Poor fuel consumption - Petrol V6 X-Type

[charlier6790]

My wife's recently-purchased 3-litre SE Estate struggles to manage more than 26mpg or 27mpg on open rural roads at 50mph to 60mph. We'd expected nearer 35mpg. Even my 4-litre XJ8 can exceed 27mpg in these conditions! Tyre pressures are fine and I've had wheel alignment checked. There's no obvious sign of sticking brakes and the car's engine performance seems fine with no hesitation, misfiring, etc. The car has just 54,000 miles on the clock. Can anyone suggest what the problem might be, please?

[J44EAG]

I wonder just how well this car might have been serviced in the past?

Several things come to mind from my experience with this engine in an S-type.

Firstly, the air filter may be choked with muck. This would make a significant difference to fuel consumption. You might be quite horrified with what you find! I`ve seen one filter taken from a car, main dealer serviced three times in 30k miles. It looked like the inside contents of a vacuum cleaner. Clearly it had not been changed since it was installed at the factory. Also replace the fuel filter. A partially blocked filter will make you work harder on the throttle pedal and your fuel consumption will increase. This filter is often overlooked at service time.

Secondly, the throttle plate may be sluggish in operation within the throttle body. This is often down to crankcase breather residue building up around the spindle bearings, throat and indeed the plate itself. Clean out this sticky, oily mess with carburettor cleaner, soft lint free rag and a very soft tooth brush. Do not scrub the throat coating off the throttle body. This material needs carefully treatment. Check also that the part breather hoses are clear of similar residue.

Thirdly, remove the Mass Air Flow sensor from the induction trunking just upstream from the airflow meter. The hot wire should not be touched with an item such as a screw driver blade. Again use spray carb cleaner only to remove any sludge. Check also the airflow meter flap between the MAF and the air box is clean and moves freely.

Fourthly, your car may still have the original spark plugs fitted. I believe these are lifed for 60k miles. In my experience, spark plugs become tired after around 50k miles and it is worth changing plugs which do influence economy when they become worn.

35mpg is I think a little optimistic for rural road use where a fair number of gear changes may occur. You might just achieve that on the motorway. My 3 litre S-type could do 31mpg on the motorway but 25mpg or less on rural roads. Granted, the car is bigger and heavier. My V8 S by comparison would perhaps achieve 24mpg at best under similar rural conditions.

The other thing to remember is that on board fuel and mileage counters are seldom accurate. When compared to a stand alone sat nav, I`ve always found that the car over reads distance by 5% and economy by 10%. That makes the figures even worse!

Try those ideas to begin with. Then you are working from a known and logical starting point for any other investigation that may be needed.

Mike

[PaulRidley]

This is interesting.

The most fantastic thing about my Police X-Type is the service history. It's impeccable. No expense spared servicing and maintenance.

Now whilst I accept that with "all the kit" and "4wd" you can't expect miracles, I will say, without checking, that economy ( V6? 4WD? Economy ? ) is pretty fair - so I agree with Mike. Servicing, real "proper" servicing, may do you right....

Paul

[J44EAG]

I forgot to ask Charlie, is your car fitted with AWD?

If it does have that then again mpg will be affected slightly. More power train friction and drag equals a loss of fuel economy.

I`ll also add that running on fully synthetic oil improves my indicated mpg by just over one mpg to the good.

Mike

[gwj]

Mike, all 3ltr and 2.5ltr were AWD. The 2.1 v6 and the diesels were FWD.

Also to check would be binding brakes, its not clear what vintage the O/P car is but the early ones (pre 2004) could also suffer from binding rear calipers.

G

[charlier6790]

Thanks to Mike, Paul and GWJ for such helpful comments about V6 poor fuel consumption.

I've just changed the engine oil (I've used a 5/30 full synthetic) and replaced the air filter.

I'll now work through all the remaining suggestions and see how I get on.

Incidentally, the car has a full service history - most of it by Jaguar. Yes - it is an AWD model.

Thanks again for all your help.