Miss under pressure....

[Shannen Durphey <shannen@grolen.com>]

Had a strange one in the shop the last few days.  TPI Vette, 89, ARAR
calibration.  Engine has about 20K on reman, stock specs with a small
twist.  It was cheaper to buy ZZ3 heads than to have the original
heads rebuilt.

The car had a pretty severe miss at idle, seemed like a lean skip.  No
ign. scope available.  Replaced plugs, ohm checked wires, bench tested
coil, changed cap + rotor.  Replaced coil for testing.  Injector
balance test showed no problems, fuel pressure was at the low end of
specs but matched several other cars on the lot.  Richening engine
seemed to help, but not cure.  Problem was apparent at all temps but
worse with heat, maf connected or no, open or closed loop.  The miss
was bad enough that the ecm would sometimes increase idle speed at a
stoplight, making the driver work to keep the vehicle stopped.  The
problem went away as loads and rpm increased above idle.  Richening
fuel via ecm did not change the symptoms, only the BL #s. 8 )  EGR
valve was not sealing correctly, blocking the EGR passage did not
improve anything.  

The final solution was to increase idle speeds at temps above 160 deg
F from 600 to 625, and to retard timing from 20 degrees to 15 @ low
load, 600 rpm.  From 15/600/low load, I increased timing gradually as
load or rpm increased, back to the factory value of 20 deg.  I found
that setting the timing as low as 12 degrees @ idle made improvements
in the shop, but I didn't have time to drive it enough to be sure that
setting would be good.

Anybody hear of anything that might provide clues?  ZZ3 heads have
stiffer springs, any possibility these could be the cause?  Any other
thoughts or stories?

Shannen
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