Fuel mileage issues

**00SilverGTP** · 11-02-2011

Hey guys/gals.
I just bought my first GTP.
Its a 2000 GTP, with 207k kms on it. Seems quite well maintained other than the previous owner was a smoker.
However I am having issues with gas mileage. My GTP is getting about the same as my Denali. abot 17-18mpg. I just had a full service done on it.
Oil change mobil1 10w30 with mobile1 filter, tranny flush, rad flush, new Belts, new air filter and fuel filter. And yet not improvement what so ever. I added an STP fuel treatment as well as an STP Oil treatment, and nothing. It seems to be running great though, smooth, loads of power, sounds good, no hesitations. Just bad fuel mileage.
Any suggestions as to what it could be? I have heard it could be something as simple as an O2 sensor, or something as big as the supercharger bearings

Any help is appreciated

**Fivefingerdeathpunch** · 11-02-2011

what kind of driving do you do?

If you are strictly city 17-20 is normal, if your straight highway you should be around 27-30 avg and 24ish combined.

**NR92** · 11-02-2011

my 04 gtp does the same thing, it averaged about 18.2 mpg for the longest time (with a good 85% highway driving) did lower intake gaskets and it went all the way up to 19.1, i suspect my upstream 02 sensor, as everything else related to a major tune up has been done

not trying to hijack the thread but i replaced the plugs and wires in my gtp

**00SilverGTP** · 11-02-2011

Thanks
its about 80% highway as well for me.

I was kinda leaning toward the O2 sensor, so maybe I will try that. If that doesnt do it maybe I will take it in for a once over

**Fivefingerdeathpunch** · 11-02-2011

did you do spark plugs and wires?

cuz if those are super old, you will get crap mpgs

**00SilverGTP** · 11-02-2011

I have not had a chance to check the plugs themselves yet, The wires are brand new. They were on the car when I bought it. New NGK wires.

**QUICKSILVER462** · 11-03-2011

Another thing you might want to do is check that the engine is reading the proper coolant temperature.
Dont forget that tire pressure, and even the type of tire will effect your gas milage along with a good wheel alignment.
I picked up a solid 3mpg on the highway after installing 4 new tires and a wheel alighnment. Also make sure you e-brake is fully realeased.
Hope this helps!

**Fivefingerdeathpunch** · 11-03-2011

change the plugs with new delco plugs if your staying stock. New plugs will help a ton with how the car runs.

**00SilverGTP** · 11-03-2011

I did notice a good wrotten egg smell today so some fuel is making it to the cat. The temp guage is staying really low for the most part as well.

**Fivefingerdeathpunch** · 11-03-2011

then it may be time for a coolant flush and new thermostat.

If your handy, plugs and a coolant change are pretty easy.

**00SilverGTP** · 11-03-2011

Coolant/rad flush was just done. Thermostat was checked and they said was good. I think it may actually be my gauge thats off. I took it for a spin this evening and the gauge went up to about normal temp. then stopped at the store came out and the gauge didnt register again. Likely the gauge thats faulty.

Still doesnt help me with my initial problem. I ran some seafoam through it today, vacuum hose, gas tank and in the oil. So we will see if I see any improvement off that. And I am gonna try a new o2 sensor early next week

**00SilverGTP** · 11-06-2011

Changed the thermostat today and got a new O2 sensor I am having a friend throw in tomorrow since I dont have a sensor socket. Hopefully this will solve my issues. If not anybody want to buy a 2000 GTP? haha

**dsmuts** · 11-06-2011

If you have a rotten egg smell your cat could be bad

**00SilverGTP** · 11-06-2011

The wrotten egg smell has only happened once. Chances are its from the bad O2 sensor running the fuel to air mix too rich, so unburned fuel would be passed into the cat. Its only happened the one time so the cat should be fine.

**z3r0** · 11-07-2011

pop out your front o2 and if it runs better it's the cat thats bad

**00SilverGTP** · 11-08-2011

Little update. Replaced the O2 sensor today. since the thermostat yesterday and O2 sensor today my mileage has gone up to about 20mpg 75% highway. Still seems a little low to me. What do you guys think?

**rynoman03** · 11-08-2011

Have you sea-foamed your car yet? That might help as well.

Sea Foam | Motor Treatment Do this through the vac. line not the gas tank or oil.

Change those plugs as well, check all of your tires pressure, ect.

**00SilverGTP** · 11-08-2011

Yeah I did the seafoam, but I only let it sit for a few minutes. I think I will do it again. Tire pressure is good. I checked the spark plugs and they seem to be brand new. changed when the wires were. But I am gonna pull all of them and check the gap on them make sure its right