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Audi 5000 Fuel Distributor

About a month ago, my 85-Audi 5000, 5-speed manual, 240,000 miles, developed
an abrupt "mis-fire" when starting out from a stop when lugging at low RPM
only. I did a complete tuneup, plug wires, plugs, cap, rotor as it was due.
I found bits of plastic from a wire connector that had broken off and fallen
into the distributor so I blew out the distributor as well. Also checked for
vacuum leaks. Nothing seemed to have a permanent effect. The mis-fire would
come and go and Techron seemed to ease the problem for a bit. 

Finally I pulled the bellows off the intake and airflow venturi. The airflow
sensor plate was very dirty and free movement was being restricted when the
plate was at the bottom of travel. I also noted that the plate itself did
not move free like it did several years ago. I cleaned this and most of the
problem was gone but the plate movement was getting stiff when pushing it
up, probably from age.

Since mileage was high I decided to replace the fuel distributor assembly
with a newer used one. I wish I hadn't.

The fuel distributor replacement was off an 86 VW Quattro, model numbers
matching. After installation the now occasional mis-fire completely

The car was fine for a day or so but another low RPM mis-fire developed,
this one not so abrupt more like an occasional sputter. But after a week it
was there all the time, also at low RPM, whether lugging or not up to 1500
to 1800 RPM.

I figured I got a defective distributor and replaced it with another used
model. No change at all!

Here is what happens now. Start up from cold is fine, flawless operation,
power good until the engine warms and mixture leans out a bit. Then I have
rough idle and mildly rough acceleration up to about 1600 RPM. Then it runs
smooth at driving speeds.

Some possibilities - maybe a fuel injector collected some dirt from the
newer used distributor, maybe a fuel injector just decided to go south,
maybe I have gotten hold of two used and defective distributors, maybe its
something I haven't thought of yet.

Ideas out there?

Otherwise the car in its advanced age are fine, as usual an excellent
high-speed road car winter and summer.

Dennis [Minnesota/BN Highline]