I'm getting a terrible gas mileage on my 2000 XJ8 at only 14-15 mph. No CEL and had replaced O2 sensors about 2 months ago. Any ideas anyone? Thank you.

 

i have got even worse than that mpg. did you up the size on your tires? i get worse than that for mpg, but i didnt buy the car for mpg

 

Wayne, thank you for your reply. No I didn't change anything on the car at all including the tire size. I used to get 17 mpg, then the coil went bad on #7 which killed my upstream O2 sensors and my mileage went down to 11-13 mpg. I had since replaced the sensors but the mileage only got up to 14-15 mpg. I was hoping I'd get 17 mpg back...... Oh well.

Alan.

 

Any change if fuel? I know here in Ohio/Kentucky they have started adding more Ethanol which drops my fuel economy 1 - 2 mpg...

 

Joey,

Thanks, buddy! Come to think of it I'm not sure if Southern California uses Oxygenated fuel or not. I know our temp drops down to the high 30's sometimes but you're definitely right, this could change my mileage by 1-2 mpg.

Alan.

 

There was a post a while back about poor fuel consumption because the auto box wasn't shifting into top gear. At anything above ~40mph you should be able to pull the J gate across to the left and into "4" and you should feel the car change down..... otherwise it's not shifting into top.

It may sound strange that you wouldn't notice it but I drove the whole way from San Fran to San Jose a few days ago in "4" because I was "driving with spirit" briefly coming down the on ramp and once I joined the freeway I didn't change back into "D". My girlfriend was chattering a lot so maybe thats why I didn't notice :-) anyway, I lost about 3mpg on the run because of it.

Also second Joeys comment about different gas. I've become a creature of habit and always use at least the same brand, if not the same gas station if I can help it, as I have seen noticable differences bteween brands before

 

IceBreaker, you may want to try a bottle or two of fuel injector cleaner. What you may be experiencing is that due to your bad coil, the one cat has gotten some carbon in it and you need to get something in there to help the cat clean itself out due to it restricting the flow of the exhaust. Some may even recommend doing what is called a "seafoam" treatment. There are pros and cons to this too. If you want to know more about that, let me know. I have done seafoam to other engines and had decent results. haven't done my Jag yet.

 

Don't mean to hijack the post, but I'm interested in learning more Thermo about your thoughts of using seafoam with the Jags, particularly a '99. I've used seafoam on all my other vehicles as a preventive maintenance, and was considering doing the same with the newly purchased Jag with 79K on the clock.

I was also considering doing the LubraMoly complete treatment that can be had thru Bavarian Autosport. Sort of a 80$-$100 tune-up in several cans.

Thoughts?

 

I'm with you Crosbo, I'd like to know what it is! I've been working on cars since 1970 and have never heard of this miracle treatment. What is "seafoam"? At the same time I will definitely run a few cans of injector cleaners in the tank. BTW I've been through about 5 tanks since O2 replacement so I don't think I would have the trans in 4th all that time. I usually look at my tach, 60 should be a little below 2000, if it's higher I usually glance down at the shifter to make sure....

Alan.

 

Check the air intake tubing for leaks. The little plenum to air pipe joint is a common problem. I discovered recently that the MAF to air pipe seal was not fitted!

 

I do know that Seafoam is highly thought of on the Mercedes forum of which I've been a member and frequented quite often over the last 8 years or so. I was just more concerned about potential issues of using Seafoam on a Niksail engine.

Checking Seafoam's website, I'm reminded of all the proposed improvements that use of Seafoam provides.

I guess I will go ahead and add a can to the tank, before we take off on our trip from San Antonio to Oklahoma this weekend.

 

Thanks, Dan. You know I've been taking this tube off, putting it back on so many times you could be right! The car's in the shop for some body work but as soon as I get it back this Friday that's going to be the first thing I'm going to attack! Thanks again.

Alan.

 

Crosbo, I don't know if you remembered the infamous Vega of Chevrolet in the 70's but Mercedes uses the same concept in that the cylinder is Aluminum and the way they hone the cylinders are by impregnate the walls with Silicon by way of some electrolysis procedure. Nikasil is new to me so I'm not sure how that is done but seems to me the Mercedes had succeeded at what Chevrolet had failed. Chevrolet's fix was to bore the cylinders and press steel sleeves (by shrinking the liners with cold temperature) into the cylinders. Once this is done the engine was a much better performer and more reliable. If Nikasil is similar then seafoam should work, right? Let me know how the stuff works out for you, thank you.

Alan.

 

Icebreaker, you may want to try a hard reset on your car, the ECU may be still running (and slowly adapting) from the coil pack and 02 sensor changes. Give it a push by making it start from scratch. Disconnect negative terminal on the battery and briefly touch the positive to clear all memory. Give it another couple tanks and see if mpg has improved. Also, if you're city driving more and less highway miles...that'll show a lower average. A quick check on your 'avg speed' on the computer will determine that. For example, I do about 75% highway driving, averaging 35-40mph on a tank, and I get 22+ mpg. I'd suspect anything below 30mph average is weighing more towards 'city' driving.

 

Thanx for the suggestions H20, but I think you are referring to Icebreaker's questions about his fuel economy. Thus far, our Jag's V8 is actually getting better mileage than my BMW's big 6. I'm quite happy with the fuel economy thus far.

 

You are right Crosbo, I didn't look all the way back to the OP. My bad. I edited it to correct.

 

Thank you for the suggestion H2O, I'll try that as soon as I get it back (tomorrow hopefully!). I did reset the RAM with an OBD tool but did not do a hard reset as you had mentioned. My driving is mostly 30 mph or less average speed so it's more of a city driving. But this same driving used to get me 17 mpg before my coil went bad. Like I had mentioned, replacing the O2 only got me 1-2 mpg better.

Alan.

 

I get easy 20mpg when I'm driving my XJ round town, and on the motorway I easily maintain 32mpg whilst sat at 80 mph. In fact when I drove to france I managed to get near 500 miles from one tank of fuel! That was with the AC on, and cruising at 85 on the Autoroutes!

 

^^ impressive Stu!

 

Stu, are you using Imperial or US gallons in your MPG calculations? I ask because it's all too easy for the US and UK members to forget that the other country uses a different sized gallon, so your higher MPG numbers may actually be in line with American XJ8s. No disputing your 500 miles on one tank though; that's a number that my XJR, and my conservative refueling habits, can only dream of achieving.