[20v] Coupe Quattro 20V Carbon Fouling Plugs

Ben Swann benswann at verizon.net
Fri Sep 11 13:39:32 PDT 2009

Concur - forgot about that one, of all things.  As John at westcoastgarage indicates, check
the throttle body switches are operating first.  I forgot these early 7A 20Vs use
throttle body switch combination nearly identical to earlier CIS for 4000, CGT and
others.  The switch in question is found at bottom of the throttle, so throttle needs to
be removed to replace.  They are usually bad after all these years.  Operation can be
tested through the connector before going through trouble to replace or adjust.

If the throttle closed position switch is not operating properly the the idle
stabilization circuit will not be engabled and engine will likely be stalling.  Also if
not enabled into Idle Mode the engine may not be adjusting fuel mix properly - I don't
know details.  

The 7A was a very one-off system from what I gleaned when poring over the wiring
harness, and there are variations on the control units, so some of this may not apply.

I did mention the Fuel Pressure Regulator because I know of two instances of the FPR
causing flooding - friends cars, not mine.  That is out of three cars I new of recently,
so apparently this is common, but so is the Throttle switch problem.


-----Original Message-----
From: john at westcoastgarage.net [mailto:john at westcoastgarage.net] 
Sent: Friday, September 11, 2009 3:49 PM
To: Ben Swann
Subject: Re: Coupe Quattro 20V Carbon Fouling Plugs

Ben Swann wrote:
> I almost sounds like the fuel pressure regulator is working overtime 
> and may not be regulating fuel properly to the rail - too much fuel 
> pressure at idle.  This is at least one possibility and has been seen before on the
> You definitely want to pressurize the rail with fuel whil not 
> installed in the head (jumper pump on with relay, or just turn engine 
> over to bump FP for a few sec.) and see if there is some sort of leak when engine not
> Be careful with fuel and put rail over large container to collect any spry or leak.
> Fire Extinquisher on hand!
> Ben
> [Date: Thu, 10 Sep 2009 22:56:17 -0400
> From: "Brett Fairbanks" <brettfairbanks at verizon.net>
> Subject: Coupe Quattro 20V Carbon Fouling Plugs
> To: <quattro at audifans.com>
> Message-ID: <004e01ca328b$70b78a00$52269e00$@net>
> Content-Type: text/plain;	charset="us-ascii"
> I can't seem to get my early build '90 Coupe Quattro project sorted so 
> I can drive it.  The car starts O.K. when cold, but doesn't want to 
> idle.  I've swapped in two other idle stabilizer valves, but they behave the same way.
> It seems like the car is choking on fuel.  I've found fuel puddles in 
> the intake and in the rubber intake boot.  Checked spray patterns on 
> the injectors when I installed new fuel injector seals and all the 
> injectors were pulsing and spraying a decent pattern.  I've tried to 
> clean up all the vacuum leaks I could find.  Air intake is unrestricted.
> The car will rev fine, but a little stumbly until it warms up.  Makes 
> good power underway, and the idle cleans up a little after it's hot.  
> Still, as the plugs continue to foul, the idle situation degrades 
> until the plugs have to be replaced.  I have to put the pedal to the 
> floor (flood clear mode) to get the car to re-start when it's hot.
> Any ideas?  I'm getting frustrated as I need to be able to get this 
> car on the road.  Anyone run into this on their 20 Valve in the past?  
> Any suggestions are truly appreciated.
> Thanks!
> Brett Fairbanks
> Delaware, Ohio]
Failed/melted/loose/dislodged throttle switches have been known to cause this problem.
Look at the appropriate pages on 20v.org for testing/adjusting instructions.  John

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