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If your fuel system was full of trash, you are in trouble with the system. It is very hard to get it clean. All lines must be flushed, hoses replaced,etc. If you are getting a pulsation at the injectors, the injectors must be plugged up. It will continue to get cantamination when the fuel pump runs. Is the fuel pump running when cranking?
The most economical thing to do is a carborator conversion. If you want more information on this I can give it to you.
You need to see what the voltage is at the injector. There is a voltage reducer pack on the right side on the side of the engine compartment. This reduces the voltage. It is small silver box with white wires and a plug in. This could be faulty. There are many things on the fuel injection system that can be faulty. That is why most people will do a change over to carboration. There are very few people who can diagnose the issues without some sort of experience because the system is so old. I could figure it out but I also have an ample supply of parts to try on it. It can get expensive as you are finding out to repair this fuel injection and frustrating. You need a good OEM VW manual that covers the fuel injection so you can troubleshoot. There is usually no simple solution.
Here's a list of things that could cause it.
I am at home now but if you can wait till morning, I will check my manual and see. What voltage do you have?
I have the book in front of me now. There should be a constant supply of current to the injectors. That supply comes from terminal 88b on the double relay. The control unit supplies a negative ground to all 4 injectors simultaneously. The negative ground breaks with each opening and closing of the points. THe series resistant block keeps a constant voltage. It does not give me the voltage specs but I don't think 1 volt is enough. You should have 12 volts to the restance block Do you have access to VW official repair manual? It gives all the testing procedures. Have you check the temp sensor on the cylinder head. At 68' it should read 2500 ohms. I think you either have a faulty resistance block or temp sensor. I am not allowed to give out phone numbers or email addresses on this site or I will be booted off as an expert. I'll try to figure something out.