Not too long ago my 01 XJR was throwing the classic "the banks are running too lean" OBD codes. I bought some MAF cleaner and gave it a good once-over, reset the codes and have not had any trouble with the check engine light since.

However, I have noticed my miles per gallon is noticeably down since the cleaning. Whereas before I was easily hitting around 20mpg, now I am lucky to break 17!

I haven't changed anything. Same fuel, same driving habits and patterns, same everything. It seems maybe running too lean was plus! But seriously, what could be going on now?

 

It's exactly as you speculate.

Same speed means same number of cylinder fills, but with
a mixture that has more fuel in it.

Urban mileage is not really comparable outside of a given
urban area because of varying conditions. The only good reference
point is highway mileage at a given cruise speed.

 

Well yes, I made that assumption.. but three miles per gallon difference? That seems to be quite an adjustment.

 

A Prius has better mileage, a Ferrari worse.

 

Check your long and sort term fuel trims!


Check the pipes are properly seated and tight.


You may have cracked one of the pipes, the plastic gets brittle with age.

 

Sensors reporting lean means it adds fuel. But things such as small misfires report as lean and adding fuel is wrong but it doesn't know that. It's trying to protect the engine from being written off - that's why it adds fuel. So, look at fuel trims at idle then rev to 2500 and see what happens.

 

its always good practice to perform a hard reset when the MAF is cleaned, this puts the engine map and throttle calibration back to base.

 

I think my MPG is dropping so bad now, a full tank only runs just a little over 200 miles.

no check engine light nor any warnings, I don't know what to begin with other than cleaning up the MAF

 

How is a hard reset done? Didn't I do that with the scanner?

 

no you just checked and removed some codes.

A hard reset, some reading for you, it should fix the gas mileage.

https://www.jaguarforums.com/forum/x...t-38112/page1/

hope that helps, if it doesn't do as Andrew states and check fuel trims, maybe an intermittent O2 sensor...