what would cause this idle behavior - 84 GTI

I have an 84 GTI, ~230k miles. Car starts great, idles around 900 rpm
for about 30 sec, then idle drops off to about 600-700, so it almost
stalls (and occasionally does). Over the next 15 minutes, the idle
will slowly creep back up to 800-850, where it remains. She has new
plugs, wires, rotor, dist. cap, o2 sensor, and a few new vacuum
lines. I've checked vacuum lines for leaks, found none. Anything
obvious to check first? Bentley notes a number of things (thermo-time
switch, cold start valve, aux. air valve, CPR for instance), but not
in any particular order for this kind of problem. I'm hoping someone
has seen this symptom before and can say what causes it. Thanks.

There is a valve on the right side strut tower that boosts the idle. I
think there is one for AC and one for general purposes idle bumpup.
You need to set the idle speed when warm with both of those power
plugs unplugged. That idle speed slowly comes up as the system control
pressure adjusts. It takes a bit of time for the Control Pressure
Regulator to warm up with electrical heat and oil warm up.

Check for vacuum leaks with unlit propane torch or carb cleaner.

On Mon, 22 Sep 2008 05:45:32 -0700 (PDT), wrote:

>I have an 84 GTI, ~230k miles. Car starts great, idles around 900 rpm
>for about 30 sec, then idle drops off to about 600-700, so it almost
>stalls (and occasionally does). Over the next 15 minutes, the idle
>will slowly creep back up to 800-850, where it remains. She has new
>plugs, wires, rotor, dist. cap, o2 sensor, and a few new vacuum
>lines. I've checked vacuum lines for leaks, found none. Anything
>obvious to check first? Bentley notes a number of things (thermo-time
>switch, cold start valve, aux. air valve, CPR for instance), but not
>in any particular order for this kind of problem. I'm hoping someone
>has seen this symptom before and can say what causes it. Thanks.

On Sep 22, 9:07*am, Jim Behning
> wrote:
> There is a valve on the right side strut tower that boosts the idle. I
> think there is one for AC and one for general purposes idle bumpup.
> You need to set the idle speed when warm with both of those power
> plugs unplugged. That idle speed slowly comes up as the system control
> pressure adjusts. It takes a bit of time for the Control Pressure
> Regulator to warm up with electrical heat and oil warm up.
>
> Check for vacuum leaks with unlit propane torch or carb cleaner.
>
>
>
> On Mon, 22 Sep 2008 05:45:32 -0700 (PDT), wrote:
> >I have an 84 GTI, ~230k miles. *Car starts great, idles around 900 rpm
> >for about 30 sec, then idle drops off to about 600-700, so it almost
> >stalls (and occasionally does). *Over the next 15 minutes, the idle
> >will slowly creep back up to 800-850, where it remains. *She has new
> >plugs, wires, rotor, dist. cap, o2 sensor, and a few new vacuum
> >lines. *I've checked vacuum lines for leaks, found none. *Anything
> >obvious to check first? *Bentley notes a number of things (thermo-time
> >switch, cold start valve, aux. air valve, CPR for instance), but not
> >in any particular order for this kind of problem. *I'm hoping someone
> >has seen this symptom before and can say what causes it. *Thanks.- Hide quoted text -
>
> - Show quoted text -

Are you talking about the "thing" next to the cold-start valve, that
has a couple vacuum hoses coming into the bottom of it and a couple of
2-wire terminals at the top? This car has (non-functioning) A/C,
btw. What is that valve called - I have not seen it described in
Bentley, and never knew what it did. How does it work?

And by adjusting the idle, I assume you just mean by turning the screw
in back of the throttle body, right? Thanks for the quick response
Jim, this sounds easy to try.

they did that when new off the truck!!! NO kidding! Try adjusting the CO%
slightly richer.


> wrote in message
...
>I have an 84 GTI, ~230k miles. Car starts great, idles around 900 rpm
> for about 30 sec, then idle drops off to about 600-700, so it almost
> stalls (and occasionally does). Over the next 15 minutes, the idle
> will slowly creep back up to 800-850, where it remains. She has new
> plugs, wires, rotor, dist. cap, o2 sensor, and a few new vacuum
> lines. I've checked vacuum lines for leaks, found none. Anything
> obvious to check first? Bentley notes a number of things (thermo-time
> switch, cold start valve, aux. air valve, CPR for instance), but not
> in any particular order for this kind of problem. I'm hoping someone
> has seen this symptom before and can say what causes it. Thanks.

Also the Aux Air Valve can be adjusted by loosening the little nut and
sliding it one way or the other and retightening it.
Try idling the engine at 1K when it is warm and see how the idle is during
its next warm up.
Do you have good power during warm up?

