Audi 100 Won't Start: Breakthrough

[Edward R. Wendell IV]

I can't remember whether or not you've done a proper fuel pressure 
test. Having resurrected two type 44s that had been sitting 
non-functional for one to two years; I found that the fuel system 
problems were limited to the pump. It's near impossible for 
contamination to get into the fuel distributor without it having been 
taken apart because the fuel filter will stop it. I'm afraid that you 
are getting too deep into this problem. The simplest answer is usually 
the right one.

I recall that you did a flow test but that you did it by disconnecting 
the supply line. The problem with that is that most any pump will push 
fuel through a line with no restriction in it but once you block off 
the flow enough to create 90 or so psi the flow rate will fall off 
drastically. A proper flow test would be done by disconnecting the 
return line and measuring the flow rate there. Because the pressure 
regulator is just upstream of the return line, you will then be 
measuring pump flow rate at full system pressure. The volume of fuel 
flow through the pump and supply line is constant. What changes is the 
amount of fuel that enters the engine, with the remainder returning to 
the tank. With the engine not running and the FPR jumpered or actuated 
via the ECU output test, all the fuel from the pump should cycle 
through the fuel pressure regulator and back to the tank.

A proper flow rate test still won't tell you what the DPR is doing but 
even if it had failed I would think that you would have gotten fuel out 
of the main injectors. As another note, the cold start injector doesn't 
require as much fuel pressure to spray fuel as the main injection 
system does because it has an internal solenoid valve that opens up. 
Most EFI cars with fuel injectors remarkably similar to the cold start 
injector operate at 35 to 40 psi which is about half of what the main 
part of the CIS system operates at.