I used my HP Tuners as a scan tool on my 96 LT1 in an Impala SS. The "Advance" as read by the tool sits at -12 deg at idle, then goes down to -35 deg if I rev it (no load) to about 2k RPM. I'm interpreting the negative numbers as meaning there is a lot of spark retard. Why is it pulling so much spark? Shouldn't there be more advance with more RPM?

Also, the short term fuel trims show very different #s for bank 1 and bank 2. Bank 2 shows a higher numerical % trim. Does this indicate the PCM is fueling bank 2 more, possibly because I have failing injector(s) on that side? Which is bank 2, passenger or driver?

Or could I have fouled O2s on one side (dual exhaust, pre and post cat O2 equipped) which is telling the pcm that the bank is running too rich when in fact it is fine?

1 Answer 1


The timing is advanced. -12 means 12 degrees before TDC (Top Dead Center) and likewise -35 means the timing is advanced 35 degrees.

I wouldn't worry much about the short term trim, unless you are having driveability problems. If you post the actually numbers including the long term trim I may be able to give you more information.

  • I've never seen a negative number for spark advanceexcept for on the 96 impala l_t 1 that I'm working on now I'm curious also
    – user9237
    Commented Jan 28, 2015 at 1:49
  • 1
    -12? As in 12 degrees Before Top Dead Center (BTDC)? Every engine I've ever dealt with has advance in the negative numbers ... that's why it's *advanced". It will not run very well if it isn't. Commented Jan 28, 2015 at 2:39

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