Hi all, as the post said: the following has occurred in my 98 XJR- when replacing the hood (don't get me started), we noticed that the air hose that connects to the MAF sensor was VERY brittle and appeared to have cracks in it. It literally fell apart in my hand. Cleaned the throttle body and washed the replacement hose, thinking that the mpg should go up from 13.5 mpg to hopefully (HOPEFULLY) 16 or even 18 mpg! Sigh. So... long story short, it has decreased to 10.3 mpg average... Quite a drop, to say the least.

Notes: 1) replacement of hose and cleaning of throttle body occurred 10 days ago. 2) Car needs an oil change (about 1000 miles overdue) 3) car's NEW hood has an 'air scoop' that is bondo'd on and hasn't been removed yet... I don't believe the hood was cut. 4) Tires are new stock pirelli asymettricals on the stock wheels

I'm wondering the following: Should I do a 'hard reset' of the car by touching negative terminal to positive, briefly and reconnecting?

A soft reset? (removing connections to battery entirely for an hour?)

Collaterally, the trunk appears to be locked in valet mode (thinking hard reset might help that too), and the windshield wiper occasionally comes to life on its own (also hoping hard reset will help that too, although I care less about that- I wanted to share with you that small fender-benders have strange consequences).

Any input would be extra awesome

 

How much fuel have you run through since, and are you using old fashioned math, or relying on the cars computer display for the mileage? Something definitely seems amiss to have been giving you 13 mpg to start with, I assume you've also cleaned the MAF at the same time? Anything else acting strange? sluggish performance, or skipping?

 

Definately do a hard reset. The car needs to relearn what the proper air intake amount is minus the probable cracks you had for a while and the ecu had been compensating for. A hard reset is a good start anyhow. Good luck

 

I would second CharlzO. I'd start with cleaning the MAF and do a hard reset (disconnect the earth lead from the battery and touch it to the positive lead on the battery).

And if it is running rough, idling rough, sputtering, skipping, sluggish, etc, it could be a vacuum leak. These cars DO NOT LIKE the air duct to be loose anywhere, if you take it off the TB the car will die completely... Spray brake cleaner on it anywhere that it connects and listen to see if it speeds up or slows down, this would indicate a vacuum leak.

 

Is there an engine warning light on? If not then it could be an unmetered air leak, as others have suggested a decaying snorkel will give the ecu extra air to deal with and down goes fuel consumption.
The hard reset will load the base engine and throttle maps, when you perform the hard reset start the car and let the throttle recalibrate, the revs should climb to 1500rpm-ish then back down to almost stall point, then it will settle at 600rpm. LET IT IDLE for ten minutes to allow it to store the new values. Take it for a spin.

Have you only the green valet key? If you have a black key it cancels the valet button. If you have a black key does the trunk open from the remote button on the dash, or from the keyfob, finally can you open it with the key you have at the lock? A wiring fault usually found at the trunk lid hinge rhs wiring loom is a place to look for fractured wires.

How old is the battery? These cars need good voltage, 12.5V min. The wiper can have a couple of faults, dry joints in the silicon blob that holds the wiring going into the unit as water gets in behind that can be fixed. Do a search on wipers.

Welcome to the forum

 

First, let me apologize for the delayed responses and thank you all for responding. @CharlzO, I did NOT clean the MAF, as the connection wire clip (Idk what it's actually called) broke, so we chose not to open the MAF itself... Definitely will at some point though. Also, it is NOT riding rough, not sluggish, no skipping. (either before or after accident and subsequent hood scoop. This was the first tank of gas since the replacement and after I drove about 240 miles yesterday, I ultimately got 333 miles out of the tank. I was confused however, that the computer said I burned 21.4 gallons of fuel (rode to empty and refilled when it said I had only 12 miles of range left)- the pump filled her up with only 17.5 gallons of gas. Which confuses me, b/c the stated fuel tank capacity per the internets is 21.4 gal. So this is a new question that needs an answer- as if I actually got 19.25 mpg on a tank, I'm pleased as punch- if the tank is indeed only 17.5 gallons. But if it were 21.4 gal, then I'm rocking a much more depressing 15.5 mpg.

Anyhow, @greverr, I have not had a chance to do the hard reset yet but I will, and I'll let y'all know how that goes. @BcCrary, it does not (currently) appear to suffer from any of those issues. Again, at this time lol

@SeanB, that's very good to know, I didn't know about the 10 min Idle. Will do that too. No engine lights on, although now I suspect I have a bad connection somewhere as I am getting 'low washer fluid' and 'low coolant' warnings, both were full when checked.

As to the key/trunk issue: I have the fob, but that hasn't worked for quite some time. I previously was able to open the trunk from inside, but it no longer does. I have the black key, so I can open manually. The trunk light is not working, so I will try and fix that, as well as look for the fractured wiring where you suggest.

