so i use to get 24-25 city and highway never changed which was kinda weird, and since i have been getting p1133 code it dropped to 21 city and highway so i put new o2sensor in did nothing then put new plugs did nothing then checked fuel pressure to see if pump was bad perfect took it to GM they did the test couldn figure it out. just cleaned the maf sensor and its at 18 mpg now can someone please have a answer to my question WHY CANT I GET GOOD MPG??????????????????????????????? and all i have is gm stage 2 with 2.8 pulley and vibrant exhaust, injen intake

 

how is the cat? Maybe run some sea foam through it but put the old o2 sensor back in before if you can cause sometimes if your car has alot of carbon buildup might clog the sensor. maybe a sticky injector?

 

Its cold out side and you are looking at your average fuel consumption.

These cars run rich as it is and worse when its cold,if you letting it warm up then froget about checking mileage.
The average meter still count up when your standing still and have the car running.

But if your doing the check by filling up the car then filling it up again and then recording the distance traveled ,then doing the math miles traveled /gallons then I take back what I said.

 

true so your saying that basically the mpg meter on dash really is not that accurate???? now what could be making the idel so bad just cold weather and it has a p1133 that i can not figure out whats causing it im leaning toward the maf sensor being bad i cleaned it and that made mpg worse but dealer says 02 sensor which i already replaced?????

 

Checking by filling up and going distance travel will still show less when you let your car warm up. While your car is warming up, your burning fuel out of your tank. Meaning your next fill up will be more. Wich is why it SHOULD match your display anyway. (But mine doesnt)

Check your tire pressure? Are you driving with your windows down more? AC on more? How about your brakes? Your parking brake might not be releasing all the way, could be dragging. Is your engine properuly lubricated? (Check oil level) Maybe your clutch is slipping or your manual transmition fluid is low/wearing out. Is your Boost gauge showing less vaccume at idle then normal? Is it showing higher then normal while your at cruise. The boost gauge is fairly accurate in displaying the workload of the engine. Higher pressure (less vacume) means the engine is working harder.

 

If you didnt use a OEM O2 sensor then the car is still running in a loop (because of the code)
It will likely throw that code until you put in the oem sensor.
Reason for this is that the pcm looks at millivolts when checking to see if its working properly.
Any resistace difference will throw it off.
When the car is running looped(only looks at certain sensors and runs off of a set table )you will use a lot of gas and this is also not good for your cat.