Check those fuel injector seals for vacuum leaks.
--
later,
(One out of many daves)

> wrote in message
...
>I have an 84 GTI, ~230k miles. Car starts great, idles around 900 rpm
> for about 30 sec, then idle drops off to about 600-700, so it almost
> stalls (and occasionally does). Over the next 15 minutes, the idle
> will slowly creep back up to 800-850, where it remains. She has new
> plugs, wires, rotor, dist. cap, o2 sensor, and a few new vacuum
> lines. I've checked vacuum lines for leaks, found none. Anything
> obvious to check first? Bentley notes a number of things (thermo-time
> switch, cold start valve, aux. air valve, CPR for instance), but not
> in any particular order for this kind of problem. I'm hoping someone
> has seen this symptom before and can say what causes it. Thanks.

On Sep 22, 9:37*pm, "dave AKA vwdoc1" >
wrote:
> Also the Aux Air Valve can be adjusted by loosening the little nut and
> sliding it one way or the other and retightening it.
> Try idling the engine at 1K when it is warm and see how the idle is during
> its next warm up.
> Do you have good power during warm up?
>
> Check those fuel injector seals for vacuum leaks.
> --
> later,
> (One out of many daves)
>
> > wrote in message
>
> ...
>
>
>
> >I have an 84 GTI, ~230k miles. *Car starts great, idles around 900 rpm
> > for about 30 sec, then idle drops off to about 600-700, so it almost
> > stalls (and occasionally does). *Over the next 15 minutes, the idle
> > will slowly creep back up to 800-850, where it remains. *She has new
> > plugs, wires, rotor, dist. cap, o2 sensor, and a few new vacuum
> > lines. *I've checked vacuum lines for leaks, found none. *Anything
> > obvious to check first? *Bentley notes a number of things (thermo-time
> > switch, cold start valve, aux. air valve, CPR for instance), but not
> > in any particular order for this kind of problem. *I'm hoping someone
> > has seen this symptom before and can say what causes it. *Thanks.- Hide quoted text -
>
> - Show quoted text -

So I tried Jim's suggestion, and tweaked the idle screw a little.
Idle is more stable now, but I don't know how disconnecting the idle
air bypass valve (finally found that in the Bentley) would have made a
difference.
Checked for vacuum leaks with a propane torch, including the injector
seals, and got no reaction from that.
As to Woodchuck's comment, THIS car didn't do that new off the truck.
I've had it for 22 years and this a new phenomenon for me.
I will also try to see what happens if I adjust the aux air valve as
Dave suggests. Power and response is good once I put my foot on the
gas, even when cold. Could it be a sticking throttle plate, that gets
less sticky as the engine warms up?
Thanks for all your suggestions...

> wrote in message
...
Are you talking about the "thing" next to the cold-start valve

Dunno about a GTI, but on my '81 Scirocco the thing below the cold-start
valve is the auxiliary air regulator, and when it went bad the engine would
not idle at all when cold (or even after sitting for a short time).

One person spent $1200 to get a Scirocco saved from the junkyard to start
and idle perfectly at anytime! OUCH!
Almost everything was needed/addressed even the residual pressure device
(fuel accumulator) located by the fuel pump.

So do you want perfection, or are you willing to compromise with a couple of
quirks with a 24+ year old car?
I prefer spraying carb cleaner at those injector seals, but I don't use
propane anyway! One day I might! ;-)

When setting the warm idle BTB ()By The Book), you might have to disconnect
and/or plug components. Some VWs you have to create a small vacuum leak by
inserting/flipping a fitting. 8^o
--
later,
(One out of many daves)

> wrote in message
...
On Sep 22, 9:37 pm, "dave AKA vwdoc1" >
wrote:
> Also the Aux Air Valve can be adjusted by loosening the little nut and
> sliding it one way or the other and retightening it.
> Try idling the engine at 1K when it is warm and see how the idle is during
> its next warm up.
> Do you have good power during warm up?
>
> Check those fuel injector seals for vacuum leaks.
> --
> later,
> (One out of many daves)
>
> > wrote in message
>
> ...
>
>
>
> >I have an 84 GTI, ~230k miles. Car starts great, idles around 900 rpm
> > for about 30 sec, then idle drops off to about 600-700, so it almost
> > stalls (and occasionally does). Over the next 15 minutes, the idle
> > will slowly creep back up to 800-850, where it remains. She has new
> > plugs, wires, rotor, dist. cap, o2 sensor, and a few new vacuum
> > lines. I've checked vacuum lines for leaks, found none. Anything
> > obvious to check first? Bentley notes a number of things (thermo-time
> > switch, cold start valve, aux. air valve, CPR for instance), but not
> > in any particular order for this kind of problem. I'm hoping someone
> > has seen this symptom before and can say what causes it. Thanks.- Hide
> > quoted text -
>
> - Show quoted text -

