Hi all,

I’ve just bought a 2015 3.0 XJ with 51000 on the clock.

On a 70mile drive back at 70mph (cruise control), I averaged 40mpg, which is a concern.

Is there a guide of things I can clean to improve this figure? I don’t mind doing something that’s easy and has a low risk of breaking.

I own the 3.0 DIESEL engine.

So far, my plan is to:
1. Use the STP 5 in 1 cleaner before I head to the motorway.
2. Assess the throttle body to see if it needs a clean

 

asjd, I would first start wtih simply changing brands of gasoline/petrol. Every manufacturer has "their blend" and no blend gets 100%. So, you switch from brand A to brand B, you wlll get some different detergents and that should do what you are after. I am not necessarily an additive guy because if you start looking, if you get say Chevron gas and you buy the Techron addititive, you are going to get 0% return because your fuel already has the Techron in it (Techron is made by Chevron). So, you need to do a little bit of research into which companies are owned by others. You find 2 different companies, you should be good.

If you clean the throttlebody, use your finger to open it up and then spray the hell out of it. If you go to start the car and find it flooded (which is very possible doing this), start wtih the car off and depress the gas pedal fully (put your foot to the front bumper). With your foot still on the gas pedal, use your other foot to depress the brake and then press the START button. Let the car crank for a few seconds and then ease off of the gas some. As the motor catches, feather the gas as needed to maintain the engine RPM. The engine WILL NOT overspeed because built into the car is a "Flooded Feature". By depressing the gas pedal all the way and then attempting to start the car, that is a clue to the ECU that you as the operator are trying to clear a problem and when the gas pedal is depressed fully on start, it turns OFF the fuel injectors. Then as you ease off the gas pedal, it will start admitting fuel again. So, spray away and get rid of any brown that you see. I would also say to get a rag and put a little on that so you can wipe the edge of the silver disk in the center. That can cause a lot of heartaches having the sticky varnish on the edge of the throttle plate.

 

What?

What am I missing here?

 

And the fuel additives only clean the system through the injectors, and the combustion chamber - it never touches the intake manifold or intake valves
Be careful choosing what you use to clean the throttle body, some cleaners can damage the factory coatings that prevent corrosion & ruin your throttle body.

But I still don't get the 40 MPG problem. I don't think that's possible unless you're coasting all the way.

 

thanks for the advice.
I own a 3.0 Diesel engine where I should be achieving high 40/50s.

 

take it back get your money back , 40mpg and your moaning try 20 in a 5 ltr petrol , buy a corsa

 

I see that constructive feedback is not your forte

 

I wouldn't have ever imagined, even in a diesel.

 

Adding thinner rear wheels will have an effect but will effect handling too. Best what i have reached in 300km journey were 5,2l/100km, temp -5C (lot of oxygen, good intercool), no AirCon, with narrower spiked wintertyres. (245/45-18 winter tyres all around. These have less contact area, so less friction in pavement, but better penetration on snow/ice)
Roads were plain ice by supercooled rain where rain drops frose immedidly when they hit on any surface (https://en.wikipedia.org/wiki/Supercooling), that i could not drive faster than 70-80km/h in highway. Full heat to windshield and only mid low part stayed open. The onboard computer showed 5.0l/100km after 300km, but it lies a bit like every other system do. (not as much as VAG:s used to do)

 

A single test is usually not indicative of average fuel economy. Maybe it was very warm outside ... or you were traveling slightly uphill or upwind ... or one of your tires is a couple psi low ... or better yet, you put in a few more ounces of fuel at your destination when you did your calculations.
The official Jaguar spec for your car is 6 litres/100km, or 39 mpg of mixed driving. Yes, you should do slightly better on the highway (at 100 kph, not 112) under ideal conditions.
When you start talking about cars that get 40+ mpg, you need very accurate measuring techniques to get an accurate value. At these efficiencies, 3 oz. of fuel equals 1 whole mpg. You can't expect every fuel nozzle to shut off at the exact same point.
I suggest you zero one of your trip meters, then run several tankfuls of fuel over the coming weeks and months. Then compare your driving style, conditions and terrain to the hypothetical "mixed" condition Jaguar used to calculate mpg. If you're more than 10% off, maybe begin diving deeper. But not before then.
YMMV (sorry, couldn't resist!)