My 1988 Audi 100 Avant (post-revision with RT 5 cyl. engine, non-cat)
broke down at the weekend, an hour from home :-(
I was going about 20mph when it suddenly splutted and cut out
altogether. Tried to restart it but although I could sometimes get it
running it would only last a few seconds.
Anyway, have been checking things out today and struggling to determine
It will start from cold  but after about 5-10mins it just cuts out.
There is no misfiring, but the idle speed gets into a rhythm hunting
between 900 & 1300rpm before it packs up.
I've tested the cold start valve and warm up valve as per Haynes manual
and they are ok. Throttle body switch appears to work ok. I've bypassed
the Fuel pump relay and it makes no difference. I can see the idle
stabliser valve moves when the ignition is switched on and it looks
clean inside. All vacuum hoses appear to be in good condition and not
blocked. The air sensor plate is positioned ok. The diaphragm device
near the coil is open circuit. Coil resistance is ok. I get a spark -
all plugs, leads & distributor cap are fairly new).
What other tests can I carry out? There seems to be several things that
would prevent starting but what would make it cut out? The Haynes talks
about a fuel cut-out on the air-box but I can see no connections on it.
Haven't checked the voltages at the Ignition control unit as I can't
find that either . There are 2 connectors on the bell housing (near
driveshaft) - one is speedo I believe, what is the other?
 after about 10 seconds cranking but this has always been the case so
may be a red herring.
 It's near the heater blower below the windsceen on my other A100
but not here! Can't see it under the front passenger carpet either.
Thanks for any help anyone can offer.