Re: 87 5ktq boost problems and questions

[Gisli Ottarsson <gisli@master.adams.com>]

I have recently gone through this same process.  My car is now showing
the occasional 1.3 bar on the in-dash gauges and I have come to
believe that this gauge is not very accurate, to be verified in the
next few days by installing a second gauge.  A friend already added a
gauge and found occasional discrepencies to the tune of 1.2 vs. 1.35.

I found one of your comments quite interesting.  It had been my
understanding that the disintegration of the lower wastegate hose
would cause an over-boost condition.  My way of thinking about it is
that the purpose of the wastegate is to control the boost by bleeding
off exessive boost and if the lower wastegate hose is shot, the
wastegate cannot detect the boost level.  This leads me to believe
that your car may be incapable of generating high boost.  You should
test this first.  This can be done by pinching the lower wastegate
hose shut.  The hose will now keep the boost in the intake manifold
but the wastegate cannot detect it.  The result is uncontrolled boost.
When I do this on my car it quickly produces 1.5 bar at which point
the ECU shuts of the fuel injection.  Very exciting.

I found this to be one of the most important experiments because I
think it eliminates so many mechanical causes of low boost, e.g. bad
turbo, blocked exaust, leaks, etc.  It is also a wonderful morale
builder to feel the high boost kick in :-)  I suspect that the reason
the gurus don't plug this experiment more is that it is not entirely
without risk.  For instance, I understand that there is a slim
possiblity that the turbo may overspin.

Try this and post your results.  

Gisli



>>>>> On Fri, 30 Jul 1999 08:08:42 PDT, you said:
 
  SM> I have a 87 5ktq that is not producing max boost according to
  SM> the onboard gauge.  I have been through the SJM's incredible
  SM> site on the this subject found and fixed many of the usual
  SM> problems, but the boost has not come up at all.  Does anyone
  SM> know how accurate the onboard gauge is?  My next step is to get
  SM> a boost gauge I don't trust the onboard gauge.

  SM> should the lower waste gate hose hold vacuum and/or pressure?
  SM> That was not clear to me on SJM's site.  Mine holds pressure but
  SM> not vacuum.  The hose to it was totally rotted out, I replaced
  SM> it, nothing changed.  The upper waste gate hold vacuum like it
  SM> should, no problems there.  I have checked out the control
  SM> solinoid it seems to function per SJM's instruction and the temp
  SM> sensor checks out.

  SM> One thing I have not checked out is the cat, and exhasut back
  SM> pressure.  As soon as i get a pressure gauge (probably today) i
  SM> will investigate that.

  SM> Thanks for the help...

  SM> sm


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