Jess, Disregard my reference to marelli and fuel strategy jumper. 1989 was point of change for ignition, it may be a good idea to check your timing though.

 

yes the timing needs to be spot on for any fuel to burn correct .So if mine will burn 87 I think a 99 should also .You can go to local airport and get aviation grade fuel I think that is 110 now that has to make a difference.

 

My 90 XJS V12 coupe uses Premium unleaded which is 91 or 93 in Wisconsin. It also states below the gas cap to use Premium Fuel...

 

Only if your compression is greater than 12.5:1

 

That's not how it works at all.

You'll see zero difference, not to mention that almost all avgas has lead in it which will destroy the catalytic converters in short order.

 

Make a difference in.....what?

Cheers
DD

 

Emptier wallet.

 

My gas gauge clearly states "Premium Fuel Only".

The fill has a sticker on it that says the same.

You can move the jumper to go down to 87, but that simply retards the timing.

Newer cars can retard the timing automatically (no jumper). But that does the same thing.

 

In pre engine management systems that employ knock sensors (aka XJS), there is a minimum octane rating. In these vehicles going to a higher octane fuel will have no effect as the ECU can not adjust timing to take advantage of the increased octane. Even in knock sensed vehicles using 110 will have no effect as the ECU will not adjust timing to that extent.

In the Marelli car Jaguar incorporated an ignition strategy link to have the ECU adjust timing for 95RON. We have 98RON available at the pump but there is no difference in performance in my car between 95RON and 98RON. There is however a big difference between 91 and 95 if I change the link.

 

Hey Jess you need to check all the ignition stuff. If the car is down 1 cylinder you will only feel it at idle once under way say 100km/h on the open road you will NOT feel it. I know this from experience 1 bad plug lead on a trip it KILLED my RH Cat.

Change ALL 12 plugs
Check/change all 13 ignition leads.

As a start.