I have a 1996 Jaguar XJ6 X300 4.0. I a have not had the car long. It was good on petrol before it reported between 19 to 23 mpg round town and on a run a lot better. Now it registers about 12-14 mpg on the computer. This is the second X300 4.0 I have had with the same problems. I get about 200 miles to a tank of petrol which cannot be right. I noticed above the alternator there was a device that was not on my 1995 XJ6 4.0 sport. A air hose was not connected I connected this back but not sure why it was not connected. Also wondered if anyone knows what this device is and if this could cause the fuel consumption problem ?. Also sometimes you get a slight petrol smell when driving but not most of the time. Any ideas on what I can check or if maybe the plugs need changing not sure what to check ?. Also it seems to run smooth and it accelerates really well even better than my old X300 4.0 Sport. Also the petrol needle seems to drop drastically after a short distance when completely filled with fuel. But my old X300 done the same. Any ideas what I can check or do to try to improve the fuel consumption would be greatly appreciated. I love the car but its to expensive to run when its doing 200 miles to a tank. If any one who has a X300 4.0 can please let me know what kind of mpg they get and how many miles to a tank of fuel roughly. I know someone who has a XJ8 3.2 and he said it shows 10 mpg on the computer. I thought a straight 6 should be a lot better.

Many Thanks
Mat

 

Mat:
I have to believe you must have a "check engine light" on, right? You really need to have the OBD codes read. Then we can offer solutions!

 

Hi there. On my 96 4litre I get an average of 28mpg. I don't tend to race it or push hard at acceleration. I drive in a relaxed sort of way. I don't tend to use the "Sport Mode" either, but will cruise at motorway speeds (120kph/ 75mph) when I can. I drive mixed, but with plenty of short journeys also.


A smell of petrol is something I would investigate, from both a safety and financial perspective. Have you considered the possibility of a leak somewhere?


Aside from that, there will be plenty of knowledgeable individuals on here who can fill you in on the complexities and operation of the fuel system. I think it is possible to "overfeed" the engine with petrol if the pressure regulator/s are malfunctioning, but others better versed will chime in.

 

Engine light is not on. Its just both of my X300 XJ6 4.0 have been really bad on fuel. I changed the spark plugs today. Even if I got 20 mpg I would be really happy. The old spark plugs where really black and dirty. I also noticed that all the spark plugs where loose in the plug holes so that must be what the petrol smell was. Also that would cause a compression problem if im correct ?. I will drive it over next few days and hopefully the fuel consumption is better.

 

Well, even if the light is not on, I would scan fro stored codes. And then check the STFT and LTFT

 

Thanks for advice!. What is the STFT and LTFT ?.

 

That is the "Long Term Fuel Trim" and the "Short Term Fuel Trim". It reflects what the engine is doing to correct the fuel mixture using the lambda sensors. You can read the trims with an OBD scanner which displays "Mode 1" data. You can also read the actual lambda sensor feedback.

In the mean time, a good cleaning of the MAF sensor would probably be a good idea.