I understand that a 1998 XJ8 requires premium gasoline but heard that if you used 87 octane or 89 octane mid grade gasoline the engine would de-tune itself, performance would be worse and mpg worse, but would regular 87 gasoline hurt the engine? Would it pink or knock or is it just a no no with this years engine. Any advice would be great,
Thankyou,
Michigan Man.

 

I'm not trying to step on anyone's knowledgable toes here...But IMO...Yes the ECM can de-tune itself (retards ignition timing, among other adjustments)to compensate for low octane fuel..the predetonation sensors will signal the possible knocking or pinging potential before it happens..but inturn the engine is not running optimumly and there is no room for Heavy loads or accelleration...My advise is to stick with what the engine is suppose to run on...You have a nice investment there..treat her right

 

LOL, my toes disappeared long ago.
Use premium only. Cheaper gas will clog your injectors, especially in the Southern US with the high sulpher content.
If you figure the cost, say 30 cents a gallon and what, 18 gallons. Only about $5.00 so definitley worth saving that $14,000 to get the engine replaced when the detonation blows a hole in your pistons. Seen it happen.
Dennis is correct, the knock sensors use a piezo crystal and signals ECM which then adjusts air fuel ratios. Car will run for quite a while on cheap fuel, but.......
Also, put a bottle of techron in tank every service! It really does keep injectors cleaner.