Re: Miss under pressure....

[Shannen Durphey <shannen@grolen.com>]

Thanks Rodney.  The car's not at the shop now.  If I ever get to do a
leakage test on it, I will.
Shannen

Rodney Davis wrote:
> 
> Try running a cylinder leakage test to see where the cylinder pressure
> loss is and how close the cylinder pressures are and I bet you will find
> it.
> 
> Shannen Durphey 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
> > ----------------------------------------------------------------------------
> > To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
> > in the body of a message (not the subject) to majordomo@lists.diy-efi.org
> ----------------------------------------------------------------------------
> To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
> in the body of a message (not the subject) to majordomo@lists.diy-efi.org
----------------------------------------------------------------------------
To unsubscribe from gmecm, send "unsubscribe gmecm" (without the quotes)
in the body of a message (not the subject) to majordomo@lists.diy-efi.org