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Low boost problems -- at wits end -- HELP!



Gurus:

I recently purchased an '87 5KTQ with 180k miles on clock.  Condition
is much better than milage suggests.

When purchased, the car had a punctured Michelin-man hose between the
intercooler and intake manifold.  After replacing this part, the car
suffers from low boost, which maxes out at 1.1 bar according to
in-dash pressure gauge.

In troubleshooting this problem I have relied on Scott Mockry's
excellent Troubleshooting Tips <http://www.sjmautotechnik.com>
Is this guy *king* or what?!

A list of my experiments follows in the hope that it will benefit
someone and that someone might spot a hole in my reasoning.

 1) ECU fault codes:  

    Test drove vehicle for > 15 minutes.  Wide Open Throttle in
    third gear.  Performed ECU fault code dump.  Only 4444 was
    blinked.  Ergo:  No faults were stored.

 2) Does Waste Gate work?

  a) Applied 20 in Hg vacuum to Waste Gate.  Waste Gate held the
     pressure.  Ergo:  Diaphragm does not leak.

  b) Quickly removed vacuum.  Heard an audible "CLUNK".  Ergo: Waste
     Gate is not stuck open.

 3) Does Waste Gate Solenoid work?

    Connected vacuum gauge to the hose that connects WGS to WG.
    Unplugged Throttle Switch Connector and shorted two lower
    connector pins.  Observed large swings on vacuum gauge.
    Ergo:  ECU is speaking to WGS.

 4) Multi-function temp sensor defective?

  a) Based on experiment 3) I conclude that pin 2 is not sending false
     ground to ECU.

  b) Coolant temperature gauge holds steady at midspan.  Over-heat
     warning symbol is ordinarily off but proved itself to be working
     during a near overheat incident.  Ergo:  All seems well.

    Are there other tests available for Multi-function temp sensor? 

 5) Does Air Temp Sensor work?

  a) Disconnected ECU and measured resistance across sensor.  I
     measured 500+ ohms at ca. 77 degrees Fahrenheit.  Ergo: Sensor is
     good (?)

  b) Reconnected ECO and measured again.  Resistance had dropped to
     420 ohms.  Ergo:  Sensor has a good connection to ECU.

 6) Vacuum/Boost leak in hoses?

    Connected vacuum pump to two rubber hoses at rear of intake
    manifold distribution pipe.  Both hoses held 20 in Hg vacuum.
    Vacuum in one hose caused pressure gauge to read 0.2 bar.
    Ergo: No leak here.

 7) Problem in Turbo or plumbing?

    I removed turbo cold side intake and turbo exhaust hoses.  Hoses
    were solid and had no internal obstructions.  No play was
    noticable in turbo rotors on the axis.  A significant improvement
    in performance was observed when Michelin-man hose was replaced
    and boost comes up quicky.  Ergo:  All is well?


I would really appreciate any feedback.  What did I miss?  Where did
I over-interpret?  What next?

Thanks in advance and thank you very much Scott Mockry!

Gisli



Gisli Ottarsson, PhD                     
Mechanical Dynamics Inc.            gisli@adams.com    
2301 Commonwealth Blvd.             Tel: 1-734-913-2550
Ann Arbor, Michigan 48105           Fax: 1-734-994-6418