Good point 2kta 
Is my diagnosis correct?
Thread Starter
Staging Lane
Joined: Apr 2005
Posts: 59
Likes: 0
From: Pittsburgh, PA
Is my diagnosis correct? Here we go.
Car: 1999 Z28 m6 61,000 miles
All stock except for borla catback exhaust.
I bought the car 3 weeks ago. It is in really good shape and runs fairly well, however it has one problem. It is guzzling gas like there is no tomorrow. My math is being based on a 16.9 gallon tank (correct?) and with that i get around 200 miles. That comes to around 11mpg. I do not drive the car hard and have actually been babying it to see if i can crank up my mpg. I had a SES light, got the codes read and it returned EGR valve and bank 1 sensor 1 and bank 1 sensor 2 o2 sensors. This light has since gone away. I still think it is the o2 sensors being that the car sometimes idles rough, falls on its face in 2nd and third gear and is getting terrible gas mileage. I have pulled a plug and they look fine. I have ran injector cleaner and sea foam through the gas with no help. Even with the SES light not on could my o2 sensors being screwing me? Any advice would be appreciated.
Car: 1999 Z28 m6 61,000 miles
All stock except for borla catback exhaust.
I bought the car 3 weeks ago. It is in really good shape and runs fairly well, however it has one problem. It is guzzling gas like there is no tomorrow. My math is being based on a 16.9 gallon tank (correct?) and with that i get around 200 miles. That comes to around 11mpg. I do not drive the car hard and have actually been babying it to see if i can crank up my mpg. I had a SES light, got the codes read and it returned EGR valve and bank 1 sensor 1 and bank 1 sensor 2 o2 sensors. This light has since gone away. I still think it is the o2 sensors being that the car sometimes idles rough, falls on its face in 2nd and third gear and is getting terrible gas mileage. I have pulled a plug and they look fine. I have ran injector cleaner and sea foam through the gas with no help. Even with the SES light not on could my o2 sensors being screwing me? Any advice would be appreciated.
11 Second Club
Joined: Feb 2003
Posts: 5,524
Likes: 1
From: The South
It's possible that it could be a maf problem, but based on your symptoms and the codes I am 99% certain it's the O2's. When mine went out it would behave poorly on and off, and the light would come on and then go away. Bad performance did not always coincide with the light either. It got worse with time. The bad performance/running rich would last longer and occur more frequently, until it finally got really really bad. I got 7.8 mpg the final tank before replacement. 
Your math is a little off on your MPG. Fill your tank 100%......reset trip odometer......drive car until you're ready to fill up. Fill it up 100%. Divide the miles driven as indicated by the trip odometer by the # of gallons you fill your tank with. That'll give you an actual MPG. I'll second the O2 sensors though....replace the fronts first, as they are what your computer uses to run on (the backs are for monitoring Catalyst efficiency).
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