Detonation at 0.8 bar

["Steven Barnes" <sbarnes@acs.ucalgary.ca>]

My '87 5kTQ just got its Charlie Smith spring installed, plus a
new head gasket.  With the fuel pump grounded per instructed I've
seen 1.6 bar.  (Without the ground, boy that sucker cuts out
suddenly!).

Anyway, I don't drive this thing much cuz it's got an open
exhaust from the downpipe back.  Not popular with the neighbors!
(Its also got a full cage, race seats, Michelin L4's etc).

The problem:  Yesterday the picture of the engine came on (the
"check engine" light), presumably to say that it was detonating,
but at only 0.8 bar.  Very consistent at this level.

Ideas?

Steve in Calgary