Scott,
Could that possibly be P0172 and P0175? Those are rich system faults. Overly rich conditions can be caused by fuel pressure regulator failure [fuel pressure too high], Mass Air Flow Sensor over reporting incoming air, or possibly a purge system fault causing fuel/vapor to be drawn in straight from the fuel tank.
If the incoming air is reported as too high, the ECM fuels more to match that air; the oxygen sensors say "we're runnin' too rich" and the ECM subtracts fuel accordingly. The ECM says, "if I gotta take away fuel, we must be runnin' rich, set the code".

Good luck!

 

I was told that the engine can adjust the timing for different octane fuels. The mileage will vary, but it should run fine.

 

You are correct about the code. I replaced the FPR last summer (pressure too low) with an OEM unit, so I'm drawn toward looking elsewhere. MAFS is clean and duct work tight... I've never looked over the pruge system; that's worth a try. As I recall there is a smallish tube crossing over the front of the engine that gets plugged...there is a post here on the topic that I'll look up.

Thanks, also for the part number on the other post.

 

You are correct in that. However, I have so many ****ling engine management problems with the XK8 that I deceded to run Premium just to remove one variable from the equation. I have to say that the car ran much better - crisper in terms of response - and although the problems didn't go away, they were less frequent.

My Corvette and various Northstar equipped Ed Dorados ran fine on regular gas. I could never sense a difference at all other than MPG. Interestingly, the decrease in MPG was not enough to offset the increase in $PG at the time...

 

I always run premium too. The only time I put regular in it was last year in Raleigh when all the premium was sold out in the area I was in and I needed fuel now. Car ran fine. Nice to have the option.