Miss under pressure....

[Bruce Plecan]

Compression test, or leak sown?.
Fuel pressure?.
Heads ever retorqued or need to have been, and valve adjustment?.
Just startin to lose a valve seal (burnt valve),   maybe just too early in
the self distruct mode to diagnose.
Grumpy


> 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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