[s-cars] Weird boost issue

[Theodore Chen]

i'm following this with interest, as i seem to have the same
problem.  my car is a '92 with MTM stage II+.  boost is
inconsistent, sometimes peaking at an indicated 1.0-1.2 bar
on the digital boost gauge (this gauge likely reads a bit
low because the MTM stage II+ kit changes the pressure
transducer from 2.5 bar to 3.0 bar).  other times, i get
full boost (indicated 2.0-2.1 bar).  sometimes this happens
during the same acceleration run.  first i get weak boost,
and then all of a sudden it kicks in with full boost.  i
cannot reliably replicate this.  engine starts and runs 
smoothly.

i installed a forge bypass valve several years ago.  the
intercooler-TB hose is a samco.  i just replaced the
crossover-intercooler hose with a samco.  i'm not sure if 
the crossover hose was stock - it's some kind of black fabric
with reinforcing rings on it.  it seems to be in decent shape
but i replaced it with the samco anyway.  the turbo-crossover 
hose is orange and seems to be made of a similar material, and
i believe this piece is from hoppen.  i have a samco hose to
replace it but haven't done that yet.  the u-bend hose from
the turbo-crossover hose to the bypass valve is rubber.

efforts so far: i replaced the clamps on both sides of the
moisture trap, swapped in the samco crossover-IC hose, and
tightened various clamps.  the lower clamp on the 
turbo-crossover hose snapped off when i tried to tighten
it.  it may have been broken already.  i replaced it with
another gear clamp.

where else should i be checking?  considering the symptoms
that the car is showing (going from low to full boost in
the same acceleration run), i'm inclined to think it's a
boost sensing/control problem rather than a boost leak in
the intake path.  what do you think?

-teddy

--- Robert Myers <Bob at chips-ur-s.com> wrote:

> 
> I have three ECUs each with a 3 bar Pressure Transducer of one form 
> or another.  One with a Bosch 3 bar PT and two with VMAPs.  I have 
> been inside all three within the last two days.  They are 
> OK.  Changing one ECU for another does not have any effect on the problem.
> 
> My boost gauge registers something on the order of 0.5 atm vacuum at 
> idle and responds to changing manifold pressure.  At present te 
> maximum observed boost si on the order of 6 to 8 psi.  There is an 
> approximate 20 psi missing.
> 
> I'm tempted to whack the wastegate with a BFH to see if I can jar 
> anything loose.  Hmmmm...  Possible Forge BPV acting up?
> 
> At 10:57 AM 11/28/2005, chris chambers wrote:
> 
> >*This message was transferred with a trial version of CommuniGate(tm) Pro*
> >damn too bad....
> >
> >My thoughts are more towards boost sensing not loss of boost.
> >If you rule out a leak at the trap, I'd check the lines to the
> >ecu and maybe the VMAP, do have a VMAP right?
> >
> >Your Boost gauge doesn't indicate a loss of boost does it?
> >
> >
> >
> >--- Robert Myers <robert at s-cars.org> wrote:
> >
> > >
> > >
> > > I wish...  :-(
> > >
> > > The water trap hoses are in excellent condition and the system hold
> > > pressure so I'm convinced that this is not the problem.  Gotta admit,
> > >
> > > though, that that was my first item to check.
> > >
> > > At 10:35 AM 11/28/2005, chris chambers wrote:
> > >
> > > >Bob,
> > > >
> > > >I had the same problem earlier this summer.
> > > >
> > > >After ALOT of searching finally figured out the water trap hose
> > > >somehow got deformed and even though I would clamp down tight it
> > > >wasn't sealing completely. I shortened the hose removing the
> > > >deformed end and reattached it to the water trap, problem fixed.
> > > >
> > > >Hope yours is as easy to fix as mine.
> > > >
> > > >
> > > >HTH
> > > >Chris
> > > >
> > > >
> > > >
> > > >Make April 15th just another day http://www.fairtax.org
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >__________________________________
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> >
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> Bob
> Http://Chips-Ur-S.com
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