[200q20v] Lack of boost very puzzling... long winded question

Pete Kunzler pck at gte.net
Wed Mar 21 09:12:37 EST 2001

I got mine from Rod at Parts Connection for $135 or so.


-----Original Message-----
From: Forhan, Thomas <Thomas.Forhan at mail.house.gov>
To: 200q20v at audifans.com <200q20v at audifans.com>
Date: Wednesday, March 21, 2001 5:43 AM
Subject: RE: [200q20v] Lack of boost very puzzling... long winded question

>I just replaced my idle switch/throttle pot which restored full boost and
>eliminated a stalling problem that a turbo bypass valve replacement had
>helped but not cured.
>Testing following Scotts instructions the idle switch seemed to be OK but I
>did not have a scope and could not really see the curve of the
>The part was about $160 from Mac at Clair. I never got into the old one to
>see if it had any broken solder joints, but its on my workbench. Someday...
>Tom F.
>-----Original Message-----
>From: C1J1Miller at aol.com [mailto:C1J1Miller at aol.com]
>Sent: Wednesday, March 21, 2001 7:42 AM
>To: Alan.Cordeiro at mts.com
>Cc: 200q20v at audifans.com
>Subject: Re: [200q20v] Lack of boost very puzzling... long winded
>Hi Alan,
>Looks like you're doing the right diagnosis path.
>If you haven't reviewed Scott Mockry's "The ABC's of Running High Boost for
>the 1991 200TQ 20V Turbo"
>it's a good place to start.
>The idle switch may be dropping out and your meter may be sampling too
>slowly to catch it.  An ocilliscope would be better.  Might want to change
>it out; don't know the cost, but the part number is:
>Bosch 0 280 120 410, Audi part number 034 133 154F
>see: http://www.sjmautotechnik.com/ecu20v.html
>Scott says "I have found that the idle switch can have cracked solder
>inside this throttle switch and cause an intermittent 2121 fault code. The
>photo shows the inside connections that can crack and cause intermittent
>problems. If you are careful, you can use a razor knife to cut the plastic
>top along the seam and pop the top off the throttle switch and then
>re-solder the cracked idle connections."
>That page also mentions the potentiometer that measures throttle angle;
>could also be your problem.
>You should probably also buy an analog boost gauge for diagnostic purposes.
>HTH< Chris Miller, c1j1miller at aol.com, Windham NH
>In a message dated Tue, 20 Mar 2001 11:28:30 PM Eastern Standard Time,
>"Cordeiro, Alan" <Alan.Cordeiro at mts.com> writes:
>Hi everyone,
>I have been trying to figure out a lack of boost for the last many
>months, not urgent, since the car has considerable performance even
>at 1.5 bar... however I have run out of ideas.
>The patient is a 200q20v, which seems to refuse to allow me more than
>1.5 bar boost. When I purchased the car it only had 1.3-1.4, which
>was cured by replacing the wastegate solenoid (open circuit failure).
>Boost improved to 1.5 immediately, which caused failure of the accordion
>hose shortly thereafter (weak with age, low boost prolonged its life).
>In trying to cure an occasional stalling problem, I discovered a leak
>in the bypass valve, replaced that, but still only see 1.6 bar on rare
>The only code that occasionally reappears is the idle switch, sometimes
>it takes two or three days for this code to re-appear. I have put a
>meter across the throttle position potentiometer, and watch it run smoothly
>from 0.3 to 4.7 volts. I also checked the engine temp sensor and the
>multifunction sensor was replaced by me to fix a "too low" temperature
>reading after I replaced the thermostat with no noticeable improvement
>in running closer to normal engine temperatures.
>I have measured the output of the altimeter, it sits just over 4 volts,
>seems about right for the Michigan area. By temporarily putting in a much
>stronger spring, the gauge goes to 1.9 almost right away and the boost
>shuts me down, so the turbo is just fine. (It spools right up to 1.3 almost
>as soon as the rpm gets over 1500, another good indication).
>I have a 1.2 watt lamp across the wastegate solenoid, and watch the lamp
>glow solid when I floor the loud pedal. Around 1.3 it begins to flicker
>and by 1.5 it is pretty much totally off, eliminating all further boost.
>So it would appear the CPU feels that for some reason it should limit the
>boost to 1.5. BTW the gauge seems right, it reads 0.9 or 1.0 when I first
>turn on the ignition, and runs 0.5 to 0.6 at idle, 0.7-0.8 cruising at
>"typical freeway speeds".
>I am using 93 octane, but I have experimented with lowering it to 87
>just to see if the boost control point goes down some more, but it has no
>effect. And the dash (added) check engine light never comes on after
>the engine starts, and there is no evidence of knocking (even when I
>put in 87 octane).
>Any ideas, does this ring a bell with someone???
>'91 200q20v, 150k
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