02 Grand Prix GT Fuel Mileage

[thekidinbuff]

I originally posted this over in the 3800 forums but still at a lost so im going to try over here.

I have a 2002 Pontiac Grand Prix GT with a 3800 motor in it. The car just rolled over to 104,000 miles. I've had the car for about a year now and I've always gotten about 19 to 22 to the gallon. I don't drive very aggressive at all. Every once in a while I'll jump on it, but not enough to affect my fuel mileage.

I've done/replaced/gone through the following:
- Spark plug
- Spark plug wires
- Pvc valve
- Has newer tires with all psi above 30.
- Transmission pan drop/ fluid change (shifts hard sometimes)
- Checked the upper oxygen sensor (unplugged, checked voltage while running. Showed a steady .8ish and when I gave throttle, jumped to .1ish then back to .8ish. I think thats incorrect?.. )


The car runs great. It has power. The car moves but doesn't get great gas mileage at all.

Any ideas anyone? And should the reading from an upper oxygen sensor always be constantly going back and forth from .1 to .9 volts, even when it is not connected to the ECU?

Thank you in advance

 

[Fivefingerdeathpunch]

I've never tested the plug on the O2 sensor with it not connected, I've only ever tested it with it plugged in and on a scan tool.

It is suppose to swing up and down all the time, it's how it works. Goes from rich to lean and rich to lean and so on and so forth, always trying to achive stoich. If the sensor has never been replaced, it's wise to do so. The original o2 sensor is outdated and isnt as good as the new design denso sensor are the market now.

Also, what brand plugs are you running?? Hopefully AC delco for the best overall performance.


And figure if you drive 30-40% city and 60% highway the avg should be around low 20's, maybe 22-25 for a good avg. Traffic and city driving kill mpgs fast.

 

[thekidinbuff]

Thank you for your respond.

Im currently working on getting a scan tool. I just have to find a cheaper priced one with the live data.

It always going from rich to lean makes sense. Because the computer attempts to correct itself, right? I thought i read somewhere where when the computer doesnt have a o2 sensor reading, it defaults to a preset thats real rich. Which would make sense to it always being .8 unplugged. But im stilling testing.

I do believe i have the NGK V-power plugs in it currently. I know theyre not the best but they did feel like they made a difference when i put them in. Theyve been in there about a year now. I pulled Cylinder 1's plug out, and it was greyish, whiteish, which i believe is good. I dont smell gas. I definitely smelled strong, rich exhaust fumes though when i unplugged the oxygen sensor, while it was just idling and i was testing lol.

I live mostly in the country. 45 mph speed limit on most roads. I do have days though i catch every damn red light.. Theyre isnt close to enough to be considered city though. A buddy of mine has an 05 Impala with the 3400 in it, same transmission, and he gets about 25 mpg. And drives similar routes as i do.