Put another $100 in the Jag today, didn't even fill the tank (almost did) ...wondering what we are getting on average out of our XJ40's? (4 litre motor)

Mine is in litres/100 km and the readout averages between 16 and 18 litres around town - I'm sure that's pretty bad - with a feather touch on the gas pedal I can get it down to about 15.5, which I still think is pretty awful having read that some folks get way better mileage.

Just to compare,

15.5 litres/100k = approx 15MPG (US gallons)
15.5 litres/100k = approx 18MPG (UK gallons)

What are you guys getting?

 

here's a calculator for that:

fuel economy: gas mileage, fuel efficiency & fuel mileage calculator

 

I get around 14-15 US miles/gallon in town on the '94 and about 24 mpg highway. The '91 is a little bit better: 16-ish in town and 29-30 highway. Not exactly hybrids, these cars; but not bad either for their weight!

Scott

 

Well Scott, that's pretty respectable - I'd be pretty happy if I got 24 on the highway ...plum's calculator converts that to 9.80 liters/100 km - on my '94, the readout on the dash on the highway has never got close to that low number - 16.1 is the lowest figure I've seen - 14.61 miles per gallon

I knew it was running rich when I got her back in Sept 2012 (check engine light + a DTC 23 fault, replaced stuck thermostat and put in a new temp sensor; apparently fixed) but got the 23 code again this week. Engine management pdf gives a few possibles, fuel pressure excessive, grounds, relay, o2 sensor etc., and the purge valve.

I'm thinking if I'm going to do my own maintenance I should know something about how the evap system works and how it can affect the running of the engine ...so some more reading up to do - From what I've read so far, you need a vacuum pump and gauge to check it out so I bought a vacuum pump today and once I figure out to use it I'll be checking out that evap system and the darn purge valve.

Who knows? Maybe I can get your 24 mpg hwy one day!!

 

compared to my last 2.8, my 4.0 is fantastic.
around town driving (U.K) i get around 20-23mpg, dependant on how i drive, but on motorway/highway driving, i get around 30-35mpg. i even took a photo once to prove it

 

Wow! I guess you don't have an emission control system on that rig?

 

i would imagine there are many factors though, that dictate the figures. regular servicing, tyre pressures, boot/trunks, full of tools and junk or not, driving style etc etc.
i think the cars with poor mpg are in the minority on these cars from what i`ve read over the years. i`ve actually been surprised by just how many obtain 30+ mpg on highway driving. i have to say though, mine is maintained, engine wise, to an almost `anorak` standard!

i even managed to find the photo i took;

 

Is your sovereign an automatic transmission?

 

sorry for the delay.
automatic.

 

In addition to tyre pressures, wheel alignment can be a factor. In particular, excessive toe at either front or rear will drag down fuel economy.

 

Same here on the highway 26-32 less in town 20-24. I notice also the brand of gas makes a difference and whether I buy premium or go cheap and get the midpriced. I'm not real happy about the ethanol blends but in my area almost all the brands have added ethanol so its hard to avoid it. Cary

 

Yeah well, that's pretty great mileage - over the last 3 days of town driving, lots of traffic lights and barely getting into 4th gear, my readout has been hovering around 18.8litres/100km - in US gallons that's 12.51 mpg.
I pulled the plugs today and they looked fine, gaps were bang on, no carbon buildup and not oily - basically looked as I they should ...vacuum tested the purge valve as per the engine management procedure and I THINK valve failed test so I'm starting a new thread on the "purge valve" - hope someone can point me in the right direction!

 

I get about 24 mpg in mostly highway driving 80% + and that's running the snot out it 75-90ish mph. I tried driving more conservatively but the mileage was worse

The last time I filled the tank it was almost $80 and it wasn't empty. Ive had people tell me I should get rid of it and buy a car that gets better mileage like a Camry or a Civic but my response is always the same...Life it too short to drive a shitbox.

 

'06 S-type 4.2, normally aspirated. Over the last 1,215 miles I averaged 23.0 mpg with an average speed of 31.0 mph per the car's computer. With a fresh start and on the freeway doing 75mph on cruise (just under 2,000 rpm) I've seen the overall average mpg get up towards 28mpg; not bad. Low mpg around town doesn't bother me - if all I'm driving is around town, I will hardly use a tankful a month.

 

Well I think I may have found a solution to my high gas consumption!

I replaced the o2 sensor a few days ago but my mileage remained the same - Last night I'd read that the ECU had to be reset after fitting a new o2 sensor and hadn't done that so that was the first order of biz. I disconnected the neg terminal and reset the system.

Next I removed the idle air valve from the manifold to gain access to the EGR port from above. When I took a look down into the manifold I could see the EGR port was completely plugged up with carbon - totally blocked!
Using a long 1/8" steel rod cut with a chamfer I was able to chip through the carbon and slowly open up the port, then, wrapping the rod in a damp shop towel removed all the loose carbon residue sitting in the port. I also removed the stepper motor from the idle valve and decoked it too.

After reassembling the idle air valve I fired up the car and after about 5-10 secs of rough idle, the ECU re-learned the idle and the car idled smoothly again.

On a short test drive around town, maybe a couple of miles or so, I saw avg. gas consumption numbers I'd never even come close to before; when I returned to the garage my readout (litres/100km) showed 12.9, which translates to about 18.5 mpg (US) or about 22mpg (imperial) - roughly a 30% improvement!.

Going for a spin on the highway later ...

Ok just got back from a 25 mile trip, town and highway, averaged 11.2 litres /100k so about 25mpg imperial or 21mpg US - I can live with that!

 

Nice one Lawrence, ah the joys of EGR.

None of that stuff on my 40, on a steady long run it would drop to the 8 - 9 region.

 

Last week I took a trip from the Los Angeles area to the San Francisco area in my 2004 XJ8 4.2. I drove the coast route going up, and the central valley route, (I-5) returning.

The trip was mostly highway driving, but I did some in town driving and one run on some curvy roads out by the ocean. My over-all mpg for the whole 1100 miles was 27.8 mpg. On my return trip of 385 miles down the central valley to my house, I got 30.5 mpg. This route involves going over one 4000 foot pass and has some 70 and 75 mph sections, so I was moving along.

What a car.

Tom