Deteriorating MPG?
 

Ok so I figured Ill post and see. When I first got the 1997 Xk8 I was averaging about 20.5 mpg. I decided to clean out the throttle body and knocked out my air filter (only a couple months old) and cleaned out the little nipple on the left cam cover that takes the place of the pcv valve. I then struggled to get 19mpg! Argh! I did some looking and decided to try resetting the computer by disconnecting the neg cable and holding it to the positive terminal for a little bit. Now Im struggling to get 18mpg! ARGH! Anyone have any thoughts?

You need to get a code reader that can read STFT (short term fuel trims) and LTFT (log term fuel trims) and post the numbers. The readings need to be at idle and at 2500 rpm's. It is possible that you have disturbed something that is crating a vacuum leak that is effecting fueling but not severe enough to set a code.

I havent looked, does it have an obd2 connector or a different type. I have a bluetooth obd2 that works well.

The first four are at idle (660ish rpm) the other 4 are at 2500rpm

Have you checked tire pressure lately? Slow leak maybe, or temp change. Another thing would be a fuel system service. Start with a new fuel filter if a bit old or of unknown age. Then use a good fuel additive (folks seem to vouch for Techron). Check for dragging brakes. OTOH, a difference of 1 MPG seems within the margin of error...

Cleaning the TB and air filter should have either done nothing or improved things. A negative effect suggests the possibility of introducing a leak on the MAF to TB air pipe.

How are you obtaining MPG - calculating by actual miles driven and fuel volume used fill to fill or relying on the trip computer?

The "hard reset" does, among other things, return fuel trims to default values and flatten trip memory. It will take several miles of mixed roads driving for the adaptations to set again. It's not surprising that fuel consumption will initially be higher.

Graham

I did buy a new fuel filter, but have yet to fit it (I keep messing with my xjs). I am going by the "average mpg" display so I know its not 100% correct. I did check my tire pressure the other day, and a couple were low, but I haven't rechecked since then. I will say that Im not super concerned right now, but part of me does wonder about an air leak because it seems to me to be, as far as i can tell since this is the only one Ive ever ridden in, running a little lumpy. Which Im sure a new fuel filter may help with too

The 20.5 your computer read was the average since the previous time you had reset the computer, so could be a truer average over various conditions. The 18 it shows now is presumably for fewer miles, so a few high-consumption accelerations could have a noticeable impact on the average. It is possible that the 20.5 included more high-gas-mileage driving, such as more highway, and the recent 18 includes more short-distance driving. Also, gas stations typically sell a higher-ethanol blend starting in September, which I have read could reduce your mileage by up to 5%, or 1 MPG. Bottom line, it may be "innocent", and not due to any problem with the car.

Personally I would disconnect the earth lead of the battery and leave overnight. Basically letting the cars computer re-learn your driving and setting the correct MPG.

My other thought was, as above, an air leak around one of the vacuum pipes. These pipes are VERY brittle. Get a can of carb cleaner and when the engine is running spray a little around the pipes. If there is a leak the vacuum will suck in the cleaner and you will hear a change in tone from the engine.

If you still have the original MAF I would change that too. No need for the expensive Jaguar one there are identical OEM models on the market.

Hope this helps. Let us know how you get on won't you?