First off Im new to the Forum so Hello everyone.

I have a 92 Bonneville SSEI and im only averaging about 15 mpg. To me this seems horrible. Ive changed plugs, wires, fuel filter, coil packs, brand new timing chain gear set (had to change the stupid magnet on the cam gear, lot of work for a $10 part) Im not sure if this is average mileage for the vehicle or not but seems like a v6 should do better supercharged or not. Any Ideas?

If you're doing all short city trips, it's not bad. But longer, faster driving should get you better mileage.

What's your boost gauge say at idle? It should be pretty much planted near the bottom of the range. -6ish or higher (less negative) could indicate vacuum leaks or a clogging cat.

Is there any hissing sound? Could be a vacuum leak. You can spray around gasketed areas and vacuum lines with TB cleaner while it's idleing. If the idle changes when you hit a certain spot, there's a leak.

Try cleaning the MAF. Take it out of the throttle body, and clean the elements carefully with rubbing alcohol on a Q-tip.

Could be that your O2 sensor is getting weak/slow. There isn't a real good way to test them unless you have a live data scan tool.

Can you borrow/rent a fuel pressure tester? Test it at idle with and without the vacuum line on the fuel pressure regulator. Should be 42-47 psi, IIRC.

Maybe dragging brakes, low tire pressures, not running premium gas, lead foot...

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WHITE WHINE - 1992 SSE Supercharged 236.26 ci (.040 Over) 15.090 at 90.2 MPH on old engine w/ slipping trans & melted O2 sensor - Gen 3 M62 and matching TB, Gen 2 Pully, Zillamotorsports Ported LIM, YT 1.72 Roller Rockers, SII FPR & Injectors, Hypertech Thermomaster chip w/ 160 Thermo, TransGo Shift Kit, Infinity/Pioneer Speakers & a 10" Alpine Type R Sub, all the watts, 140 amp Alternator, Ricepipe CAI w/ heatshield, Pilot Angel Eye Foglights, Clear Corners, '02 17" Chrome Bent 5's, Magnaflow F-Body Muffler and Hi-flo Cat, Ceramic Coated Ported Exhaust Manifolds, Fan Override, Monroe Reflex struts, red calipers
2009 G8 GT - Sport Red Metallic, loaded, SOLO Axlebacks, Rotofab Intake, Tuned, autodim mirror, removed intake manifold cover, HSV GTS triple gauge pod, two tone red-hot shifter and HSV SuperSport steering wheel, GXP rear sway bar and diffuser, 3.45 diff and various Camaro suspension bits, LED Taillights

Everything mentioned above is right on.

In addition to that,

I see your in Indiana, which means your fuel (like ours in Michigan) gets blended differently for the winter months to keep the gas from freezing. We have all noticed decreased MPG during winter months due to this.

Also, I tend to let my car warm up a few minutes in the morning, particularly when its cold. This really reduces MPG as well. I just like to know the oil is somewhat thinned up by the time I need to leave my apartment and (possibly) need to accelerate hard to merge with traffic.

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Eric

2005 SLE, Greystone Metallic, 152K.

A few mods..

most of my driving is down country roads. Im not hearing any hissing and boost guage stays idle at about -8. It just doesnt make since to me why this engine combo would get such horrible gas mileage ive had 5 differnt v8 cars that did better on gas when I did have a lead foot. Love the car but 15 miles a gallon on a v6 is just rediculous looks like its time to sell her

If you're getting lousy fuel mileage only from time to time, it's the "crappy" gas. Otherwise, Mr Wikoff would be able to tell you more than I. - BC

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In Memory of Brad - 1/21/1977 .. 10/23/2013 ...... Aaron - 1977 .. 2017 .....
2017 Hyundai Santa Fe Sport ..... 1992 Bonneville SSE 1SB 170 HP L27 4T60E retired/sold to MattStrike ..... 2005 Bonneville SE 1SC 205 HP L36 4T65E - retired/salvage yard ..... PBCF user 2321

Something else to try would be to remove the fuel rail, leaving the wiring and fuel lines connected, and turn the key to on (don't try to start it). The fuel pump will bring the pressure up and it will show you if any injectors are leaking. You can also pull all the spark plugs and see if one looks different than the others, showing you different combustion (too much or not enough fuel, fouling, etc).

Unless you are going to buy something for less than you can sell your car for, it's worth some diagnosis first.

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WHITE WHINE - 1992 SSE Supercharged 236.26 ci (.040 Over) 15.090 at 90.2 MPH on old engine w/ slipping trans & melted O2 sensor - Gen 3 M62 and matching TB, Gen 2 Pully, Zillamotorsports Ported LIM, YT 1.72 Roller Rockers, SII FPR & Injectors, Hypertech Thermomaster chip w/ 160 Thermo, TransGo Shift Kit, Infinity/Pioneer Speakers & a 10" Alpine Type R Sub, all the watts, 140 amp Alternator, Ricepipe CAI w/ heatshield, Pilot Angel Eye Foglights, Clear Corners, '02 17" Chrome Bent 5's, Magnaflow F-Body Muffler and Hi-flo Cat, Ceramic Coated Ported Exhaust Manifolds, Fan Override, Monroe Reflex struts, red calipers
2009 G8 GT - Sport Red Metallic, loaded, SOLO Axlebacks, Rotofab Intake, Tuned, autodim mirror, removed intake manifold cover, HSV GTS triple gauge pod, two tone red-hot shifter and HSV SuperSport steering wheel, GXP rear sway bar and diffuser, 3.45 diff and various Camaro suspension bits, LED Taillights