Fuel consumption

[sotheran57441]

Hi, I have a 2007 XJ 350 TDVi. At the beginning of 'lock-down' and anticipating little use I put the car on SORN. I started it up frequently and ran it until the engine reached operating temperature. I recently re- taxed it, insured it and renewed the MoT. I have been using it, intermittently, for two or three weeks and I have noticed, from the dash read-out, that the fuel consumption is very low; showing, at best around town, mid to high teens. This is a substantial drop from the normal mid-to high twenties that I normally achieve. I haven't done any motorway or similar driving yet. I changed the fuel filter cartridge last year well before lock down. Is there any specific reason for this and what can I do to remedy it? Thanks. David

[davidr]

Hi David - I would suggest you give it a good long run in the very near future. My suspicion is that the car is trying to regenerate the DPF but is unable to as it hasn't had long enough.
Starting and running it without going anywhere followed by short town runs will exacerbate the issue.
Athe DPF is regenerated by over-fuelling the car so the excess fuel burns in the DPF so clearing out the soot etc. If it keeps doing this unsuccessfully then the danger is that the excess fuel finds its way into the engine oil and dilutes it causing possibly terminal bearing issues.
Keep an eye on the oil level. If it is going up then that is fuel diluting your oil.
I hope that helps

[PaulGover]

Hi, I have a 2007 XJ 350 TDVi. ... the fuel consumption is very low; showing, at best around town, mid to high teens. This is a substantial drop from the normal mid-to high twenties that I normally achieve. ...

David,

Did you really mean "mid- to hi-twenties"? That's seriously low for the TDVi - I get nearly 40mpg, and never less than 35 mpg in normal use. (That's the average reading from the trip computer, not the instantaneous one - the instantaneous reading varies tremendously depending on circumstances - you can "see" climbing hills, accelerating and cruising; the mpg can drop to 10 or so on hard acceleration, and rise to 55 or so up to 90+ descending a hill.)

As davidr says, you need to give the Jaguar Diesel engines a regular long run; I think the handbook says something like 40 miles, keeping the speed above 50mph all the time, for the engine management system (EMS) to clear the particulate filter (DPF). If you have cruise control, a blast up the motorway for a couple of junctions at a constant speed is best. Basically the EMS has to generate heat in the DPF, and then pass enough gas to burn off the excess carbon over a period of time.

If you're already down to 20 mpg and below, there's serious blockage. Early experience with my 2008 TDVi suggests (but doesn't prove) that if the DPF is particularly bad, it's essential that the throttle never closes during the 40 mile trip, otherwise the EMS "cancels" that run, and tries again, harder, next time! Hence the suggestion of using cruise control.

[sotheran57441]

Thanks for your helpful comments. I will take the car out for a run tomorrow and see what happens. If I suspect that the oil is contaminated should I change it? It was changed (with filter) shortly before it was garaged. I realise that because of lock-down and limited movement the car has not been used as it should, normally if the car is not used for a long time do you suggest starting it regularly or not at all? Thanks David.

[davidr]

As far as changing the oil goes, I would check the level to see if it is higher than it should be. I'd also smell the end of the dipstick to see if you get a whiff of diesel. If either of these lead you to suspect fuel in the oil then change it. If you are at all unsure then change the oil. If the filter is relatively new then that can stay. You need to weigh up the cost of a few litres of oil versus peace of mind I guess.
My personal opinion would be that starting the engine of a stored car does more harm than good, especially on a diesel with a DPF. It's more important in my mind to keep the battery charged up. Others may have another view!

[naki]

Very good advice there.
Just to add my 2p’s worth. It may be questionable whether you are feeling any benefit from owning a TD, if you are not covering large distances on a regular basis anymore? It may be arguable that the TD is costing more to run than a petrol engine?
It may be a can of worms starting this conversation though?