Deteriorating MPG?

(Jason Miller 1993 XJS 4.0 convertible, 1997 XK8 coupe ) #1

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?

(Grahame Loader) #2

The best test of mileage is to measure it over several tanks of gas. That is what I use the trip B setting for. You might try cleaning the MAF sensor.

(Eric Capron) #3

Grahame is right. If you are reading your mpg from the computer this will not necessarily be accurate (although in my experience, usually optimistic). The only reliable way is to brim the tank, drive the car and then brim the tank again. Over a few tankfulls you will be a very good idea of your fuel consumption.

In any case, it depends on the style and type of your driving. On motorways and long drives out of town I get mid to high 20s but if I use the car mainly around town and on short journeys I struggle to see 15mpg. It’s a 4.0 litre after all.

Shropshire, UK

(Jason Miller 1993 XJS 4.0 convertible, 1997 XK8 coupe ) #4

Yes I understand about different driving styles and locations having an impact on mpg. My drive is pretty much the same most of the time. Usually 12mi starting out going through a small (1/2mile long) town at 40mph, then the rest of the drive has several small hills at 55mph. I am reading the average mpg from the computer and do agree that the best way is to fill the tank, run it down refill and check mileage, but I figured if the computer does an average if should be fairly close.