Great Results So Far with Vapor Purge Line
 

I plumbed in a return line from the fuel pump output back to the tank. It runs through a valve that is just *barely* cracked.
So far so good - fuel pressure is rock steady :D
What used to happen after a long time running on a hot day is the fuel pressure would get erratic, dropping to 0 or close and bouncing back up to 4. On rare occasion fuel would quit feeding altogether :eek:
One odd side effect is the engine is a lot easier to start. I think the carb primes a lot faster now.

Joe-
Assuming a Facet pump...
Have you noticed any difference in the temp of the pump?
Maybe running cooler?

(Neil knows where this is going) ;)

I did not measure the before and after temp. I am assuming it is at least a little bit cooler. If I cracked the valve more it would probably be a lot cooler.
edit - My buddies old 67 Corvette has this same system with a tiny drilled hole to meter the return fuel.

Update - still no sign at all of vapor lock and the engine remains easier to start :D
My theory on starting is with my habit of running the fuel out at the end of the day, the vapor return allows the carb to prime again much easier.

I am thinking the fuel pump will also live a longer life being kept a bit cooler.

Still working well. We ran for several hours at 100% power and not a hint of fuel pressure fluctuations. I wish I had done this years ago!

* general hint, many "A4 problems" are not specific to the A4, many a diesel has fuel feed problems and exhaust problems too ;)