[s-cars] Stumbling and stalling

[Paul Gailus]

I would first check if there are any trouble codes:
http://www.sjmautotechnik.com/TROUBLE_SHOOTING/ecuf20v.html

It's a long shot, but since you mentioned the jerking
action , it's possible that you could have a certain problem
in the ignition coils. One or more of the coils could have a
defective blocking diode in series with the secondary winding.
The purpose of this diode is to prevent a possible spark
from occuring when a positive output voltage is produced
at the coil output when the POS transistor first turns on.
Such a false spark would be more likely with the relatively
small 0.025" spark plug gaps in the AAN.
The diode will conduct and produce the desired spark when
a negative output voltage is induced when the POS turns off.
The unwanted positive voltage is at most a few KV,
which is not enough to cause a spark at the plug when
the cylinder pressure (and also maniford pressure) is
relatively high . However, when the engine is throttled
it may be possible once in a while to get an undisired
spark when the POS turns on. This spark would be advanced
way too much, and cause a jerk. Luckily, such a false spark
is unlikely at higher cylinder pressures where it could
cause a lot of damage.

It would be easiest to check for such a defective coil
by swapping coil packs with another car, if possible.
Otherwise, you can make some measurements on
the coil outputs, but they will not be very definitive.
If you put positive 27 volts DC on the coil output
(relative to the coil grounded core) it should
conduct about 3-5 milliamps. Essentially no measureable
current should flow in the other direction (it should be able
to block 1-2KV).
If you can find a shop with an ignition scope and all the
appropriate adaapters, that would be another good option.


Paul


----- Original Message -----
From: EB <rebank at sbcglobal.net>
To: <s-car-list at audifans.com>
Sent: Thursday, May 06, 2004 6:42 PM
Subject: [s-cars] Stumbling and stalling


> Guys,
>
> My car recently started to stumble, as in momentary loss of power.  A few
times it happened at the traffic light, resulting in stalling.  What is
worse, the stumble also occurs at a high speed, when it is no longer a mild
stumble, but a very hard jerk.  Seems to idle OK, though.  Also, sometimes
the CHECK ENGINE light comes on and goes away with a light application of
throttle.  I also have what has been described lately by a few of us, i.e.
sharp drop of RPMs when declutching while coming to a stop.
> Replacing O2 sensor did not help any of the above.
>
> Any ideas?
>
> Thanks a lot,
> Edward
>
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