I have a 1996 P Reg XJ X300 4.0. Its really bad on fuel and not sure what to do about it. I had a 1995 N Reg XJ sport 4.0 before and that was the same. I get between 150 - 200 miles to a full tank. The onboard computer says its doing 12.2 mpg on average. I had cleaned the EGR valve using a cleaner. It seems to warm up really quickly even after starting the heating is warm nearly straight away. Not sure if this is linked or not. It seems to run smoothly when driving and does not hesitate when you put your foot down. I wanted some advice on what to check and do. I was wondering if getting it put on a computer will find the faults. I was thinking maybe airflow meter which I had a problem with on a old car years ago. I do love Jags and would never change to another car but the petrol consumption is a really big problem.

Any advice would be greatly appreciated!.
Mat

 

Mat

Getting the car on a computer is the first thing you should do. But don't go to a garage, buy one. If you are serious about DIYing this car (or any other for that matter) then some kind of Diagnostic capability is as important as a 13mm spanner.

There's lot's of info on what you can buy on the forum from cheap as chips bluetooth connectivity for your phone up to expensive All Singing All Dancing dealer level kit. Get the best you can afford - it's an investment.

As a first step I would get any stored codes and get Fuel Trim numbers (STFT and LTFT), and I just saw from your previous post that Sparkenzap already suggested that. You can do some research on what these mean and what effect they have on the car. Loads of info on the net.

Also, to the best of my knowledge no UK X300s were fitted with an EGR valve so not sure what it was you have cleaned. Or if you do have an EGR I'd be interested to see that.

 

Are you sure the onboard computer is correct? Have you checked by computing when you fill up by dividing mile driver by gallons taken on board? With that consumption you should be seeing black sooty exhaust pipes and maybe affecting your O2 sensors. Mine is always between 22 and 25 mpg. But I don't drive hard. I love the experience and extend it as much as I can.

 

It done roughly 175 miles to a full tank. I know this because I completely filled it up before and it was nearly completely empty when I filled it up again. I put in 67.07 Litres of fuel. Its weird its the second x300 4.0 I have had that has been the same. I did suspect faulty air flow meter which on a old car affected the fuel consumption really badly. Sometimes you can smell petrol outside the car and inside but not all the time. I was wondering if it was injecting to much petrol into the engine that's where the smell was coming from sometimes. There are no fuel leaks anyway either have checked this. I was also wondering if it was possible If a coil pack was not working properly even though it runs smoothly and idles nicely. I love Jags If I could get even 20 mpg I would be very happy. I have never driven a car that drives so nice and so smooth. It drives better than my BMW 750il used to drive. I always drive it sensibly as you do not need the power it has to offer.

 

It seems particularly odd that your previous X300 returned similar consumption, which is as you would have gathered much worse than what people would consider the bottom of the normal 4 litre consumption range. I take it you never got to the bottom of the problem on your old car?
Did you check for leaks with the engine running? Easier said than done I know, but I am wondering about fuel leaking under pressure, perhaps from a pinhole in the supply or return pipes. These are prone to corrosion, and could allow fuel to be pumped off of the car leaving very little trace, especially if it was the return pipe.
For the car to be burning so much extra fuel, I would not expect it to be running as well as you suggest.

 

On my old car it was doing bad on fuel so had it put on computer and one coil pack was faulty. Which I changed it increased slightly but it was never good on fuel. It seems to be ever so slightly missing when idling but hardly noticeable. When you put your foot down it does not hesitate at all. My new car also seems a lot more responsive than the old one. I have checked around the engine bay and underneath while it was on a ramp for fuel leaks but could not see any leaks at all. Where do you think I could notice leaks in the return hose does this hose go underneath the car then up into the top of the petrol tank in the boot ?.

 

Im totally confused with my car!. I filled the fuel tank twice and It was doing nearly 20 mpg for a while. But then I let it run low on fuel and put £10 in. Then I filled the tank completely. It started to smell of petrol inside and outside again and its doing bad on fuel again. Im totally confused in what is going on with this car. Any help or suggestions would be greatly appreciated.

 

Do you have a "Check Engine Light" or other dash warning during any of this? I assume you do not have an OBD scanner. For keeping any older post MY 96 car running, a scanner is an essential tool and a pretty good arrangement can be put together with an Android Tablet ($79.00 or use your smart phone), a Chinese ELM module ($15.00) and the application "Torque" ($4.95). With that, you read and reset fault codes but most importantly for your situation, you can read and graph Fuel Trims.

The X-300 cars are notoriously poor for giving good OBD diagnostics codes, but even as poor as they are, you are much better trying to diagnose with something rather than nothing, and it is difficult to diagnose fuel mapping problems like yours any other way.

 

My 4.0 liter returns an average of 28 mpg, but that's European/UK gallons. However, if I were smelling petrol I would be looking for leaks, for both economical and safety reasons. Your car isn't in "Sport Mode" by any chance? Have you tried using "Sport" to see if it's actually operating, or possibly "stuck" in Sport?


Just a thought.

 

Isn't there some filter that lets air into the tank but not out? I can't recall the name but that might be faulty and explain the smell but not poor consumption.

I agree if I could smell petrol I'd be having a good look and sniff round. First stop would be the rear wheel arch where Jaguar so sensibly placed the steel fuel lines and the filter both of which I believe can corrode.

 

Sports mode does not seem to work. But not 100% sure. On other cars the revs used to go up when putting sports mode on. On this car it does not. When you turn it on. But also the light comes on the dashboard when you turn it on. The car does seem quite fast compared to my old xj4.0 sport.

 

According to the computer on my XJR, my average city/highway MPG (US) is a steady 18MPG, with 21-24 on the highway instantaneous.

 

You ask for help, but you seem to ignore the suggestions to get the fuel trims tested. Any other procedure is hit and miss.

 

The "sport" mode changes the shift points, it delays them.

 

a) I smell petrol inside and outside
b) fuel consumption is down.

I vote for a fuel leak somewhere - that would account for (a) and (b)