Weird Idle problem. (idle kickdown causes stall on cis-e)

[amalventano1@comcast.net (Allyn)]

The rieger does the same exact thing:
    our cars have an idle fuel cutout. if the throttle is at the idle position,
it closes the switch on the throttle body. if the idle switch is closed and rpm
> ~1600, this results in the jetronic box applying ~-50ma to the DPR until rpm
drops below ~1400 rpm. This lets the engine coast down with no fuel (as opposed
to the fuel injection attempting to supply correct fuel during a coast).
    as a side note, there is a time delay. the ecu must see the idle switch
closed for >1/4 second in order for it to start the idle kickdown process.

the reason we get our stall is a combination of any of the following:

- dirty idle contact, resulting in longer than 1/4 sec lag for ecu to see the
switch closed (switch debouncing is accomplished by a r/c circuit in the
jetronic box), resulting in the kickdown occuring right at 1400 rpm or slightly
below, possibly resulting in a stall if you get it just right.
- sluggish/clogged fuel system, resulting in a lag as the DPR attempts to get
back to normal control pressure (<1400 rpm). if it cant get pressure back quick
enough, you stall.
- vacuum leaks (may not be enough to mess with idle, but can be enough to cause
a stall in this kickdown condition).
- tuning: the kickdown circuit recovers to the standard +5 to +10 ma DPR current
range. if you adjusted/tweaked your DPR current out of this range, your fuel/air
will be out of whack immediately following the kickdown. this may be enough to
cause a stall.
- idle speed set screw adjusted too low. if the ISV is already at near max open
dwell during a normal idle, it cant open any further to correct for the downward
swing during a kickdown, meaning its harder for the system to recover a low idle
speed, and you stall.

    and finally, the reason the problem goes away when the a/c is on: idle
kickdown is disabled with the a/c on to prevent compressor loading from stalling
the engine during a kickdown event.

    I plan on messing with these things one at a time until i nail it, but i am
fairly certain that its a combination of the above thats causing it right now. i
have a feeling simply opening the idle speed set screw a bit will fix the
problem.

HTH
Al

Allyn Malventano, ETC(SS), USN
87 Rieger GTO Scirocco 16v (daily driver, 180k, rocco #6)
86 Kamei Twin 16V Turbo Scirocco GTX ('it has begun', rocco #7)
86.5 Occo 16v Trailer (dotting 'i's across the country, rocco #8)
87 Jetta 8v Wolfsburg 2dr (daily driver, 260k, 0 rattles, original clutch,
driveshafts, wheels :)


----- Original Message -----
From: "Euroroc II" <flaatr@yahoo.com>
To: <scirocco-l@scirocco.org>
Sent: Friday, September 20, 2002 4:45 PM
Subject: Weird Idle problem.


> Heres one for ya...
>
> My car idles perfectly, closed loop @ 870 RPM, smooth, stock cam. new
> distributor & Cap & Rotor, differential pressure regulator & fuel
> accumulator are new too along with plugs, filter, and oil. The alternator
> is working fine and the battery is happy, 13.5-14V from alternator, no hard
> cranking when starting,  etc. etc. etc.
>
> When I drive the car and come to a stop, slowing in gear to below 2K before
> letting the clutch out the engine speed will drop to 500 RPM'ish bog for a
> split second then return to perfect idle (or lower and occasionally stalls
> although not that often) . This is repeatable and annoying. Revving up and
> letting it drop is fine, holding at a higher RPM and letting it drop is
> fine. the problem only seems to happen when the car is driven, higher RPM
> under load and let it out of gear under 2K. Letting it out of gear above 2K
> is good, engine returns to idle without dipping.
>
> Strange thing is that if I have the A/C on the entire problem goes away.
>
> I've had it in the shop several times, for many days, and they can't seem
> to figure it out. The problem remains so I ask the group...
>
> Anyone have this problem before and how did you fix it? I may have left out
> a detail or two so ask me any questions you have.
>
> -Raffi
>
>
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