I guess my new questions are: 1) what is the actual fuel capacity of the engine- and why does it say 21.4 gallons, both on the internet and in the car, and then takes only 17.5 gallons, and 2) what do you think the dealer would charge to re-program a key fob? (mind you I bought this XJR from my Jag mechanic, and he wanted nothing to do with re-programming a different fob, and told me this fob only worked intermittently- we split the fob open and cleaned it, but that didn't help for more than a week- I will buy a new battery and give that a try, maybe I'm crazy). I will do the hard reset and report back. Thank you guys again.

 

@SeanB, I'm not sure how old the battery is... I will take a peak when I do the hard reset- I've had the car since October 2014 and have not purchased a new one yet, but haven't had any issues with it (fingers crossed).

 

The only thing I can say, in regards to the fuel capacity.. is I ALMOST wonder if there's a small reserve of fuel of approx a gallon or so that's below the E level in the car. I've also seen that it lists as 21g, but even when I have pulled it down VERY close to E, it took just under 19g to fill up, which leads me to believe that either there's a documentation error, or that there's a small reserve that the gauge and computer 'ignore', as a safety net. Kind of like a backup - "Ok, here's a little light to warn you. Ok, well you totally ignored the warning light, but lucky for you were were smart enough to know you would ignore it, so here's another gallon to bail you out." Totally a guess, but just a theory.

Otherwise, I've not compared the onboard computer with real world myself, but I'm sure there could easily be a discrepancy there. But I would fill the tank, reset your trip odometer, drive a while normally, say a couple hundred miles, and then fill back up, and calculate that way. Gives a real world number to rule out the computer, and might surprise you and be better than you might have expected.

 

333 miles to the tank is about average... I get about 330 before filling up (typically around 19ish gallons).... Mind you these cars are ranked for 17 city and 24 highway, so expect 1.5-2mpg less than what that states. So 15.5 MPG is pretty average.

However I have noticed that whenever I go to fill up, it always takes more than what the computer says I have used...maybe this is an issue with the fuel level indicator sending an incorrect measurement to the ECU?

 

Program the fob yourself:
- Insert key in ignition (do not turn key)
- Pull the beam/long distance light into full beam
- While holding the full beam turn key into position 1
- Release beam and pull it again 4 times
- Push the "lock" button on the fob
If you hear a beep now the fob should work with your car.
Vector

 

I will add though, my light came on about 5-10 miles ago, and I'm planning on filling up completely after work tonight, so I'll post back how many gallons it takes to full, what it said remaining, etc.. just for reference.

Edit: computer showed 21 miles remaining, and an average of 18.5, so by it's mark just over a gallon until zero to go. Filled up and took 17.8 gallons, which would make a total of 19 gallons from full to zero, so if it's really a 21g tank, the difference us a reserve below E. Next time I get low I'll carry a couple gallons in a spare tank and run before zero mark and see if it sputters.

 

Let us focus on what, inexpensively, causes poor mileage scores: air, fuel, spark. That is basic for engine start. You have too much/little of one/two/third of each. This makes me think, which is causing the missing benefit? Poor ECM control will devalue idle/run; poor engine management can cause as well. Since it was quite abrupt, I would thing ECM translation and would focus on this to resolve your problem.

 

Jaguars normally have a fuel reserve. Handbook will say, I expect.

Filling and mileage calc by hand is a good way of checking mpg, especially done a few times.

If you've done the hard reset and given it a chance to relearn then the above is probably good enough but I'd check fuel trims as they are what the car is really doing and will warn of pending issues.

 

I will check the following then: ECM, Fuel Trim, wiper wires (she goin nuts now, and drivin me nuts!). Hard Reset to come later today... then will re-fill, and see what happens. Tomorrow will be a tweeter replacement on the A-Pillar... and fuel torching the ghastly blower off the hood. As to milage, which was the original question I had, I will see what she really gets by hand. @CharlzO, so weird this fuel capacity thing! I'm looking forward to this.

 

The Vehicle Care handbook ( JJM 10 16 12/80 ) for my 98 XJR, states


Indicated refill capacity (E to F on the gauge) 75 litres, 16.5 imp gals, 19.8 US gals


Unindicated capacity (Below E on the gauge) 6.0 litres, 1.3 imp gals, 1.6 US gals


Total refill capacity ................................... 81 litres, 17.8 imp gals, 21.4 US gals


Hopefully, this will clear up the fuel capacity issues for you.

 

Hi
A few days ago my XJR start to use a lot of gas, i drive all the time in the city, usually 18.1l/100km (12.9 US gallons or 16 imperial) a few days ago start to show me 25l/100km (9.4 US or 11 imperial) It was a lot so what to do?

Today I clean the MAF, the throttle body and change the air filter, the car now show me as before 18.1l/100km.

So if any one find that the car start to use a lot of gas this is the first thing to do. I check the fuel trim and all are ok.
Good luck