Hi all, a newbie question, my first post. I bought my 1998 XJ84.0L when it was 9 months old, about 9000 on the clock, it had been a Jaguar-owned vehicle. Overall fuel consumption since I bought it has been over 23mpg, it now has 106k on the clock. Last summer the fuel consumption suddenly dropped to 19 - 20mpg. No other symptoms: it starts and runs as well as ever and has always been regularly serviced. There are no faults showing on diagnostics.

I have changed the plugs, replaced the air filter and cleaned the MAF sensor. The MAF sensor is the old type that is integral with its housing, so although I think I have fully cleaned it, I can't be certain. I have read about cleaning the full load and part load breathers, throttle body and air intake hose, they're the next jobs.

Any other suggestions will be gratefully received. Thanks in advance.

 

just went through this...its possible that you have an o2 sensor going out. My o2 sensor went out on the driver side, upstream, and after doing all that you just said and changing it, everything was back to normal. But if you aren't getting a rough idle or a smell from exhaust then I'd say that 2 mpg isn't worth changing it. After all the cleaning, did you reboot the computer? Disconnect your battery for an hour or 2 and reconnect and everything will readjust.

 

Yup,....if there is no codes (like running rich), I wouldn't worry about it. These "fuel consumption computers" are not very accurate anyway. The best thing is to fill the tank, set the trip, and next time you fill it, use the simple calculator. Reseting the system will also help.

 

With the price of fuel in the UK at the moment, even a 3mpg increase in fuel consumption can hurt

The upstream O2 sensors spring to mind here-whenever I've had a catalyst-equipped car that suddenly uses around 10-20% more fuel with no other obvious symptoms, it's always been the O2 sensors.

The only way to be certain is to have a scanner connected to the diagnostic socket which can read the long-term fuel trims to see if they're running excessively rich.

Or you can just wait for the MOT test & see if it fails the emissions-which it will do if the O2 sensors have failed as the ECU defaults to a 'base' fuel map that is overly rich.

If your O2 sensors have failed & the engine is running too rich, it won't be good for the catalysts which may overheat with a constantly rich mixture.

 

believe me you will know when the o2 sensors fail because the car will hardly run, transmission will act funny and your car will be sucking gas down like a cold beer lol

 

Thanks for the replies. I did a hard reboot by disconnecting the battery and leaving it for an hour or so. The car is in for its MOT in a week or two, so I'll take Red October's advice and see what the emissions tests come up with. At the moment, there are no faults showing on the diagnostics, but if it is the O2 sensors, surely it would flag a fault of some sort?

 

When the O2 sensor failed on my XJR the car still started & ran fine, the transmission worked in the same manner & the only symptoms were the EML light being on, the fuel consumption being about 15% higher & the 'edge' had gone off the performance.

The O2 sensors were changed to a different design around the 2000 model year & work in a completely different manner to the earlier types. When they do fail, the ECU defaults to a 'basic' fuelling map which allows the car to run, but not at it's most efficient.

 

I would expect a fault code to be flagged up-especially if it's a straight heater element failure on an O2 sensor. They can fail for other reasons-though, such as slow switching speed on the earlier types-as on your 1998 model year car.

The biggest indication will come from the MOT emissions results, because if the fuel consumption has gone up for no good reason, then that normally produces a rich mixture which will fail the MOT emissions test.

Since you've only got a few days until the MOT test now, the easiest option is to just see what happens when they do the emissions test. A rich mixture will produce a high CO level, typically above 1-2% CO. The MOT limit is 0.3%.

It will also produce a higher HC level of around a hundred ppm, although may not actually fail the 200pm MOT limit. Also, the 'Lamda' value on the test printout will show a value of less than 1 & typically around 0.95 or less with a rich mixture.

The UK MOT limits for Lambda (excess air) value are betwen 0.970 to 1.030. Values of less than 0.970 indicate a rich mixture, which usually signifies O2 sensor or catalyst failure. Values of greater than 1.030 indicate a weak mixture which can indicate intake air leaks.

These are the UK MOT emissions results for one of my cars which had a failed O2 sensor & failed the emission tests-I've given the results for before the test, and then when the new O2 sensor was fitted & the car then passed the emissions test:

Before------------------After

CO 1.78% (limit: 0.30%) 0.00%

HC 121ppm (limit: 200ppm) 11ppm

Lambda 0.946 (limits: 0.970-1.030) 1.005
Value

 

It starts, idles smoothly and runs fine, no lights on the dash during driving and no faults coming up when it is plugged into the diagnostics at the garage, the only issue is the sudden increase in fuel consumption measured from fill-up to fill-up at the pump.

