Hi All!

I'm sure this was asked a million times, but couldn't find it searching. What should be the MPG on a 2000 XJ8L? I got it about a month ago, and getting 16 MPG -- on the highway.

It drives well, but starting feels a bit soft: if it were manual, I'd say the clutch stays on too long... I never drove a Jag so not sure if this is how it should be.

What tunings/check would you recommend to make fuel efficiency better?

When starting it, it smokes a lot for a split second. I read something in the forum about this - but can't find what it was...

Thank you for the help!

Miki

 

If it helps, I've had my XJ8L 1998 since it was 9 months old; it now has 118k UK miles on it. It averages 23.2 miles per UK gallon over 110,000 miles, which should be about 19.3 per US gallon.

 

to the OP, that's a little low from what it seems. What speed are you running on the hwy, and how are the other bits? oil changed on time, not like 50k overdue? how about spark plugs, oil filter, air pressure in the tires?

My '03 XJR, I average about 18ish on mixed driving to work, and that's with quite a few hills. On the highway however, I can average 22,23+ if I keep the speed around 65. I've noticed that it drops by a good 1-2 mpg if I up it to 72-75ish though. Still, that's still 19 mpg at 75. Makes me wonder if you're getting some blowby with the smoking at start up, low compression, which could certainly do it. Loss of power, heavy car, gonna drop the mileage. Would be worth having someone test for losses to see if there's something potentially major looming ahead.

 

Good questions from CharlzO. How long did you drive it to get the 16MPG for highway? What speed, Weather, etc? What tire pressure?

I get 24 to 26 MPG (US gallons) in a 200 mile run at 70 - 80 miles per hour. 16 around town. But at 73 I ain't hot rodding it.

Clean the MAF sensor, check the air filter, clean the throttle bore and plate, get new plugs, and get an OBDII reader that can give you Long Term Fuel Trims. At some point, change the fuel filter. Run the tires at 32 front, 34 rear. Might get better.

What is the color of the smoke? At start up or decelerating or accelerating (rings, valve seals, water vapor)? Change the oil -- 5w30 gives best gas mileage.

 

Going about 60 on the hwy. Oils just changed and air pressure checked before I took off.

Spark plugs etc - good call. Will change those. Any specific type you recommend? I'll check the oil filter. These are all serviceable by me, right? I never had a Jag before but I'm pretty handy...

 

I did about 200 miles at 60-65, all highway except the beginning.

The smoke is blueish - and only when I start the car. After that there's some white smoke that goes away when the engine heats up.

Thanks for the tips! I'll do these maintenance tasks.

Any idea about how to check the transmission - clutch?

I have an OBDII reader that's a plain vanilla ELM 327. I'm reading it with a phone app. What do you use?

I got high grade 5w30 before I went on the highway...

Thanks!

 

If the elm327 works then it's good enough for many things. Check sensors are plausible and then watch fuel trims in case they're a bit out.

 

I had poor fuel consumption on my XJR for ages after I first got it. Only realised there was something wrong when it failed emissions testing. The fuel trims showed it to be a bad O2 sensor - no warning or other indication of a problem and a standard multimeter confirmed which one had failed, replaced it and improved fuel consumption by almost 33% so worth looking at.

 

Hi
Can any one tell me the normal value in Torque pro for Fuel trim Long term?

 

Near zero. What are they?

 

Hi
Fuel trim bank 1 Long term -1.56%
Fuel trim bank 1 sensor 2 -0.78%
Fuel trim bank 1 short term 0%
Fuel trim bank 2 Long term -4.69%
Fuel trim bank 2 sensor 2 -0.78%
Fuel trim bank 2 short term -1.56%

 

It looks like your O2 sensors are probably fine... Have you given the MAF a good cleaning? What your trims show is that the ECU is having to correct a small amount for too much fuel. That means you MIGHT have a leaky injector, slightly off fuel regulator (unlikely), bad temperature sensor, or imperfect MAF readings.

But again, your numbers are not too bad all in all. The same readings under load would be helpful. That's why a scanner that trends is valuable. You need the numbers when you are driving at highway speed.

BTW, I woulds say your mileage numbers are not too bad. Some folks claim much better, but lots of us get just less that 20 mpg. And... what is "highway speed" to you? I can get 24+ mpg on my wife's XJ8 with cruise control at 60 mph on flat ground, but the way I normally drive I get closer to your numbers. The mileage number on the dash has many things that can affect accuracy both directions. I know a guy who claimed extremely good mileage, then had a rude awakening when he checked pump fuel input vs. Mileage. He even tried to tell me that all the gas pumps MUST be inaccurate.

 

I use Fuelly.com they you can browse by car and year and see what others get for fuel mileage. Mine is on there 1999 XJR UK Cat

 

Hi
In the city (12.57-12.37 mpg) 18.7l-19l/km,
highway (24.28-20.63 mpg) 9.7l to 11.4l at 120km/h.
If outside the temerature is around 15Celsius
on the highway i have (24.28-22.61 mpg) 9.7l to 10.4l cruise speed 130km/h

 

Thanks all for your replies! I cleaned the MAF and saw that it looked a little burnt so I changed it. Also one coil was bad. Fuel consumption is much better now - about 16 MPG in the city.

 

Mine felt a bit soft in the pedal and slow to shift. I tightened up the throttle cable under the hood and its much better.

 

I avg about 17 to 18 in the city and as high as 23.4 on the freeway at 60.