High idle, sticking throttle, throttle body questions - Conclusion

[SJ]

The problem turned out to be sticking throttle plates in the throttle body.
The whole throttle body insides was pretty well gummed up. Keep in mind that
the crankcase fumes are fed into the intake bellows just in front of the
throttle body. After 13 years . . .it gets pretty yucky in there.

I removed the throttle body for a complete cleaning and lube. You can do
some of this while its on the car, but you will not do a thorough job. It's
best to remove it and work on a bench.

Its not that much of a problem to also remove the upper half of the Intake
manifold and check the ISV hoses. My hoses no cracks and were not stiff with
age, so I reused them.

I also cleaned the Idle Stabilization Valve and checked it with a 1.5v and a
9v battery for proper operation.

When you have the throttle body out, might as well check the idle and WOT
switches for resistance. My idle switch read 8 ohms. But, it was a
consistent reading. No matter how many times I played with the switch button
. .. the resistance was a steady 8 ohms. I decided to keep using this
switch.

The idle switch feeds 12V to the Ignition Control Unit. The input resistance
at the ICU is probably very high . . .which means very little current flow.
Therefore, the 8 ohms resistance is nothing compared to the input
resistance(100k or more) of the ICU circuit. Whatever voltage drop there is
across the switch contacts will be insignificant. Think of a 1 meg resistor
with one end connected to the battery. The other end will read 12 volts. 25
meg resistance will also read 12 volts . . .this is true as long as you have
no current flow or very low current flow.

As soon as you connect a load to this resistor(100k to ground), you will
read the voltage across the 100k resistor . .. (100,000/1,000,000)(12v) =
1.2 v . . .
What does this mean? It means you have to have a pretty shot up switch that
is either completely open or has an extremely high resistance in order to
affect the ICU.

If  my idle switch contacts were intermittant or the resistance was high or
varied greatly, I would have changed the switch assembly.

On re-assembly, I first chased out and cleaned all the threads in the
mounting holes in the aluminum manifold. I replaced the throttle body gasket
and the maniflold gasket. $5 for both.

It took about 5 seconds of cranking to get the engine started up. Took it
for a ride, shut off engine, restarted . .fired right up in less than 1
second. Hmmmmm . . .hard start when cold . .

Had a looksee . . . .turns out I hooked up the cold start injector plug into
the gas tank fumes solenoid valve. Thats what you get when you work too long
on the car . . or was it the beer?

SJ



----- Original Message -----
From: "SJ" <syljay at optonline.net>
To: <quattro at audifans.com>
Sent: Wednesday, October 01, 2003 7:02 PM
Subject: High idle, sticking throttle, throttle body questions


> Keywords: high idle, unintended acceleration
>
> Patient is a 1990 100q     non turbo, manual transmission
>
> Here are the problems I am experiencing.
>
> 1. Idle is at 1200 rpm
>
> 2. Unintended acceleration: After pressing the gas pedal past a certain
> point (secondary throttle valve activation), the throttle wont return.
> Noticeable on shifts - the RPM's climb to 5k.
> Letting out the clutch will reign in the RPM's, but sometimes the throttle
> is stuck until you stomp on the gas pedal to release the throttle. Thats a
> bit scary.
>
> I tested the ISV valve. Unplugged the electrical connection . .no change
in
> idle. I pinched off the hose . . no change in idle.
> I traced the problem to the throttle body. There is friction in the moving
> parts and the secondary throttle plate does not close completely. The
> primary throttle plate closes ok.
>
> Now the questions:
>
> 1. Idle switch - I read 8.0 ohms instead of 0.0 with contacts closed. The
> reading is consistent. I read in the archives about cleaning out the
> contacts by drilling a small hole in the switch body and spraying in
contact
> cleaner. How effective is this procedure? Does the idle switch ground a
> circuit or provides voltage to a circuit?
>
> 2. What is the purpose of the dashpot? I know it slows down the closing of
> the primary throttle plate at the end of its travel by about 1.5 seconds.
> Why is it necessary?
>
> 3. The adjusting screw in the primary/secondary linkage has no head. It
> looks like the head was sheared off deliberately after the factory
assembled
> and adjusted the linkage.
> The function of this screw appears to provide a positive mechanical stop
to
> ensure that the secondary is completely closed when the primary is in
idle.
> A lever on the primary shaft presses against this screw stop. The screw
> intself is mounted on the lever mechanism for the secondary.
> I have about a .020 gap between the screw and the lever. The only thing
that
> shuts off the secondary completely is a small spring at the end of its
> travel . . .not much leverage there.
> I have to figure out a way to make an adjustment . .I may just grab the
set
> screw with pliers. Any ideas?
>
> I stepped away from this email and did some more cleaning and lubricating
on
> the throttle shafts. I used WD-40 on the shafts and linkages.
> I got the secondary shaft and linkages freed up . . .the plates now close
> under spring tension only.
> I suspect that the set screw described above in #3 actually is the
positive
> shut-off for the secondary throttle plate. Otherwise any minor binding of
> the secondary shaft would result in a slightly open throttle plate .
.which
> gives higher idle RPM.
>
>
> SJ
> 85 Dodge PU, D-250, 318, auto
> 85 Audi 4k - - sold but still on the road
> 88 Audi 5kq
> 90 Audi 100q
>
>
>