Miss under pressure....

[Len Sabatine]

   Had similar problem on a TBI 5.7 Truck , Intake gasket leak, Removed 
"Blue" Gaskets,
   Installed a fresh set  of "  Gray " OEM intake gaskets , miss and 
"shakes" went away.
   I would think the lack of valve spring seat pressure is problematic . 
For GP s , I'd
   do a Leak down test , maybe GM forgot to  qualify valve seating. Never 
know with
   box parts these days. Also ,  Fattening up is helping , so I 
I'd  suspect air leak . One
   last thing , Maybe double check lifter pre load  adjustment  as well.

   Len

        At 12:04 AM 6/28/00 -0400, you wrote:
>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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