Bad Gas Mileage Oxygen Sensor Voltages Help

[Konner27]

Hello, I have a 1999 Pontiac Grand Prix GT with 170,000 miles. I purchased the car in March and have noticed that I am only getting around 12 mpg city. I only drive in the city. I tested the O2 sensors using the Torque android app and bank 1 sensor 1 fluctuates from .1 to .9v which I think is good right? Bank 1 sensor 2 fluctuates from .2v to .5v. I don't know if this is good or bad. However I only had the car running for about 2 or 3 minutes before I tested the O2 sensors so I don't know if that affected anything. Looking forward to your help. Thanks

[brandonl2000]

bank 1 is what counts. and if it's fluctuating that's a good thing. you want it active

[indygrandprix]

If it was the o2 sensor causing that bad of mileage then I would think you would have a code. Do you have any codes?

[Fivefingerdeathpunch]

Bank one will always bounce around, so what you see is right.

Sensor 2 will jump around a little while its warming up and waiting for the cat converter to get hot, once its hot and says its running good, the numbers will stay around 400-600mv.

If you don't have a check engine light for an O2 code, then thats likely not your issue.


You can try doing a tune up, like plugs, wires, clean the Throttle Body and MAF sensor. Run some injector cleaner etc etc.

But in the end if you only do city and its short trips you'll likely only get 15-18 mpgs. Slightly less if its cold where you live.

[Konner27]

I have no codes. I checked the plugs and wires and both look good. I have cleaned the MAF sensor recently. I also slightly cleaned the throttle body but I didn't take it off so it probably isn't very clean. I ran Chevron Fuel Injector Cleaner and also Seafoam through the system. What else could cause this?

[Scottydoggs]

my mpg drop to 12 and lower if i stay local, nothing over 30 mph for days on end. seen it go as low as 9 mpg if im beating on it.

on the highway ill see mid 20's and higher. up to 30 mpg if i dont hit a hill lol

[Konner27]

Would the pcv valve affect gas mileage? I checked mine and it rattles freely but it is covered in carbon.

[Scottydoggs]

i also checked mine out, it was dirty but not clogged, i cleaned it with brake cleaner, put it back, no change.

clean your TB up. do a tune up if its been a while. and try to drive with a light foot. let off the gas roll to red lights. make sure the tires are at the right psi. avoid jack rabbit starts and such. see if your driving habits can help any. not saying you drive it hard, i dont know you lol just suggestions.

winter time we get worse mpg too.

[indygrandprix]

Also check for dragging brakes incluing the parking break and take out any unnessesary weight you have in the car.

[Scottydoggs]

unnecessary weight, check, i got some of that. theres a sub and amp back there too now. lol

[ericspecullaas]

Scotty you have to many seats and to much padding in your car lol

[Konner27]

I would say I am pretty conservative when it comes to driving. Also, will cleaning the throttle body effect gas mileage that much? How do I take it off to clean it? I'm nervous to take it off because of the throttle cables.

[Scottydoggs]

the cables just un clip. take a good look at them. take a pic before, so you get it back on right.

the TB's all get black and carbon build up behind the flap and in the housing of the back of the TB. the edges of the flap also get gummed up. air just dont flow as well as it could. it should idle smoother too.

you'll want to take the maf off the TB before you clean it. you never want anything but maf cleaner to get on the maf.

good old cheap carb and choke cleaner works just fine for cleaning the TB. you'll want to pick up a new gasket for it too.


theres a how too on removing the TB and the cable. search around for it if you feel you need it. its pretty easy to get off tho.

[02NavyBlue]

Removing the TB is a very simple job, I didn't look up anything before cleaning it. The throttle cables unclip, and re clip with no adjustments necessary. (basically what scotty said lol)

I'm not sure cleaning it will improve mpg much though. The main thing I noticed was more throttle noise lol.

[Konner27]

I was looking at your signature and the flipped dog bones reminded me. Does this mod really do anything?

[02NavyBlue]

Well after you flip them, the motor will have much less movement as you can see in the video I made: http://www.youtube.com/watch?v=qAZJ4xxcLyI

When driving, not really. Some may notice a more instant feeling of torque but I didn't

[Scottydoggs]

i flipped mine too. it makes the engine a bit firmer in the bay. you'll notice a small difference.