I have a 1996 XJS convertible with the 4.0 6 cylinder motor. It has 108K miles and drives great with plenty of pep. I am on my 4th tank of fuel. The first one it got about 17 MPG which was baout 50/50 highway (80 MPH) and city Around town the next tank fulls delivered about 13 MPG. It seems low to me, but I have no idea...

 

you may want to consider an Andy bracket. It advance the timing 5 degrees for more power and economy.

 

Where can I find one?

 

It won't fix your economy.

First thing would be to see what you LTFT (long term fuel trim) is pegged at. You'll need something to plug into and read the OBD2 data. Do you have that?

 

Yes I have a diagnostic obd2 reader with other functions

 

OK. Can you tell me what your LTFT (long term fuel trim) for both banks is pegged at?

 

https://www.jaguarforums.com/forum/x...engines-66536/

First thing to do is to check that you have Vacuum on your FPR Fuel Pressure Regulator

If not She could be running rich and this can really mess with your Fuel Economy

 

I believe the ltftrm is 1.6 for both banks at idle

Stftrm is very different between banks

 

So, oddly enough, that’s the mileage I’m getting. 17s with mostly Highway, 13s if I’m in the city all the time. 15s if it’s mostly city, with some highway.

My LTFT is at 7.0. It was at 7.8.

The STFT should be changing all the time.

With 1.6 as the LTFT, I think you’re getting what the engine can give. It’s not overfuelling you (like it is for me).