90 100q stalls out - the final solution

[syljay]

My original post on the topic dated Dec 26 09:21:14 PST 2007

http://www.audifans.com/pipermail/quattro/2007-December/120240.html

The original problem:
My 1990 100q, manual, non turbo stalled out at the mall . . . .
would not restart . . .restarted 3 hours later.

The final solution:
Defective Hall Effect sensor in the Distributor.

Details:
I thought I had it fixed with the downpipe repair (details in the 
archive). Car ran fine for about 2 months until it started acting up 
again.
Car finally stalled out and would not restart. I had it towed back home.

I did notice some ignition missing sometimes when driving from a cold 
start. But the missing would clear up after a mile or two of driving. 
The missing got worse so I swapped out the dist cap and wiring from 
the 88 5kq to the 90 100q. Problem seemed to clear up so I thought the 
cap/wiring were the problems. I got a new cap and rotor and problem 
seemed to be solved.

Then the wife reported that car would not restart when she went 
shopping. She left the car to get a bite to eat, came back and car 
started up fine.

Finally, the car started up one day, wife drove it about half a mile, 
and it started bucking and stalled out.
I went to car, started it up . .no problem. Managed to pull it onto 
shoulder. Engine died a lingering death - started missing and losing 
RPM's and finally died.
I came back several hours later, same problem. Car would start right 
up . .and die after 15-30 seconds.
I left the car overnight - came back next morning - same problem. Had 
the car towed home.

Diagnostics:
The engine would always start, but died after about a minute or two of 
idling. Sounded like fuel supply low or cutting out. Fuel Pressure 
test ruled that out.
After much parts swapping, I narrowed the problem down to the Distributor.
With a timing light attached to the distributor center lead, I could 
see the missing spark pulse in the usually steady pulsing rhythm of 
the timing light. Everytime the timing light showed a missing spark, 
the engine would stumble.
I swapped out the distributor with the one from my 5kq and car started 
right up and kept on running. I started and restarted car over 2 days 
- no problems.
Wife has been driving car for 3 days now and its fine.

Problem solved.

Apparently, the Hall Effect sensor in the distributor was getting 
flaky and would act up under certain temperature conditions. Further 
reading on the topic of "Audi Hall Effect" in the archives and web 
searches showed that my symptoms(erratic symptoms) were typical for a 
failing/failed Hall sensor.

Next time, as another good diagnostic tool, I will remove the 
distributor cap, rotor and plastic shield and spray the Hall Effect 
sensor with electronic Freeze Spray.
http://www.allelectronics.com/cgi-bin/item/PTR-7400/163/FREEZE_SPRAY,_10_OZ_CAN_.html
  "Helps locate thermal intermittent defective resistors, capacitors 
and semiconductors. Helps locate cold solder joints and pc board cracks."

I've used this stuff in my prior life on intermittant electronic 
components. The cold would make the the parts work until they warmed up.


SJ in NJ