 

I'd just wait & see what the MOT emissions test shows-if that passes fine & there are no other obvious faults anywhere, then there's not much else to suggest really.

You can use a sophisticated scanner plugged into the diagnostic socket to read the Long Term Fuel Trims & see if that's normal or running very rich.

Did the fuel consumption suddenly increase literally overnight, or did it take a few weeks? A sudden fuel consumption increase is usually due to an electrical fault.

Does the gearbox still work correctly in all ranges & shift into 5th gear at cruising speeds? Are the tyre pressures all ok? Did you suddenly switch petrol brands?

As part of a European directive to cut the use of fossil fuels, Ethanol alcohol is being introduced in small quantities in petrol now-typically 5% but increasing to 10% in some cases. As Ethanol alcohol has a lower calorific energy value than petrol & will give a weaker mixture, the ECU will readjust the fuelling to compensate for this-giving a higher fuel consumption.

This is not widely broadcast by the authorities & amounts to 'deception'-the amount of Ethanol alcohol in petrol cannot be raised too high or the weakening of the mixture will be too excessive for the ECU to compensate for-it will go 'out of range'.

However, even just a 10% level of Ethanol in petrol-which is available in parts of Europe, particularly France-will reduce the energy content of a litre of 'petrol' by around 4% & push up fuel consumption by an equivalent amount, as Ethanol only has around 60% of the energy content of petrol.

 

Did you do a hard reset after cleaning the MAF?

Do this by unbolting the neg lead off the battery and holding it to the pos post for 10 seconds then tighen back up, np sparks, no smells, just a C-MOS reset like on any computer. You'll need your radio code afterwards.

This puts the car back to factory settings, it'll recalibrate the throttle on it's first startup, don't be alarmed as rev's climb then fall to almost stall, this is the ecu recalibrating the throttlebody (helps if you've cleaned this too before the reset. Let the car idle for 10 minutes to store the new air/fuel ratios and go for a drive.
Also stay away from the sport switch for a a hundred plus miles while the car learns the driving style.

An unmetered air leak can cause higher fuel consumption - any recent work on the induction that coincides with the rise?

 

Thanks for all the helpful replies. Sean B, I will try earthing the positive lead as you suggest, all I have done so far is disconnect both leads for an hour or so. Nothing had been done on the induction side at the time of the sudden increase, literally from one fill to another. I cleaned out the throttle body, part load and full load breathers yesterday, they were pretty grubby. No signs of any splits in any of the pipes. To answer Red October, the gearbox is fine, I use either BP or Esso petrol and the tyres are all at the recommended 32/34 pressures.

 

I can't think of anything else it could be, so the only suggestion is to see if it passes the MOT emissions test. If that fails, then I'm pretty certain it's the upstream O2 sensors-although as you say I would have expected to see a fault code.

My 2001 XJR has the different upstream O2 sensor design & when they failed the car continued to drive normally-it started & idled fine. The only symptoms were that the edge had gone off the performance, the fuel consumption had gone up by around 15% & the Check Engine light was on with the associated fault codes.

If yours passes the MOT emissions test then the fuelling must be ok & everything else you've checked is ok. It would then be worth using a more sophisticated scanner on the OBDII port to read the Long Term Fuel Trims, as these would show if the overall fuelling had gone up by double percentage amounts...

Other than that, I'm out of ideas as everything you've mentioned seems to be ok-unless the quality of wholesale fuel has changed now with increasing Ethanol content, as this pushes up fuel consumption by several percent due to the lower calorific value per litre of fuel...

 

How old are the spark plugs? Your MAF can be on the way out, but should post codes. The other suggestion of O2 sensors is valid too. There's a number of sensors that can alter the fuel air ratio. The 2 biggies are MAF and O2.

 

I changed the spark plugs and air filter as the first thing when the problem first occured last summer, so they should be good as they have only done about 5k miles.

 

Thanks for all the helpful advice, after it failed the MOT it turned out to be the lambda sensors needed to replaced.

These were the results:

CO Before 0.65% (limit 0.2%) after 0.02%
HC Before 79ppm (limit 200ppm) after 7ppm
Lambda 0.99 (limit 0.97 - 1.03) after 1.01

As an added bonus, on the way back from the garage, about 12 miles, the fuel consumption, admittedly on the trip computer, showed 24.5mpg.

Result.

 

Thats good MPG for 50/60MPH roads, glad you got it sorted