1990 golf gti knock sensor.
http://www.autozone.com/autozone/catalog/parts/partsShelf.jsp?displayName=Knock+Sensor&currentPag e=1&categoryDisplayName=Engine+Management&navValue =15900126&itemId=126-0&parentId=59-0
Most likely mass air sensor.
AutoZone.com | Vehicle Selection | Year
The mass air flow sensor on this car is more like a swinging door. At the top of the unit there is a carbon, half moon shaped, pad that the brush picks up how much air is flowing into your engine. If it is worn out it will not start at all. On my 89 golf i had the same problem you are having and that was what it was. Also check to make sure it is plugged in. sometimes it looks connected but for one reason or another the pins and sockets do not connecting inside the connector itself. Let me know how this works out for you OK.
email me if you need at: jayfroelich@gmail.com