audi won't idle

Many possibilities:

- Bad or non-functioning cold start injector

- Bad temperature sensor for ECU to operate cold start injector.

- Problem with ISV (idle stabilization valve) or its controller

- Air plate in fuel distributor is sticking

- differential fuel pressure regulator (don't remember the correct name) (on front of fuel distributor) is bad or has dirty contacts

- vacuum leak - very likely

- water in the gas

- dirty fuel filter

- clogged air filter

- clogged cat converter

- blown head gasket - run a compression test

- more but this is a start

Sounds like you have some real diagnostic fun times ahead of you.

kaqu> my 89 audi 100 suddenly started acting up. it will only start with starting

Did you check the main fuel distributor rubber boot and make sure it is not broken and there aren't any missing hoses off of it. Check for other vacuum leaks too.

If it's got a carburator, you might check if there's an electrically activated idle nozzle/valve. Most VAG carburator-engines used to have one - have experienced this on several 1.8S engines of that age. The fix is usually to re-connect the loose wire.