I have a strange situation related to gas mile on my 2003 Z71 (5.3). I normally get 15.5 in town and 16.5 to 17 mpg on the highway. I have about 97,000 miles on it now. A month or so ago, I cleaned my throttle body because it had never been cleaned and was running rough. It made a huge improvement in how clean the truck idled, and it seems to have more power now. I also changed my transmission fluid and the filter. I used the AcDelco OEM filter and Royal Purple Synthetic ATF. I had already been running Royal Purple in the tranny since about 50,000. Now my gas mileage is about 13.5 or 14 in town and 15 or 15.5 on the highway. Any ideas as to which of these changes (if either) would have caused the drop???

Thanks.

 

The throttle body cleaning will affect it but in a good way, most of the time. Sometimes these things happen and create a chain reaction and a few other things might go bad 2. Your coming up on 100k miles, I would also change the 2 upstream O2 sensors. They need to be replaced every 100k miles. Also the spark plugs if you have not already done that. I stayed with iridium but went with NGK and got much more power and efficiency out of those plugs. With all my mods and tuning on my 01 5.3, I get bout 19 in town and 23 on the highway.

 

Agreed, but it is also that you have higher mileage on the engine. The 5.3 is not like the 5.7 and get 100's of 1000's of miles on it with little to no fuel mileage change. This also COULD be a sign of a impending Tranny problem, not saying it is but its possible. Higher road mileage= Lower fuel mileage unless lots of maintenance is done. 100,000 is a good time to do some "deep" maintenance, fuel injectors, ect.

 

I was wondering about it being a tranny issue, but it just seems strange to me that the mpg dropped so suddenly. I keep everything up to date... This is the 2nd Royal Purple trans. fluid change, did spark plugs at around 50K, always run Castrol Synthetic oil, etc. Not sure where to go with this...

 

A dirty MAF sensor will cause the tranny to shift wierd. So will a faulty VSS(vehicle speed sensor). Everything on this truck is electronically controlled by the computer.