V8 Mileage Variation

[broberg@vx1.cpg.mcw.edu]

I have a 1990 V8 Automatic with 114k mi on it. Runs like a dream. It gets 
consistent 19-21 MPG on 93 RON fuel.  I just took a 600 mile trip and was 
monitoring the 3 second fuel average.  On the highway -70 mph- it was 
consistenly at 24-30 mpg, like normal, but suddenly, after a tad over 50 miles, 
mileage dropped to 12-16.  It didn't run any differently and all temps remained 
stable. This was not a road condition issue.  I stopped to shut off the engine 
and reset the computer, a process sometimes clears strange cold weather (-20F) 
maladies.  Didn't make any difference.  Since I was far from home and shop and 
there were no other indications of a problem I continued, though the gas was 
clearly dropping faster.  At about 150 miles the problem corrected and I was 
back to 24-30.  Overall average mileage on the tank should have been 22+ but was 
18.  The following day, still traveling, the same thing happened.  Several days 
later, but never driving more than 25 miles at a time, the problem does not 
recur.  Any ideas?  The first suggestion in the Ox sensor, but do they become 
intermittent?  I thought they just failed.

This car also exhibits the pulsing acceleration that someone else described.  it 
can easily demonstrated when engine is warm and accelerating up a moderate hill 
at 35-45 mph without the cruise control. It is cyclic and feels like a 
parkinsons diseased foot is on the accelerator pedal at a frequency of about 
1/second. It is only an irritation, but all occupants notice it.  Think it is 
related to the above issue?

Unrelated, this is #4 Audi for me.  It follows an 86 5000CS TQ, an 84 5000S and 
my daughter is now driving a very nice 91 100.  Loved them all, too!

Roland Broberg