Barely-running '89 90q

[Huw Powell]

> Well this has really turned into a stumper.  Some of you may recall the
> problem I'm having with my 221k mile '89 90q- in short, it runs like it is
> missing on at least one if not two cylinders.  Starts like crap, runs like
> crap when warmed up. This behavior started suddenly at highway speed, with
> no indication of this before.  No power, and the exhaust smells pretty rich.
> Here's what I have done:
> Checked spark- OK
> Checked compression- OK
> Checked TDC on cam compared to flywheel- OK
> Checked fuel system pressure(s)- OK
> Checked fuel pump delivery rate- OK
> Checked O2 sensor operation-at least attempts closed-loop
> Checked cat- not plugged, but I hollowed it out anyway
> Checked for vac leak- as well as I could, hoses look ok
> Pulled Codes- 4444
> Pulled out and tested injectors- thought I had it here....  But noooo.
> Still runs the same, maybe a little worse.  Argh!  I'm getting thin on ideas
> now.  Would a big vac leak cause this behavior?  I am just baffled by the
> fact that it ran great for a good long while and then suddenly crapped out.

Other than obvious physical issues like vacuum leaks, what you need to 
do is check and verify all the senders and mechanicals of the CIS-E3 
system.  One at a time, and don't worry if a given component could cause 
the problem, check it anyway.

People have mentioned the temp sender - on top of the upper coolant 
flange.  best way to test is to remove it, bring it inside, and immerse 
in boiling and iced water, measure resistance.  Compare to Bentley 
manual spec.

Potentiometer - on the "hidden" side of the fuel dist.  Tough to get to, 
I made a little 3 pin out jumper to get to mine.  measure while slowly 
raising air plate, should be a smooth change in resistance with no dropouts.

WOT and idle switches - connector near throttle body, check to make sure 
they open and close properly.

Pressure actuator - on front of fuel dist, little grey box with a 
connector.  measure current, when at warm idle it should be near 0 mA. 
When cold it is higher, 50-100 mA or so and dropping as the engine warms 
up.  The ECU controls this when cold based on the temp sender, when warm 
based on OXS signal ("closed loop").

OXS output - hard to measure if other things are driving the system 
rich, but if it the problem, disconnecting it should pretty much remedy 
the issue.

Uh, what else?  ISV couldn't cause the problem, but no harm in cleaning 
it anyway.  Check its duty cycle when cold to warm, should start highish 
and slowly drop to about 30%.  (at idle)

make sure the cold start valve is not firing all the time for some reason

you've checked your injectors and ignition.

make sure the engine fuses on the side of the fusebox are good.

make sure the ground wires to the intake manifold are intact and clean.

check your air filter.

well, that's about all off the top of my head.  I'm sure if anyone 
thinks of anythig else they will chime in.

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/