I have a 96 XJS with the AJ16 engine. ive been struggling with poor mpg’s. After throwing a ton of parts and getting the injectors cleaned, I suspected issues within the ECU. I sent a spare out to a shop for refurbishment, have not got it back yet.

In the meantime I’m starting to attack some leaky areas in the engine bay. First thing to go was the heavily leaking oil cooler lines along with the cooler itself. I put in the bypass part in its place.

After a few commutes to work, it seems my mileage is back to where it should be. I’m seeing an average of above 20.0mpg whereas before I was struggling to get past the 16s.

How could the removal of leaky oil cooler lines affect my mpg’s, or do I need to continue watching for a return the norm?

 

I can't easily think of any direct connection between the two.

If I s-t-r-e-t-c-h my thinking a bit....

Bypassing the cooler makes the engine run warmer which gives better fuel economy...but a whopping 4 MPG? I dunno

Cheers
DD

 

Anything to do with heavily leaking hoses?

The crimps were failing on both ends as well as well as the o-rings.

 

Personally: I doubt it.

 

How was the data gathered? Were both only based on the same number of commutes to work with the same traffic and the same weather?

Maybe the previous was over a longer period of time and included some short trips to the store, in different weather, or in slower traffic? Without the exact same conditions I doubt the info is very useful or worth comparing.

 

Data is similar. Its been the same ride to and from work, in the summer, spring, winter and spring. Freezing cold, boiling hot, the numbers seemed to remain the same.

Right now I am averaging 20.2mpg according to the trip computer. Previously I was struggling to get to 17mpg. Mostly highway miles, some traffic.

My LTFT is still pegged at 7.8, but it is something I will check in on again. I suspect if something really did change, the trim should fall.

 

20 mpg??? I don't even get that when I push mine.

 

Keep I. Mind, it’s the AJ16, 6 cylinder, not the 12.

if I were to drive straight highway miles, I should be able to average well above 24. I have done it in the car.

 

Where I used to live (totally different speeds) I had my V12 up to 18 going to and from work, totally stock (using the odometer to calculations). It is possible.

I suppose the engine could just be running hotter, as doug said, but I didn't think the oil cooler was doing that good of a job. Wonder if its worth upgrading.

 

I see your in the US so that is good for what I'm about to suggest... regardless of what the actual cause is regarding your poor mpg I'd like to strongly recommend two products which should give a noticeable improvement.

Archoil 9100 oil friction modifier

and

Chevron Techron concentrate


I can't say enough about Archoil the difference in performance and the overall reduction in wear/heat is well documented in everything from racing to industrial applications, and the concentrated techron injector cleaner is literally the best bang for your buck - I have family in the US that mail me the product because it works better then anything here aside from LiquiMoly.

My V12 XJS was night and day difference between synthetic oil and archoil, I also managed to clog my fuel filter in a week after a few rounds of techron. After replacing filters the difference in performance and milage was substantial as I was getting 4-6mpg before and 12+ after which isn't terrible for a 30 year old v12.

Used the same process in my 89' XJ6 to and averaged 24mpg on mountainous hwy conditions. Sometimes its just one clogged injector that needs a cleaner blasted through it at max PSI.