GM EFI Problems - Any Help?

[Scott Fraser]

I subscribed to this list hoping to glean some knowledge in preparation
of putting together an EFI for an old VW when the system on my Pontiac
went haywire.  I'm hoping that someone out there on the list could help
me with some pointers.

'87 Pontiac Grand AM 3.0L V6  - Port Injection

Rebuilt engine about 5000 miles ago
Car has developed an intermittent problem where it has two modes, one of
them bad and the other very bad

last week I replaced the crank angle sensor because it was loose and
rubbing against the crank pulley,  figured it was best to replace it as
the wear marks had made one of the little magnets loose and almost ready
to drop out.  Eliminated most of the back firing.

Replaced the MAF sensor which seemed to be damaged by the backfires
(mechanic friends idea)

PCV and EGR valves have been replaced

Cannot find any vacuum leaks

recent symptoms

- hesitates upon acceleration off idle, if done "right" will kill the
engine
- last 4 mornings (45 degrees F) the car runs like its in a limp home
mode, drove it 2 miles and parked.  (the very bad mode)
- four hours later runs better with what sounds like a miss during idle
about every second or two
- engine has died three times on the freeway while driving at 65mph
- engine died on the side street today and after restart it was in the
"very bad/limp home" mode.  it drove and got me home, upon getting out,
it seemed that the catalytic converter was quite a bit hotter than
should be expected, I was hit by a heat wave from under the car upon
stepping out.
- check engine light does come on during these problems

I have not been able to find the exhaust sensor yet.  With the exception
of the sensors that I have mentioned, I'm believe that they are most
likely original with 144K miles and 11 years.

Any ideas?

Thanks
Scott Fraser