So I tried Jim's suggestion, and tweaked the idle screw a little.
Idle is more stable now, but I don't know how disconnecting the idle
air bypass valve (finally found that in the Bentley) would have made a
difference.
Checked for vacuum leaks with a propane torch, including the injector
seals, and got no reaction from that.
As to Woodchuck's comment, THIS car didn't do that new off the truck.
I've had it for 22 years and this a new phenomenon for me.
I will also try to see what happens if I adjust the aux air valve as
Dave suggests. Power and response is good once I put my foot on the
gas, even when cold. Could it be a sticking throttle plate, that gets
less sticky as the engine warms up?
Thanks for all your suggestions...

On Tue, 23 Sep 2008 06:56:15 -0700 (PDT), wrote:

>On Sep 22, 9:37*pm, "dave AKA vwdoc1" >
>wrote:
>> Also the Aux Air Valve can be adjusted by loosening the little nut and
>> sliding it one way or the other and retightening it.
>> Try idling the engine at 1K when it is warm and see how the idle is during
>> its next warm up.
>> Do you have good power during warm up?
>>
>> Check those fuel injector seals for vacuum leaks.
>> --
>> later,
>> (One out of many daves)
>>
>> > wrote in message
>>
>> ...
>>
>>
>>
>> >I have an 84 GTI, ~230k miles. *Car starts great, idles around 900 rpm
>> > for about 30 sec, then idle drops off to about 600-700, so it almost
>> > stalls (and occasionally does). *Over the next 15 minutes, the idle
>> > will slowly creep back up to 800-850, where it remains. *She has new
>> > plugs, wires, rotor, dist. cap, o2 sensor, and a few new vacuum
>> > lines. *I've checked vacuum lines for leaks, found none. *Anything
>> > obvious to check first? *Bentley notes a number of things (thermo-time
>> > switch, cold start valve, aux. air valve, CPR for instance), but not
>> > in any particular order for this kind of problem. *I'm hoping someone
>> > has seen this symptom before and can say what causes it. *Thanks.- Hide quoted text -
>>
>> - Show quoted text -
>
>So I tried Jim's suggestion, and tweaked the idle screw a little.
>Idle is more stable now, but I don't know how disconnecting the idle
>air bypass valve (finally found that in the Bentley) would have made a
>difference.
>Checked for vacuum leaks with a propane torch, including the injector
>seals, and got no reaction from that.
>As to Woodchuck's comment, THIS car didn't do that new off the truck.
>I've had it for 22 years and this a new phenomenon for me.
>I will also try to see what happens if I adjust the aux air valve as
>Dave suggests. Power and response is good once I put my foot on the
>gas, even when cold. Could it be a sticking throttle plate, that gets
>less sticky as the engine warms up?
>Thanks for all your suggestions...
If you know that vintage car well you will on occasion hear a click
when the idle dips. That is that booster (intake air bypass valve) on
the strut tower opening up and letting more air in. That also lets in
more fuel bumping up the idle. If you hear your car clicking a lot
then you know you need to properly adjust the idle speed. No I was not
talking about the warm up air regulator mounted on the intake.

See pages 58-59 of your Bentley Rabbit 1980-1984 gas service manual in
the fuel section. The manual does their procedure different from what
I did. I unplug the connectors on the idle air bypass valve so it is
not trying to boost the idle speed. I adjust my idle speed and then
reconnect the electrical connector. They speak of clamping down on the
air hose so it is not supplying "false" air.

I have never played with the settings on the auxiliary air regulator
but I have often looked at it wanting to but I resisted.

Well if it is running better then you may be done. I did have plenty
of crud in all my fuel screens. I bought my 84 from a junkyard with no
hood, gas cap or seats. I replaced a few injection things due to
clogging over those yeras of ownership. Having a vehicle most its
whole life is much easier to maintain as there hopefully is less jerry
rigging to undo.

"Jim Behning" wrote in message

snip

> I have never played with the settings on the auxiliary air regulator
> but I have often looked at it wanting to but I resisted.

Only if you need/want to! ;-)

>
> Well if it is running better then you may be done. I did have plenty
> of crud in all my fuel screens. I bought my 84 from a junkyard with no
> hood, gas cap or seats. I replaced a few injection things due to
> clogging over those yeras of ownership. Having a vehicle most its
> whole life is much easier to maintain as there hopefully is less jerry
> rigging to undo.

I seem to remember that some inlet bolts for the fuel distributor could have
a screen on them too.