full tune up of oil change, plugs, wires, pull off throttle body and clean, O2 sensor and some nice premium fuel. If you happen to be in the CT area..I'm usually available to help.