Volkswagen: vw van FI question, air flow meter, chop stick

Question
hi there----
i asked you a question a week or 2 back and have been working at my 75 type 2 w/ a 78 motor since with no luck. it consistently starts but dies within a moment. the fuel pump works but not diagnostically...as in i can hear it when im starting the van , but when i bridge pin 36 and 39 in the airbox, the pump doesnt turn on as it should (according to my FI manual anyway) also, it doesnt turn on when i put a chop stick in the airbox and open the air intake flap with my ignition to accessories as it supposedly should. i ohm metered the air flow meter, the temp sensor, and the ECU connections and it all came out as described as 'working properly' in the manual. last night i found a neighbour w/ an old vw not running and pulled/borrowed his dual relay figuring that to be the last possibility and now it wont start at all (could be a bad part, but i opened my dual relay and it looked and smelled fine..no burns) so im at a loss... im not looking for THE ANSWER, but i guess im asking what you would do next? thank you very much in advance for offering your services/wisdom as you do.
p.s....
- i have spark
- i smell gas when i turn her over
- the points appear fine, before she dies the engine sounds quite good
- i have tried swapping airboxes and computers with exact same results
- i have redone all ther vacuum lines to the best of my ability and redone many electrical connections
- i need help

Answer
Hi Adam,

At least we know the problem!  It sounds like a wiring fault.

Here is a way to ENSURE we are dealing with a fuel pump/double relay issue:

1) Hook up the fuel pump to + on the battery and let it just run. (excess fuel will be returned to the tank)

2) As the pump is running, go start the bus.

3) It should run normally now.  

If that is the case, see below:  (If not, let me know, but I'm pretty confident it will run based on your symptoms)



This is how the fuel injection should work:

1) The starter motor activates the fuel pump as you are cranking over the engine. (so, you will get a gas smell).

2) Once the engine starts, you obviously let go of the key, and the starter is no longer operating, thus nor is the fuel pump.  This is where your problem lies.

3) The suction from the engine should pull open the flap and activate the fuel pump from the air flow sensor. This 'hand-off' is done by the double relay.

4) This is also done because if you are in an accident, once the motor kicks off, the closure of the flap kicks off the fuel pump so it doesn't spray all over the place.

5) Thus, you have a wiring problem somewhere.  The fuel pump is not receiving the message to run from the air box when the starter is de-activated.

An important note:  The computer has nothing to do with this operation, as the switch in the Air Sensor is literally an 'on-off' type.  As is the double-relay.

The telling sign was when you put the chopstick in and opened the flap, and nothing kicked on.  So, I am going to assume for now, that the air sensor is not the problem, nor is the ECU.

Now, it's time to check to see if the signal is getting to the double relay.  

- Does the relay click when you put the 'chop stick' in the air box and move the flap?

If it does not click here are some potential culprits:

- The double relay is not grounded on the air-sensor side.
- The double relay is not getting power to the air sensor side.
- The wiring between the double relay and the air flow sensor is faulty somewhere.
- One of the wires fell out of the plastic plugs under the double relay.
- A wire to the fuel pump is missing (by the starter).

The best way to attack this is to determine if there is a 'click' or not when you put the accessory to 'on' and use the chop stick.  NOTE: The double relay will click when you put the key to 'on', but we are looking for an additional click...with the chop stick.

Still diagnosing the no click problem:
Use the wiring guide in the shop manual to make sure that you can account for every wire that goes under the double relay.  Each terminal under the relay is marked with a number (ie 86a 86b) and you can see that number in the wiring diagram and can follow it to it's destination.

Every wire has a home!

If you do get the double relay to click with the chop stick - the wiring fault is between the double relay and the fuel pump.

Start at the fuel pump terminals and check the wiring diagram.

Good Luck,

You are very close.

Ron