Ever since I had the throttle body replaced, I have continued to monitor my MPG and have noted a significant decrease in MPG. I used to get 26-27 MPG on the freeway (55-75 MPH) and about 19 MPG on city driving. Now I am getting 22.5 MPG freeway and 16 MPG city.

I almost always use adaptive cruise control.

Any idea why so much of a drop? I expected it to get better not worse with the new throttle body.

 

only thing I can possibly think of it the old one was dirty and the car adapted for the slightly increased throttle opening. now clean there more air that requires more fuel (lower mpg) this could be from long term fuel trim changes. I would do a hard reset on the battery to clear adaptations and see what happens. Just thinking out loud

 

I agree with brutal; hard reset a good idea. Also I'm not sure about Florida but here in the Midwest, we switch to part ethanol in the winter I think. Ethanol produces fewer btu's (did I say that right?) so more fuel is required. Also, thinking outside the box, it's colder now - how is your tire pressure?

 

Cold air means a lot (more aerodynamic drag), event though the engine should be slightly more efficient. Tire pressure a big deal too, obviously. I usually see a 3-4 MPG decrease in winter.

 

I've never gotten more than 22.5 overall avg since I've had my XJ (2 years now.) What is everyone else averaging? I use Shell premium unleaded btw...

 

I recently had the Jag dealer do an injector cleaning and have run a couple of cans of 44k. Motor runs a lot better and fixed some other issues mentioned in another thread.

HOWEVER MPG is even worse now.

Anyway for me to get access to fuel trim values? Jaguar never has done them on their cars before and and can't provide me with that data. Go figure.

 

If the car is running well, be sure of the mileage first. Then, if it is lower, put it on a scanner and check the fuel trims. Changing the throttle body could result in an intake leak at the throttle body, or damage to the intake snorkel or the MAF or its harness. There is nothing about a new throttle body that should reduce mileage if it is installed without damaging anything. Keep in mind that plastic parts that are getting old get brittle and just moving the snorkel around can break something.You should be getting about 26 average of town and road if yours is like mine. Fuel trims on a scanner will read either plus or minus from the center so you know if it is runing lean or rich. There are no fuel trim figures needed.

 

Car runs fine. Dealer replaced throttle body and performed injector cleaning. What I was saying is they do not know how to get the fuel trim values before or after, so at the least I wanted to know if I could use something other than IDS to find out for myself.

 

Yes, you can get the trims. You will need a laptop, a cable with interface and a program for the computer. I use Proscan but there are others. This will allow you to monitor the sensors in real time and find out what is really going on. If the mileage is dropping, the possibilities are great, the scanner will get it right down to the culprit. Without a scanner, you are reduced to guesses and parts replacement, a very expensive and frustrating activity. If mileage was good right before the throttle body job, my assumption is that the tech damaged something in the process, probably the intake system. You just have to find it by retracing his steps. I typically do not let repair shops do this kind of work due to this problem. I know that not everyone has a choice, but replacing a throttle body is easy work for the shadetree guy and you might be more careful than the shop typically is.

 

I'll see what I can find out. It is down to 15 mpg freeway/city as of today and a couple of tanks...not good.

 

To see many OBDII drivetrain elements, you can get a bluetooth OBDII reader from eBay (!$15-20) and a copy of "torque" for Android phones (free- $5?) not sure about apple and then read short term (immediate) and long term fuel trims for both banks. This will show if there is something amiss. Also calculated AFR and acceleration and 0-60 times and 1/4 mile and charting and many other things.
Not other bode codes, but all drivetrain etc.

 

Jahummer,

You can spend a lot of time and money getting to the bottom of this problem. I think you need to get that car to a competent Jag tech who can diagnose the issues. I would, were it me, raise hell with the Serv Mgr at the agency who performed the most recent service that engendered the problem. As you can see from all the suggestions, you have a steep and expensive learning curve to climb just to understand what's going on.

BTW There is a something in the back of me little skull saying 02 sensor or stuck open thermostat. Something just below the level of throwing a code.

 

Hi jay hummer.
A month has gone by since your last post on the fuel mileage issue.
Have you fixed it? If so, what pray tell what was the problem.
Mine is new to me and gets close to 30 on the highway at 65.

 

Down to 14 MPG mixed driving.

Bank 1 reads 7% & bank 2 1%

Dealer wants to try reflashing the ECU just tired of throwing money at something that is getting me no where.

 

I used to screw with my motorcycle fueling maps. Got it to fly but the mileage suffered. I returned it to the original tune and it is ok again.
I am betting you just have a software problem and the dealer reflash will cure it.
Good luck.

 

I assume you mean disconnect the battery for X amount of time? What should I expect when doing this, will there be any systems that need reprogramming afterwards or will the car recover automatically? What difference would a re-flash be?

Thank you!

 

Mileage dropped to 12 MPG before I replaced the spark plugs and is now slowly creeping up, but still not good, at about 15 MPG now.